Excitable tissues and their general properties

Excitable tissues are nervous, muscular and glandular structures that are capable of being excited spontaneously or in response to the action of an irritant. Excitation is the generation of an action potential (AP) + the spread of AP + a specific tissue response to this potential, for example, contraction, release of a secret, release of a mediator quantum.

Properties of excitable tissues and indicators characterizing them:

Properties

1. Excitability - the ability to be excited

2. Conductivity - the ability to conduct excitation, i.e. conduct PD

3. Contractility - the ability to develop force or tension when excited

4. Lability - or functional mobility - the ability to rhythmic activity

5. The ability to secrete a secret (secretory activity), mediator

Indicators

Threshold of irritation, rheobase, chronaxy, duration of the absolute refractory phase, rate of accommodation.

The speed of AP conduction, for example, in a nerve, it can reach 120 m/s (about 600 km/h).

The maximum value of force (voltage) developed during excitation.

The maximum number of excitations per unit of time, for example, a nerve is capable of generating 1000 APs in 1 s.

Electrical phenomena in excitable tissues

Classification:

Biopotentials- the general name of all types of electrical processes in living systems.

Damage potential- historically the first concept of the electrical activity of the living (demarcation potential). This is the potential difference between intact and damaged surfaces of living excitable tissues (muscles, nerves). The clue to its nature led to the creation of the membrane theory of biopotentials.

Membrane potential(MP) is the potential difference between the outer and inner surfaces of the cell (muscle fiber) at rest. Typically, the MP, or resting potential, is 50-80 mV, with a "-" sign inside the cell. When a cell is excited, an action potential is recorded (its phases: peak, trace negativity, trace positivity) - a rapid change in the membrane potential during excitation.

Extracellularly registered action potential, intracellularly registered action potential- these are variants of action potentials, the shape of which depends on the method of assignment (see below).

Receptor (generator) potential– change in the magnetic field of receptor cells during their excitation.

Postsynaptic potentials(options: excitatory postsynaptic potential - EPSP, inhibitory postsynaptic potential - IPSP, a special case of excitatory postsynaptic potential - EPP - end plate potential).

Evoked Potential is the action potential of a neuron that occurs in response to the excitation of a receptor that carries information to this neuron.

History of arousal physiology research

L. Galvani was the first to be convinced of the existence of "living electricity". His first (balcony) experience was that the preparation of the hind legs of frogs on a copper hook was suspended from an iron balcony. From the wind, he touched the balcony railing, and this caused muscle contraction. According to Galvani, this was the result of closing the current circuit, as a result of which "living electricity" caused contraction. Volta (Italian physicist) refuted this explanation. He believed that the reduction was due to the presence of a "galvanic pair" - iron-copper. In response, Galvani set up a second experiment (experiment without metal), which proved the author's idea: a nerve was thrown between the damaged and undamaged muscle surfaces and, in response, the intact muscle contracted.

Membrane potential and its origin

MP, or resting potential, is the potential difference between the outer and inner surfaces of the membrane at rest. On average, in cells of excitable tissues, it reaches 50–80 mV, with a “–” sign inside the cell. It is mainly due to potassium ions. As is known, in the cells of excitable tissues, the concentration of potassium ions reaches 150 mmol / l, in the environment - 4-5 mmol (potassium ions are much more in the cell than in the environment). Therefore, along the concentration gradient, potassium can leave the cell, and this occurs with the participation of potassium channels, some of which are open at rest. As a result, due to the fact that the membrane is impermeable to cell anions (glutamate, aspartate, organic phosphates), an excess of negatively charged particles is formed on the inner surface of the cell, and an excess of positively charged particles is formed on the outer surface. There is a potential difference. The higher the concentration of potassium in the medium, the lower this ratio, the lower the value of the membrane potential. However, the calculated value is usually lower than the actual value. For example, according to calculations, the MP should be -90 mV, but in reality -70 mV. This discrepancy is due to the fact that sodium and chloride ions also contribute to the creation of the magnetic field. In particular, it is known that there is more sodium in the medium (140 mmol/l versus 14 mmol/l intracellular). So sodium can enter the cell. But most of the sodium channels are closed at rest. Therefore, only a small part of sodium ions enter the cell. But even this is enough to at least partially compensate for the excess of anions. Chlorine ions, on the contrary, enter the cell (partially) and introduce negative charges. As a result, the value of the membrane potential is determined mainly by potassium, as well as by sodium and chlorine.

In order for the magnetic field to be maintained at a constant level, it is necessary to maintain ionic heterogeneity - ionic asymmetry. For this purpose, in particular, the potassium-sodium pump (and chloride) is used, which restores the ionic asymmetry, especially after the act of excitation. The proof of the potassium nature of the magnetic field is the dependence: the higher the concentration of potassium in the medium, the lower the value of the magnetic field. For further presentation, the concept is important: depolarization (reduction of the magnetic field, for example, from minus 90 mV to minus 70 mV) and hyperpolarization are the opposite phenomenon.

action potential

action potential- this is a short-term change in the potential difference between the outer and inner surfaces of the membrane (or between two points in the tissue), which occurs at the moment of excitation. When registering the action potential using microelectrode technology, a typical peak-shaped potential is observed. It has the following phases or components:

1. Local response - the initial stage of depolarization.

2. The phase of depolarization - a rapid decrease in the membrane potential to zero and recharging of the membrane (reversion, or overshoot).

3. Phase of repolarization - restoration of the initial level of the membrane potential;

in it, the phase of fast renolarization and the phase of slow repolarization are distinguished, in turn, the phase of slow repolarization is represented by trace processes (potentials):

trace negativity (trace depolarization) and trace positivity (trace hyperpolarization). The amplitude-temporal characteristics of the action potential of the nerve, skeletal muscle are as follows: the amplitude of the action potential is 140-150 mV; the duration of the action potential peak (depolarization phase + repolarization phase) is 1–2 ms, the duration of trace potentials is 10–50 ms.

The shape of the action potential (during intracellular recording) depends on the type of excitable tissue: in the axon of a neuron, skeletal muscle - peak-like potentials, in smooth muscles in some cases peak-like, in others - plateau-like (for example, the action potential of the smooth muscles of the uterus of a pregnant woman is plateau-like, and its duration is almost 1 minute). In the heart muscle, the action potential has a plateau shape.

The nature of the action potential

When studying the AP of the axons and soma of the nerve cell, the AP of the skeletal muscle, it was found that the depolarization phase is due to a significant increase in the permeability for sodium ions that enter the cell at the beginning of the excitation process and thus reduce the existing potential difference (depolarization). Moreover, the higher the degree of depolarization, the higher the permeability of sodium channels becomes, the more sodium ions enter the cell and the higher the degree of depolarization. During this period, there is not only a decrease in the potential difference to zero, but also a change in the polarization of the membrane - at the height of the AP peak, the inner surface of the membrane is positively charged relative to the outer one (the phenomenon of reversion, or overshoot). However, this process cannot go on indefinitely: as a result of the closing of the inactivation gate, the sodium channels close, and the influx of sodium into the cell stops. Then comes the repolarization phase. It is associated with an increase in the release of potassium ions from the cell. This is due to the fact that as a result of depolarization, most of the potassium channels, which were closed under resting conditions, open and “+” charges go outside the cell. At first, this process goes very quickly, then slowly, so the repolarization phase first proceeds quickly (descending part of the AP peak), and then slowly (trace negativity). The same process underlies the trace hyperpolarization phase. Against the background of trace potentials, the potassium-sodium pump is activated. If it operates in an electrically neutral mode (2 sodium ions are removed from the cell in exchange for 2 potassium ions introduced into the cell), then this process is not reflected in the form of AP. If the pump operates in an electrogenic mode, when 3 sodium ions are taken out of the cell in exchange for 2 potassium ions introduced into the cell, then as a result, for each cycle of the pump, 1 less cation is introduced into the cell than is taken out, therefore, the excess in the cell gradually increases. anions, t.s. in this mode, the pump contributes to the appearance of an additional potential difference. This phenomenon may underlie the trace hyperpolarization phase.

In the cardiac muscle, the nature of AP is different: the process of depolarization is caused by sodium and calcium ions - these ions enter the cell at the beginning of the depolarization phase.

In the smooth muscles of blood vessels, stomach, intestines, uterus, and other formations, AP generation is associated with the fact that at the moment of excitation, calcium ions rather than sodium ions enter the cell.

Laws of irritation of excitable tissues

Before considering these laws, it is necessary to imagine how the excitation occurs, i.e. what conditions must arise in an excitable tissue in order for it to realize its ability to be excited. The main condition is a decrease in the membrane potential to a critical level of depolarization (CDL). Any agent, if it is able to do so, simultaneously causes tissue excitation. For example, MP -70 mV. FAC = -50 mV. To cause excitation, it is necessary to depolarize the membrane to -50 mV, t. by 20 mV to reduce its initial resting potential. As soon as MP reaches the level of CUD, then in the future the process (due to regenerative) will continue independently and will lead to the opening of all sodium channels, i.e. to the generation of a full-fledged PD. If the membrane potential does not reach this level, then at best a so-called local potential (local response) will arise.

All agents that cause tissue hyperpolarization will not be able to cause excitation at the moment of exposure, because in this case the MF does not reach the critical level of depolarization, but, on the contrary, leaves it.

Three remarks:

1. In a number of excitable tissues, the value of the membrane potential is not constant over time - it periodically decreases and independently reaches the FCA, resulting in spontaneous excitation (automaticity). This is typical for pacemakers of the heart, for some smooth muscles, for example, the muscles of the uterus.

2. When an irritant (in a subthreshold strength) acts on the tissue, it can cause a change in the FCA. For example, prolonged subthreshold depolarization leads to the fact that the FRA changes: for example, in the initial state it is -50 mV, and as a result of prolonged depolarization it becomes equal to -40 or -30 mV. In such a situation, it becomes more difficult to cause arousal. In general, this phenomenon is called the accommodation of excitable tissue. It underlies the gradient law (not to be confused with the concept of "accommodation of the eye").

3. To excite a tissue, it is necessary to have an external stimulus in relation to this tissue (an exception is tissues with automaticity). Such irritants in natural conditions can be a nerve impulse, the release of a mediator. In general, in physiology they speak of two types of stimuli - adequate and inadequate. Adequate stimuli are those effects that "in small doses" can cause excitation. For example, a quantum of light for a photoreceptor, a nerve impulse for a synapse. An inadequate stimulus is also capable of causing excitation, but for this it must be used in large "doses", as a result of which the tissue may be damaged.

In order for the stimulus to cause excitation, it must be: 1. strong enough (the law of force), 2. long enough (the law of time), 3. grow fast enough (the law of the gradient). If these conditions are not met, then excitation does not occur. Let us consider in more detail these laws of irritation and the consequences that follow from them.

The law of strength. For arousal to occur, the stimulus must be strong enough - threshold or above threshold. Usually, the term "threshold" refers to the minimum strength of the stimulus that can cause excitation. For example, to cause excitation of a neuron at MP = -70 mV and FSC = -50 mV, the threshold force must be -20 mV. A lower strength of the stimulus will not cause a response.

One important consequence of this law is the introduction of the concept of “stimulus threshold” (the minimum strength of a stimulus capable of causing arousal). Determining this indicator

Law of time(or the dependence of the threshold strength of the stimulus on the time of its action). This law states: the stimulus that causes excitation must be long enough to act on the tissue for some time to cause excitation. It turned out that in a certain range, the dependence of the threshold strength of the stimulus on the duration of its action has the character of an inverse relationship (hyperbole) - the less time the stimulus acts on the tissue, the higher its strength is required to initiate excitation. On the curve (Goorweg-Weiss-Lapik) there are areas that indicate that if the stimulus is long enough, then the threshold strength of the stimulus does not depend on its duration. This minimum force is called "rheobase". Starting from a certain value of the pulse duration, its threshold strength depends on the duration - the shorter the duration, the higher the strength of the stimulus should be. The concept of "useful time" is introduced - the minimum time during which the stimulus of a given force must act on the tissue in order to cause excitation. If the strength of the stimulus is equal to two rheobases, then the useful time for such an irritant receives another name - chronaxy. (So, chronaxia is the useful time of the stimulus, the strength of which is 2 rheobases).

A-threshold (rheobase); B-double rheobase; a - useful time of the current, b - chronaxy.

Gradient law. In order for the stimulus to cause excitation, it must increase quickly enough. If the stimulus increases slowly, then due to the development of accommodation (inactivation of sodium channels), the threshold of irritation increases, therefore, to obtain excitation, the magnitude of the stimulus must be greater than if it increased instantly. The dependence of the threshold strength of the stimulus on the rate of its growth is also hyperbolic in nature (it is an inversely proportional relationship). The minimum gradient is the minimum rate of increase of the stimulus at which the tissue is still able to respond with excitation to this stimulus. This indicator is also used to characterize excitability.

The ratio of the phases of the action potential and excitability

When the tissue is excited - it generates AP, then excitability changes temporarily (according to the duration of AP) in it: at first, the tissue becomes completely unexcitable (absolute refractoriness) - any stimulus of any strength is not able to cause a new attack of excitation in it. This phase is usually observed during the peak of AP. Then there is a gradual restoration of excitability to the initial state (phase of relative refractoriness) - at this moment, the stimulus can cause excitation (generation of a new AP), but for this it must be much greater than the threshold (initial). Then (in the phase of trace negativity) excitability rises (superexcitability, or exaltation phase). At this point, subthreshold stimuli can cause arousal. Finally, in tissues in which trace hyperpolarization is clearly manifested, another phase is observed - subnormal excitability (reduced excitability).



Physiology of excitable tissues

Excitable tissues include nervous tissue (peripheral and CNS), muscles (smooth, skeletal, myocardium) and glandular cells. Excitability is a property (ability) of tissues to respond to irritation. At the same time, the tissue is not yet in working condition, but only has the ability, readiness to respond to irritation. Excitation is a transition from a state of rest to activity. For the excitation of nerves, the generation of potentials (impulse) is characteristic, and for the muscle - the generation of biopotential and contraction. Tissues vary in degree of excitability. Somatic nerves have the highest excitability, but among them there are fibers that have unequal excitability and different rates of excitation. Less than that of somatic nerves, the excitability of the autonomic nervous system (sympathetic and parasympathetic). In muscles, skeletal muscles have the greatest excitability (which contract in phase, quickly - these are mainly the muscles of the limbs). Less excitability in tonic muscles (support posture, position in space) than in phase muscles. The excitability of the myocardium is even less (it has a very large absolute refractoriness, which occupies the entire systole); smooth muscles have the smallest excitability (they contract according to the principle of tonic contraction).

Excitability indicators: I) irritation threshold - this is the minimum strength of the stimulus that causes the minimum response (excitation). With high tissue excitability, the threshold is lower, and vice versa. Subthreshold irritation (strength of irritation below the threshold value) - usually does not cause visible changes, but can lead to local excitation without spreading to other areas. Suprathreshold irritation - the magnitude of the stimulus is above the threshold value, therefore the response is greater, and may be maximum to these irritations. 2) chronaxia - this is the minimum time that is necessary for the occurrence of minimal excitation at a current strength of two thresholds (2 rheobases; rheobase is otherwise the irritation threshold). Types of chronaxia: a) motor - the criterion is muscle contraction. To determine the chronaxy of any muscle, there are special tables indicating the location of the motor points, which determine the location of the entry of the motor nerve ending into any muscle. When this point (motor point) is irritated, an isolated contraction of any muscle can be obtained. Motor chronaxy is an objective research method (nothing depends on the will of a person). The higher the excitability of the muscles, the lower the chronaxia. For example, the chronaxy of the flexor muscles in humans is lower than the chronaxy of the extensor muscles, i.e. the former have more excitability.

b) sensitive chronaxia is determined by the minimum sensation of the passage of current. The method is subjective. It characterizes the state of conduction and the receptor apparatus, c) reflex chronaxia - receptors on the skin are irritated and a motor response occurs in response to irritation of the receptors. Irritation spreads along the reflex path (receptor - afferent path - center - efferent path - muscle), d) subordinate - the magnitude of chronaxy of somatic nerves can be changed under the influence of the central nervous system. If there is CNS inhibition, then chronaxia may increase, with high excitability of the CNS, all types of chronaxia are reduced, e) constitutional - when the influence of the CNS is excluded (for example, as a result of an injury, there may be a complete nerve break). In the initial period after the cessation of the influence of the central nervous system, chronaxia lengthens, but then there may be a decrease, or recovery to normal. 3) lability (functional mobility) is the rate of each excitation cycle. With high excitability - lability is higher, and vice versa.

Mouse and nerve irritants. All stimuli are of 2 groups: I) adequate (natural), for example, a nerve impulse is an adequate stimulus for muscles, central nervous system and nerves. 2) inadequate (unnatural) - exposure to electric current, chemicals, mechanical effects, temperature, dosed electric current. The type of current is of great importance. More often, a direct rectangular current is used, since irritation with a direct current appears either at the moment of closing or opening (a sharp change in the magnitude of the current). During the passage of direct current, the muscle does not contract. If the increase in current is gradual, then the current may already be above the threshold, and there will be no muscle contraction. In addition to the steepness (decline and rise), the following are also taken into account: I) the magnitude (amplitude) of the current, 2) frequency (Hz) - if it is very high, then most of the irritations will fall on absolute refractoriness, and will not be effective. 3) the duration of each stimulus (in milliseconds). Therefore, the dosed current is widely used. Other types of inadequate influences have not found application (since they are difficult to dose). Although under natural conditions such irritants as chemicals are widely represented in the body (hormones, mediators, other biologically active substances).

The relationship between the magnitude of the stimulus and the response: I) the law of force - with an increase in the strength of irritation, the response increases, but only up to a certain limit. With any large force, there may be a decrease in response. This law is characteristic of all excitable tissues. 2) the law "all or nothing" - if the value of the stimulus reaches the threshold, then there can only be a complete reaction, and if this value is low, below the threshold, then there is nothing. However, if we consider the application of this law to a whole muscle or nerve trunk, and not to a separate nerve or muscle fiber, then this law is not applicable, since the nerve trunk or muscle contains fibers that have different excitability. Therefore, some muscle or nerve fibers will respond to smaller stimulus forces, while others will respond to larger ones. Therefore, with an increase in the strength of the stimulus, the force of contraction of skeletal muscles gradually increases.

The action of direct current on excitable tissues. For direct current (galvanic excitability of tissues), the following laws are characteristic: I) the law of polar action: a) direct current acts with its poles - the cathode (K) and the anode (A), b) at the moment of closing, the cathode has an irritating effect, and at the moment of opening - anode, c) the irritating effect of the cathode is stronger than that of the anode, so the threshold for the cathode will be less than that of the anode. The law can be found on a neuromuscular preparation. At a low current, only the closing contraction (ZS) appears; if the current is of medium strength, then there is both a closing contraction and an opening contraction (PC). If a strong direct current is used, then the response to irritation depends on the location of the electrodes, i.e. from the direction of the current. If the anode is located closer to the muscle, then they say that the current is ascending. When the electrodes are placed in the reverse order (the cathode is located closer to the muscle), the current is descending. Under the action of a strong current under the anode, a blockade of the conduction of excitation occurs (hyperpolarization occurs in this place), therefore, the excitation that occurs under the cathode will reach the anode, but it will not pass to the muscle through the hyperpolarization site, and there will be no cathode-closing contraction.

Current action. medium strength. With any arrangement of the electrodes, there will be both a closing and opening contraction. If you tie the nerve between the electrodes, then depending on the location of the electrodes will be:

a) if the electrodes are located: the cathode is closer to the muscle, and the anode is located behind the ligation site, then there is a closing contraction, but there is no opening contraction, since the excitation under the anode, having reached the ligation, does not spread further, and the muscle does not contract.

b) when the electrodes are located: the anode is closer to the muscle, and the cathode is behind the nerve ligation site, then when closing, the impulses do not reach the muscle, and the closing contraction does not occur.

To determine the electrodiagnostic formula (in medicine), based on the law of the polar action of direct current, the unipolar stimulation method is used. One electrode in the form of a plate is applied to a certain area of ​​the body, and the other - a point electrode - to a motor point. The unipolar method is characterized by the fact that an electrode with a small surface (active electrode) has an irritating property, and a plate electrode has a passive, non-irritating electrode, since the current density per unit area in a point electrode is many times greater than that of a plate electrode. When these electrodes are applied, there will be 4 electrodes: I) true cathode (K), 2) true anode (A) 3) field lines go from the anode to the cathode, which cross the nerve, entering it. Then they leave the nerve, forming an additional pole - the physiological cathode (K). 4) Then the lines of force enter the nerve, and a physiological anode (A) is formed under the cathode. If a low current is used, then only the cathode-closing reduction (CPC) is determined. At current

medium strength, is determined by the GLC, anode-closing (AZS) and anode-opening reduction (ARS). With a strong current, all thresholds can be determined (GLC threshold<АЗС<АРС<КРС). Это. и есть электродиагностическая формула. Для того чтобы понять кокой порог за счет какого электрода определяется, необходимо вспомнить закон полярного действия постоянного тока. Исходя из этого закона можем констатировать: КЗС - соответствует закону полярного действия, и сокращение мышцы происходит за счет раздражающего действия истинного катода; АЗС - не соответствует этому закону полярного действия, но в данном случае раздражающим электродом является физиологический катод (К²); АРС - соответствует закону полярного действия, раздражение происходит за счёт истинного А; КРС - не соответствует закону, но под катодом образуется физиологический А и за счёт раздражения этого полюса происходит КРС. Электродиагностическая формула определяется для диагностики нарушений целостности нерва, иннервирующего мышцу, и для контроля за ходом лечения. Например, при травме нерва, если происходит ущемление или нарушение целостности нерва, то электродиагностическая формула изменяется.

Electrotone is a change in excitability and conductivity under direct current electrodes. When a direct current is closed or passed, the excitability under the cathode increases - this is a catelectroton. At the same time, excitability and conductivity are reduced under the anode - this is anelectroton. The anode can achieve complete blockade of nerve impulse conduction. With prolonged action of direct current, or the passage of a strong direct current, there may be perversions of the usual electrotonic changes: I) cathodic depression (described by Verigo) - when a strong direct current is passed, or a long passage of direct current, excitability and conductivity under the cathode decrease. 2) anodic relief - the passage of a strong current, or prolonged passage of current leads to an increase in excitability under the anode. According to the principle of electrotonic change, excitation can be carried out in some non-myelinated fibers, but its speed is lower than that of impulse. In addition to electrotonic changes, there are perielectrotonic changes that are opposite to electrotonic ones: excitability is increased near the anode, and excitability and conductivity are reduced near the cathode (the phenomenon of perielectroton was described by N.E. Vvedensky).

The value of the time factor for irritation. Direct current has an irritating effect at the moment of closing and opening, if long-term stimuli are used (in the form of closing and blurring, without taking into account the duration of each stimulus). When long stimulation intervals are used, time does not matter, but only the speed of change in the magnitude of the current at the moment of closing and opening (Dubois-Reymond's law) matters. But when the stimulation intervals are short (milliseconds), then the time factor is important in the occurrence of arousal. The duration of the stimulation time and the threshold value depend on each other: with a decrease in the stimulation time, the threshold current value increases (this can be seen on the Goorweg-Weiss curve). The French scientists Lapic and Bourguignon proposed not to determine the entire curve, but only at the chronaxy point, which is determined with a double rheobase. Therefore, the method for determining excitable tissues is facilitated. The interval from the moment the dependence between time and the threshold current value appears will be to the left of the DC, and to the right of this - infinite time - here time does not matter for the occurrence of irritation. The Dubois-Reymond law applies here, which believed that the irritating effect of direct current depends on the rate of change in the magnitude of the current: at the moment of closing, the steepness of the current increases, and at the moment of opening, it falls rapidly. The duration of the current does not have an irritating effect, here the current is equal to

A D 2.0 ms

Curve "strength - duration" (curve Goorweg - Weiss) AB - rheobase; 2 - double rheobase; AD - chronaxia. The abscissa is the duration of the stimulus, the ordinate is the value of rheobase.

threshold value (rheobase). On the left side of the curve from DC - there is a relationship between the stimulation time and the threshold current (with decreasing time, the threshold current increases). The ultra-high frequency current (UHF) does not have an irritating effect, since each subsequent irritation falls on the absolute refractoriness.

Bioelectric phenomena in nerves and muscles. In 1791, Galvani opened. biocurrents. He worked with neuromuscular preparations, hung them on a copper wire on his balcony, and the balcony railings were metal, and once, when the wind appeared, the preparations (frog legs) touched the metal railings. Each contact with the railing was accompanied by a contraction of the legs. He concluded that due to "animal electricity" (later called biocurrents), the frog's legs contract. The very next year, the physicist Volta criticized Galvani's provisions. Volta believed that in this case we are talking about the occurrence of an electromotive force between dissimilar metals (in this respect he was right), which is an irritant for a neuromuscular preparation. As proof of his correctness, Galvani set up an experiment without the use of metal. He took two neuromuscular preparations, he irritated the nerve of the first, and applied the first preparation to the muscle of the second (Matteuchi's experiment).

The irritation of the nerve of the first remedy was always accompanied by contraction of the muscle in the first and second remedies. At the moment of excitation in the muscle of the first preparation, there appears a potential difference between the sections of the muscle. Since the surface of the area of ​​excitation is charged electronegatively, and the unexcited area has a positive charge, this potential difference is an irritant for the muscle of the second drug. These discoveries were finally confirmed in the experience of Matteuchi "with secondary tetanus", when it was caused by the biocurrents of the excited muscle of another drug. In 1902, Bernstein's first hypothesis about the origin of biocurrents was formulated - the membrane-ionic hypothesis of the occurrence of excitation. This theory existed until the 40s of the 20th century, when it became possible to use amplifiers.

Membrane potential (resting potential)

The membrane of any cell consists of lipids and proteins (it was previously thought that there were pores through which electrolytes pass in and out of the cell). It turned out that these pores are functional in nature (open at a certain moment). Any living cell has the ability to create a concentration gradient: in the cytoplasm, the concentration of K + ions is ~ 50 times greater than outside the cell, and sodium is 10 times greater outside the cell than inside it.

Hodgkin-Huxley experiment on the giant squid axon; A is the form of the potential registered in the experiment..

The scheme of the Hodgkin-Huxley experiment shows a jump in the negative potential at the moment the electrode is inserted into the axon, i.e., the internal environment of the axon was negatively charged relative to the external environment.

The electrical potential of the contents of living cells is usually measured relative to the potential of the external environment, which is usually taken equal to zero. Therefore, such concepts as transmembrane potential difference at rest, resting potential, membrane potential are considered synonymous. Typically, the value of the resting potential ranges from -70 to -95 mV. The value of the resting potential depends on a number of factors, in particular, on the selective permeability of the cell membrane for various ions; different concentrations of ions of the cytoplasm of the cell and ions of the environment (ionic asymmetry); operation of active ion transport mechanisms. All these factors are closely interconnected, and their separation has a certain conventionality.

In an unexcited state, the cell membrane is highly permeable to potassium ions and less permeable to sodium ions. This was shown in experiments using sodium and potassium isotopes: some time after the introduction of radioactive potassium into the axon, it was found in the external environment. Thus, there is a passive (according to the concentration gradient) release of potassium ions from the axon. The addition of radioactive sodium to the external environment led to a slight increase in its concentration inside the axon. Passive entry of sodium into the axon slightly reduces the magnitude of the resting potential.

The difference in the concentrations of potassium ions outside and inside the cell and the high permeability of the cell membrane for potassium ions ensure the diffusion current of these ions from the cell to the outside and the accumulation of an excess of positive K + ions on the outer side of the cell membrane, which counteracts the further release of K ions from the cell. The diffusion current of potassium ions exists until their desire to move along the concentration gradient is balanced by the potential difference across the membrane. This potential difference is called the potassium equilibrium potential.

Equilibrium potential (for the corresponding ion) - the potential difference between the internal environment of the cell and the extracellular fluid, at which the entry and exit of the ion is balanced (the chemical potential difference is equal to the electrical one). It should be borne in mind: 1) the state of equilibrium occurs as a result of the diffusion of only a very small number of ions (compared to their total content); the potassium equilibrium potential is always greater (in absolute value) than the real resting potential, since the membrane at rest is not an ideal insulator, in particular, there is a small leakage of Na + ions.

At rest, the cell membrane is highly permeable not only for K+ ions. In muscle fibers, the membrane is highly permeable to Cl ions. In cells with high permeability to chloride ions, as a rule, both ions (Cl and K+) participate almost equally in the creation of the resting potential.

It is known that at any point in the electrolyte, the number of anions always corresponds to the number of cations (the principle of electrical neutrality), therefore, the internal environment of the cell at any point is electrically neutral. Indeed, in the experiments of Hodgkin, Huxley and Katz, moving the electrode inside the axon did not reveal a difference in the magnitude of the resting potential. It is impossible to maintain a constant difference in ion concentration (ionic asymmetry) without special mechanisms. In membranes, there are active transport systems that work with the expenditure of energy and move ions against a concentration gradient. Experimental proof of the existence of active transport mechanisms are the results of experiments in which the activity of ATPase was suppressed in various ways, for example, by the cardiac glycoside ouabain. In this case, the concentrations of K+ ions were equalized outside and inside the cell, and the membrane potential decreased to zero.

The most important mechanism that maintains a low intracellular concentration of Na + ions and a high concentration of K + ions is the sodium - potassium pump. It is known that the cell membrane has a system of carriers, each of which binds to 3 Na+ ions inside the cell and brings them out. From the outside, the carrier binds to 2 K+ ions located outside the cell, which are transferred to the cytoplasm. The work of carrier systems is provided by ATP. As a result, it is ensured: maintaining a high concentration of K + and a low concentration of Na + inside the cell; potassium - sodium pump promotes the coupled transport of amino acids and sugars through the cell membrane. action potential

The action potential is understood as a rapid fluctuation of the resting potential, usually accompanied by a recharge of the membrane. An action potential appears when a stimulus is applied. On the curves during the registration of the action potential are recorded: I) latent period (hidden). 2) depolarization phase - a steep rise in the curve, while the cell surface is negatively charged. 3) phase of repolarization - restoration of the previous state. Recovery to the initial level does not occur immediately, but there are 4) trace potentials (negative and positive).

Action potential of a single cell and its phases. The response of the cell membrane to an irritating stimulus; I - local response; 2 - fast depolarization; 3 - reversion; 4 - repolarization; 5 - trace (negative and positive) potentials.

Active subthreshold changes in the membrane potential are called local response.

A shift in the membrane potential to a critical level results in the generation of an action potential. The minimum current required to reach the critical potential is called the threshold current. In the experiments of Hodgkin and Huxley, a surprising effect was discovered at first glance. During the generation of the action potential, the membrane potential did not just decrease to zero, as it would follow from the Nernst equation, but changed its sign to the opposite. An analysis of the ionic nature of the action potential, originally carried out by Hodgkin, Huxley and Katz, made it possible to establish that the depolarization phase of the action potential and membrane recharging (overshoot) are due to the movement of sodium ions into the cell, i.e., sodium channels turned out to be electrically controlled. Excitation leads to the activation of sodium channels and an increase in sodium current. This provides a local response. A shift in the membrane potential to a critical level leads to a rapid depolarization of the cell membrane and provides a rise front for the action potential. If Na ions are removed from the external environment, then the action potential does not arise. A similar effect was obtained by adding a specific sodium channel blocker, tetrodoxin, to the perfusion solution. By replacing sodium ions with other ions and substances, such as choline, it was possible to show that the incoming current is provided by sodium current, i.e., in response to a depolarizing stimulus, an increase in sodium conductivity occurs. Thus, the development of the depolarization phase of the action potential is due to an increase in sodium conductivity.

Membrane recharging, or overshoot, is very characteristic of most excitable cells. The overshot amplitude characterizes the state of the membrane and depends on the composition of the extra- and intracellular environment. At the height of the overshoot, the action potential approaches the equilibrium sodium potential, so the sign of the charge on the membrane changes. It has been experimentally shown that the amplitude of the action potential practically does not depend on the strength of the stimulus if it exceeds the threshold value. Therefore, it is customary to say that the action potential obeys the all-or-nothing law.

At the peak of the action potential, the sodium ion conductivity of the membrane begins to decrease rapidly. This process is called inactivation. The rate and degree of sodium inactivation depend on the magnitude of the membrane potential, i.e., they are voltage-dependent. With a gradual decrease in the membrane potential to -50 mV (for example, with oxygen deficiency, the action of certain drugs), the sodium channel system is completely inactivated and the cell becomes unexcitable.

The potential dependence of activation and inactivation is largely due to the concentration of calcium ions. With an increase in calcium concentration, the value of the threshold potential increases, with a decrease, it decreases and approaches the resting potential. At the same time, in the first case, excitability decreases, in the second - it increases.

Under normal conditions, a delayed outward potassium current exists for some time after the generation of an action potential and this provides hyperpolarization of the cell membrane, i.e., a positive trace potential. A positive trace potential can also occur as a result of the operation of the sodium-electrogenic pump.

Inactivation of the sodium system during the generation of the action potential leads to the fact that the cell cannot be re-excited during this period, i.e., a state of absolute refractoriness is observed.

The gradual recovery of the resting potential during repolarization makes it possible to evoke a repeated action potential, but this requires a suprathreshold stimulus, since the cell is in a state of relative refractoriness.

The study of cell excitability during a local response or during a negative trace potential showed that the generation of an action potential is possible when the stimulus is below the threshold value. It is a state of supernormality, or exaltation.

Under resting conditions, the difference between the outer surface of the membrane and the cytoplasm constantly exists. If you first remove the cytoplasm of the cell and introduce a solution with a high content of sodium ions into the cell, then the potential value will change dramatically. Therefore, potassium and sodium ions are of decisive importance in the occurrence of the resting potential. All electrolytes have a hydration shell, but the hydration shell for potassium ions is less than that of sodium. Therefore, sodium ions cannot pass through the membrane at rest. In addition to these ions, chloride ions located under the membrane and calcium ions take part in the formation of the membrane potential. The decisive factor in the occurrence of the charge is the presence of protein molecules.

In a hyperpotassic solution, the action potential is significantly reduced. In a hypersodium solution, its value increases. To analyze the action potential, pharmacological substances are also used - they have the ability to block either the potassium or sodium channel. When the sodium channel is blocked, the action potential decreases. This is very important in the diagnosis of myocardial infarction, brain tumors, etc. Depending on how the electrodes are located to different healthy and diseased areas, you can register a two-phase or single-phase action potential.

A biphasic action potential is recorded if the electrodes are on a healthy, undamaged tissue site. If two points are attached to the discharge electrodes, and to the other point (shown by the arrow) - irritating electrodes, then when artificial stimulation is applied, there will be a two-phase potential oscillation. The outlet electrodes are connected to the recording equipment. The mechanism of the occurrence of a two-phase action potential is that the outer surface of the cell, muscle or nerve fiber has a positive charge, and the cytoplasm is negative.

Dynamics of electrical potentials in muscle fiber

Therefore, when registering the potential, at first there will be just a straight line (a), b) the excitation wave passes through the area under the first electrode. The outer surface of the membrane in this area becomes negative and a potential difference arises between the electrodes, the arrow deviates, the curve rises, c) then the excitation occupies the entire surface between the electrodes, the potential difference disappears and the arrow returns back to its initial state, the curve goes down. d) under the first electrode, repolarization occurs (positive charge), and under the second electrode, depolarization also takes place and the galvometer needle deviates to the other side, and the curve goes down. e) excitation leaves the limits of the second electrode, repolarization occurs under it, and the galvanometer needle returns to its initial position.

If one of the electrodes is located on the damaged area, then the arrow does not occupy the zero position, since the healthy area is positive, and the damaged one is negative, and the arrow will be rejected in advance. With this arrangement of the electrodes, a single-phase action potential is recorded.

This is important for the diagnosis of myocardial infarction, because. the damage site will have a negative surface charge earlier and the excitation wave that will propagate on its way will meet the changed area, and, consequently, the shape of the ECG will be changed.

Change in excitability of tissues during excitation.

All excitable tissues change their excitability when stimulated, the nerves immediately after the application of irritation, that is, they have a very short latent period. The figure shows: at the top, the action potential, at the bottom - the change in the excitability of the nerve fiber in different periods of excitation (absolute refractoriness corresponds to the peak of the high-voltage potential, relative refractoriness - to the repolarization phase, supernormal period - to the negative trace potential). Below in the text is the sequence of development of different phases:

local positive trace potential

supernormal period

Time, ms

local process, depolarization phase, repolarization phase, negative trace potential, positive trace potential, as well as phases of changes in the excitability of the nerve fiber: the phase of absolute refractoriness, the phase of relative refractoriness, supernormal excitability, subnormal excitability and the initial level of excitability. In the phase of absolute refractoriness, excitability drops to zero. This corresponds to the phase of depolarization. The maximum refractoriness is observed at the time of the peak of depolarization. If persistent depolarization is caused by any substance, then the tissue loses the ability to respond to the next incoming excitation. In practice, inhibition can occur with persistent hyperpolarization, with persistent depolarization and persistent polarization, when the surface positive charge does not change under the influence of any cause.

The period of repolarization corresponds to the phase of relative refractoriness. Here excitability is gradually restored. After relative refractoriness, a phase of supernormal excitability sets in - corresponds to a negative trace potential, then subnormal excitability occurs - corresponds to a positive trace potential, and then excitability comes to the initial level

Curves of single contraction (I) and changes in excitability (2) of skeletal, cardiac and smooth muscles.

:

time, 0.1 s

a) contraction period, b) relaxation period, c) absolute refractoriness phase, d) relative refractoriness phase, e) exaltation phase (supernormal excitability).

Different muscles have different refractoriness and this property largely determines the features of the contractile function of these muscles.

If we take a constant frequency of the irritating current, but gradually increase the strength of the irritation, it will be found that with an increase in the strength of the irritation, the response will increase. The same regularity is observed if the frequency of the applied irritations is increased at a constant current strength. However, the increase in contraction will occur up to what - the optimal strength or frequency of the applied irritations. To assess the ability of an excitable tissue to respond to stimuli of different frequencies, the concept of “lability” or functional mobility was introduced (N. E. Vvedensky).

Lability is understood as the rate of each excitation cycle or the ability of tissues to reproduce the frequency of the applied stimuli without distortion (for nerves, lability is ~ 1000 Hz, for skeletal muscles - ~ 250-500 Hz, for synapses - ~ 100 Hz). If the frequency of the applied irritations is greater than the lability, then not all impulses will be reproduced, but only those that do not exceed the lability value (for example, if irritations with a frequency of 2000 Hz are applied to the nerve, then we will receive only 1000 responses). With a further increase in frequency, the response may disappear. To explain this phenomenon, it is necessary to resort to the concepts of absolute and relative refractoriness. Part of the high-frequency stimuli falls on absolute refractoriness, so they do not cause a response. On the basis of lability, Vvedensky developed the concept of the optimum and pessimum of the strength and frequency of stimulation. The frequency at which the maximum response is obtained is the optimum frequency. A decrease in response, due to a further increase in the frequency of applied irritations, is called a pessimum. The pessimum is more pronounced, the higher the frequency. For example, a change in excitability during a single contraction of a skeletal muscle:

At a frequency greater than 50 Hz, a tetanic contraction occurs. If irritation is applied at intervals of one cycle, then a single contraction will be obtained each time. If the frequency is increased, the intervals between the applied irritations will decrease, and the contraction will first be in the form of a jagged, and then, with a further increase in frequency, a smooth (solid) tetanus will appear. The optimum contraction will correspond to the phase of the peak of exaltation - the highest excitability. In this case, the current strength will be the same, but since the excitability of the muscle is greater, the response will be maximum. With a further increase in frequency, the time intervals will mix into the phase of relative refractoriness, and part of the impulses at the pessimum falls into this phase. Here, the excitation is lower than in the exaltation phase, and the response will be lower. A further increase in frequency leads to the hit of impulses on the absolute refractoriness. In this case, there is no answer, because during this period, excitability is completely absent. Therefore, high-frequency currents are used for therapeutic purposes, electrodes are applied to the skin, but the muscles do not react (do not contract), because muscle lability is much lower than the frequency of the UHF current, and each stimulus falls into the period of absolute refractoriness. When there are many impulses from the central nervous system along the nerve to the muscles, then, depending on the need for the magnitude of the contraction, impulses of different frequencies arrive at the muscles (for example, to raise a piece of chalk - the flow of nerve impulses is less, and to raise a chair - more, while contracting more myofibrils, and the response increases). Vvedensky, based on the doctrine of lability, pessimum and optimum, discovered the phenomenon of parabiosis. He would take a neuromuscular preparation and irritate the nerve with various current strengths and record the contraction of the muscle. The response at the same time completely fit into the "law of force", i.e., with an increase in the strength of irritation, the response increased. After that, he applied a cotton swab moistened with cocaine to the nerve, and again irritated the nerve. He revealed that a phase change in excitability and conductivity is detected: I) an equalizing phase: here there is an equalization of all responses, for all types of irritations - the same answer, 2) a paradoxical phase - a weak irritation gives a greater response than stimuli of medium and large strength, 3) inhibitory stage - there is no response to any irritation. This is because in the area of ​​​​alteration (where the cotton wool with cocaine) is gradually reduced lability. This leads to: I) in the equalizing phase - to the passage of a certain number of impulses, and the excess number (which is greater than the lability of the altered area) is blocked and the same number of impulses reaches the muscle, 2) in the paradoxical phase, a further decrease in lability occurs, and the answer is perverted: a small number impulses pass, but with an increase in the strength and frequency of irritation, part of the impulses is blocked, the response decreases. A further increase in strength and frequency leads to a greater blockade of impulses - occurs according to the pessimum principle. 3) in the inhibitory phase, the lability decreases further and the conduction in the alteration site stops altogether, and the impulses do not reach the muscle. Novocaine anesthesia is based on this. The action of novocaine is based on the fact that the lability of receptors and afferent conductors is reduced. Impulses do not reach the center and pain is not felt.

Vvedensky was the first to substantiate the theory of the unity of excitation and inhibition. He considered inhibition as a special case of excitation, but a special one - not spreading, stationary. According to Vvedensky, there is an impulsive (ordinary) excitation, and under the influence of an altering agent, a local local non-spreading excitation occurs. The stages of parabiosis are the result of the interaction of two excitations - pulsed and local (stationary). The development of parabiosis and the occurrence of inhibition should be considered as secondary inhibition due to the interaction of two excitations. The phenomenon of parabiosis has a universal character, and it can develop in different parts of the central nervous system and in peripheral nerves under the influence of extreme factors, large doses of medicinal substances. Parabiotic phenomena (in the form of phase phenomena) can also occur in higher nervous activity.

Physiological properties of nerve fibers. Nerve fibers are divided into several groups according to the speed of conduction of excitation. The classification of nerve fibers according to Erlanger-Gasser has received the greatest recognition. According to this classification, 3 main groups of nerve fibers are distinguished - A, B, C. In turn, group A is divided into several subgroups (a - alpha, b - beta, g - gamma and d - delta). The highest speed of conduction of excitation (70-120 m / s) in group A alpha - the primary afferents of muscle spindles, motor fibers of skeletal muscles have this speed. Group A - b - these are skin afferents of touch and pressure have a speed of excitation of 30-70 m / s. Group A-gamma has a speed of 15-30 m / s - these are motor fibers of muscle spindles. Group A-delta has a speed of conduction of excitation 12 - 30 m / s, such a speed has skin afferents of temperature and pain (primary pain). Group B has a speed of 3 - 15 m/s. these are mainly sympathetic preganglionic fibers. Group C has a speed of 0.5 - 2 m/s. are cutaneous pain afferents (secondary, slow pain) and sympathetic postganglionic fibers (unmyelinated).

Aksotok. Nerve fibers have a peculiar structure - microtubules, through which substances move from the nerve cell to the periphery (anterograde flow) and from the periphery to the center (retrograde axotok). There are fast (about 410 mm per day) and slow (about 2 times slower) axotok. Due to it, biologically active substances spread from the center to the periphery. The axon, which is only a few microns in diameter, can reach a length of one meter or more, and the movement of proteins by diffusion from. nucleus to the distal end of the axon would take years. It has long been known that when any section of the axon undergoes constriction, the proximal portion of the axon expands. It looks like the centrifugal flow is blocked in the axon. Such a flow - fast axon transport can be demonstrated by the movement of radioactive markers in the experiment. Radiolabeled leucine was injected into the dorsal root ganglion, and then, from the 2nd to the 10th hour, the radioactivity was measured in the sciatic nerve at a distance of 166 mm from the neuron bodies. For 10 hours, the peak of radioactivity at the injection site did not change significantly. But the wave of radioactivity propagated along the axon at a constant speed of about 34 mm per 2 hours, or 410 mm per day. It has been shown that in all neurons of homoiothermic animals, fast axon transport occurs at the same rate, and there are no noticeable differences between thin, unmyelinated fibers and the thickest axons, as well as between motor and sensory fibers. The type of radioactive marker also does not affect the rate of fast axonal transport; Various radioactive substances can serve as markers.

If we analyze the peripheral part of the nerve in order to understand the nature of radioactivity carriers, then such carriers are found mainly in the protein fraction, as well as in the composition of mediators and free amino acids. The fast axon transport described above is anterograde, i.e. directed away from the cell body. It has been shown that some substances move from the periphery to the cell body using retrograde transport. For example, acetylcholinesterase is transported in this direction at a rate 2 times slower than the rate of fast axonal transport. A marker often used in neuroanatomy, horseradish peroxidase, also travels by retrograde transport. Retrograde transport probably plays an important role in the regulation of protein synthesis in the cell body. A few days after axon transection, chromatolysis is observed in the cell body, which indicates a violation of protein synthesis. The time required for chromatolysis correlates with the duration of retrograde transport from the site of axon transection to the cell body. Due to the anterograde current, tissue differentiation (for example, muscles) occurs. This is of great biological importance. There are phase muscles (muscles of the limbs) and tonic (support the posture). It was established in the experiment that if the nerves innervating these muscles are cut, and then the innervation crossover is made, that is, the central end of the nerve innervating the phasic muscles is sutured to the nerve innervating the tonic muscles, then after the sprouting of the nerves, the phasic muscles begin to function as tonic, and tonic - as phase. Their structure changes, since the trophic function of the motor nerves is provided due to the axotok. Due to the retrograde current, neurotropic substances flow from the periphery to the center, exerting a trophic effect on the nerve cell itself. By retrograde transport, toxins can enter the body of the nerve cell, as well as some chemicals used in industrial conditions. Rapid axon transport requires a significant concentration of ATP. Poisons such as microtubule-destroying colchicine also block fast axonal transport. It follows from this that in the transport process we are considering, vesicles and organelles move along microtubules and actin filaments; this movement is provided by small aggregates of dynein and myosin molecules, acting using the energy of ATP.

Fast axon transport can also be involved in pathological processes. Some neurotropic viruses (for example, herpes or polio viruses) penetrate the axon at the periphery and move by means of retrograde transport to the neuron body, where they multiply and exert their toxic effect. Tetanus toxin, a protein produced by bacteria that enter the body through skin lesions, is captured by nerve endings and transported to the neuron body, where it causes characteristic muscle spasms. Cases of toxic effects on the axon transport itself are known, for example, exposure to the industrial solvent acrylamide. In addition, it is believed that the pathogenesis of beriberi vitamin deficiency and alcoholic polyneuropathy includes a violation of fast axonal transport.

As mentioned above, in addition to fast axon transport in the cell, there is also a rather intense slow axon transport. Tubulin moves along the axon at a rate of about 1 mm/day, while actin moves faster, up to 3 mm/day. Other proteins also migrate with these components of the cytoskeleton; for example, enzymes appear to be associated with actin or tubulin. The rates of movement of tubulin and actin are roughly consistent with the growth rates found for the mechanism when molecules are incorporated into the active end of a microtubule or microfilament. Therefore, this mechanism may underlie slow axonal transport. The rate of slow axon transport also approximately corresponds to the rate of axon growth, which, apparently, indicates the limitations imposed by the structure of the cytoskeleton on the second process.

It should be emphasized that cells are by no means static structures, as they appear, for example, in electron microscopic photographs. The plasma membrane and especially the organelles are in constant rapid movement and constant restructuring; that is the only reason they are able to function. Further, these are not simple chambers in which chemical reactions take place, but highly organized conglomerates of membranes and fibers in which reactions proceed in an optimally organized sequence.

Physiological properties of nerve fibers: .1) excitability - the ability to respond to an incoming impulse. 2) conductivity - the ability to propagate impulses from one area to another. These properties depend on the structure of the nerve fiber. All nerve fibers are divided into I) pulpy - have a myelin sheath, Ranvier intercepts, which are important for the transmission of excitation. The myelin sheath itself is a powerful biological insulator. Through it, excitation does not jump from one nerve fiber to neighboring ones. Therefore, the transmitted pulse is inefficient for adjacent fibers. 2) unmyelinated nerve fibers - the transfer of excitation in them occurs along the surface of the nerve through a change in the surface charge. Usually the nerve trunk contains a large number of nerve fibers. The unmyelinated fibers in it are among the myelinated ones. The laws of conducting excitation along the nerve fiber: I) the law of physiological integrity. Anatomical integrity - when all structures are preserved on the nerve fiber. Functional integrity can be broken by the action of any factors without damaging the structure, for example, parabiosis. For excitation to pass through the fiber, there must be its physiological integrity. 2) the law of bilateral conduction of excitation: if two galvanometers are placed on the nerve and irritation is applied between the devices, then the resulting action potential is recorded both on the right and on the left. In any integral organism, excitation actually goes in one direction (from the afferent channel through the center to the efferent channel, since synapses conduct excitation one-way). 3) the law of non-decremental conduction of excitation (without weakening) - regardless of the size and length of the neuron, the excitation does not lose its strength - in different areas the magnitude of the action potential will be the same. 4) the law of isolated conduction of excitation - for each nerve fiber, excitation is transmitted only along this fiber. Transverse transmission from one nerve fiber to another does not occur. Without this, there would be no coordination of movements. Violation of this rule occurs when a nerve is injured and its comparison. 5) the law of spasmodic conduction of excitation (saltatory) - such transfer of excitation occurs only in myelin nerve fibers. Interceptions of Ranvier are of great importance, since excitation jumps from one to another interception or even bypassing one interception. Therefore, such nerve fibers have the highest rate of conduction of excitation. 6) myelin-free fibers conduct excitation over the surface. The excited area is charged electronegatively (depolarization) and this wave propagates along the nerve fiber.

The mechanism of transmission of excitation from the nerve to the muscle.

The excitation that occurs in the central nervous system reaches the skeletal muscles through efferent channels. There are 2 mechanisms for the transfer of excitation from the nerve to the working organ: I) chemical - as a result of the production of mediators 2) electrical, when the action potential from the presynaptic membrane jumps to the postsynaptic membrane and causes its depolarization. The main condition is the distance between the pre- and postsynaptic membranes: if it is more than 0.2 nm, then the transmission will be chemical, and if it is less, then the potential from the presynaptic membrane passes to the postsynaptic one, it depolarizes and the muscle contracts. This transmission is not common. Myoneural synapses transmit excitation to skeletal and smooth muscles, due to synapses, excitation is transmitted to the central nervous system, including the cortex. Mediators can be different. The most common: a) acetylcholine - for skeletal muscles, in parasympathetic nerves, cholinergic synapses of the central nervous system; b) adrenergic synapses transmit excitation to the periphery due to norepinephrine, which, being released at the endings of sympathetic nerves, affects the heart, blood vessels, and the gastrointestinal tract. In sympathetic ganglia, the transfer of excitation from preganglionic to postganglionic neurons occurs due to acetylcholine. Adrenergic synapses are common in the CNS (especially in the brainstem). The sympathetic innervation of the brain occurs mainly due to the blue spot, which synthesizes adrenergic substances; c) serotonergic structures. - produce serotonin (mainly the nuclei of the brain suture), d) specific synapses - are sensitive to certain neuropeptides synthesized in the structures of the brain, in the mucosa of the gastrointestinal tract, and in the adrenal glands. They act on peripheral and central structures.

Chemical transfer mechanism:

The structure of the synapse: I) terminal - the end of the motor nerve, 2) plaque, 3) vesicles containing the neurotransmitter, 4) presynaptic membrane, 5) synaptic cleft, 6) postsynaptic membrane. When the impulse propagates along the neuron, the excitation reaches the synaptic plaque through the terminal, as a result of which the neurotransmitter enters the synaptic cleft from the vesicles through the presynaptic membrane. On the postsynaptic membrane there are special cholinergic receptors (for the mediator acetylcholine) or adrenoreceptors (for norepinephrine). At the endings of the motor nerves, the mediator acetylcholine is released, when it enters the synaptic cleft, it causes an increase in the permeability of the postsynaptic membrane for Na + ions. The occurrence of a flow of Na + ions through the postsynaptic membrane causes its depolarization and an excitatory postsynaptic potential (EPSP) or end plate potential (EPP) is formed. There are active zones on the presynaptic membrane, that is, areas of the presynaptic membrane where acetylcholine is most released into the synaptic cleft. Nearby are inactive areas. Types of secretion of acetylcholine: I) quantum secretion (or caused by excitation) - with the help of an impulse - is the main factor causing depolarization of the postsynaptic membrane and muscle contraction. 2) spontaneous secretion - when the arrival of an impulse is not necessary. This gives rise to local

potential, which is negligible and does not cause muscle contraction. It is recorded as a small potential difference, c) non-quantum secretion - it accounts for about 30% of the released mediator, but it is not accompanied by depolarization of the postsynaptic membrane, and does not cause muscle contraction. Due to it, trophism is provided. Regulation: I) presynaptic autoregulation of acetylcholine secretion - there are mechanisms in the terminal plaque that can regulate the release of acetylcholine. They can have a dual effect on the functional state of the synapse: I) desensitization - a decrease in the sensitivity of synaptic receptors. When their sensitivity is reduced to a state of immunity to excitation, a state of synaptic inhibition develops. It is due to a decrease in the sensitivity of cholinergic receptors located on the postsynaptic membrane. There may be presynaptic inhibition, which is due to a decrease in the production and release of the neurotransmitter or their blockade. Therefore, in pharmacology there are blockers that inhibit presynaptic structures, or reduce the sensitivity of postsynaptic membranes. 2) synaptic potentiation - increased excitability. Spontaneous secretion of acetylcholine can lead to these phenomena: there is an increase in the excitability of cholinergic receptors, and when a quantum of the mediator is released, it acts on a state of increased excitability and causes a greater response. This happens depending on the performance of a person's work. The mediator acetylcholine, which enters the synaptic cleft, undergoes rapid destruction by the enzyme acetylcholinesterase. If he didn't exist. then acetylcholine would irritate the postsynaptic membrane receptors for a long time, and there would be no adequate coordination of muscle action. Therefore, a new contraction occurs only when a new mediator quantum arrives. A significant part of the choline formed during the destruction of acetylcholine takes part in the resynthesis of new quanta of acetylcholine. New portions of the neurotransmitter are located farther from the presynaptic membrane, and mature vesicles with acetylcholine are located closer, which then release the mediator. The ion-membrane mechanism underlies the emergence of a potential on the postsynaptic membrane:

the same patterns of transition of ions (K, Na, Ca) into the cell and the pericellular space are observed. Features of conducting excitation through synapses: I) synapses work like valves - they allow excitation to pass only in one direction: from the terminals, the presynaptic membrane to the postsynaptic one. Excitation does not propagate in the opposite direction, since there is no production of acetylcholine in the postsynaptic membrane, and there are no receptors in the presynaptic membrane that respond to the mediator. Unilateral conduction of excitation occurs in the synapses of both the peripheral and central nervous systems (from the afferent channel to the efferent channel). 2) synapses are characterized by a large latent period of excitation, in contrast to nerve fibers. This is due to the fact that it takes more time to develop the mediator.

3) synapses have high chemical activity and selective sensitivity: cholinergic synapses have cholinergic receptors that respond only to cholinomimetics, but will not respond to adrenaline and adrenomimetics. 4) synapses have the ability to sum up excitation. In peripheral synapses, summation can be temporary - sequential. If you irritate the motor nerve with a single subthreshold stimulus, then there will be no muscle response. But if frequent irritations are used with a subthreshold stimulus, then with an increase in the frequency of the applied irritations, a response occurs - to subthreshold irritations. This occurs as a result of the fact that each previous subthreshold irritation causes the phenomenon of potentiation (increases the excitability of cholinergic receptors). So when the stimulus falls into the exaltation phase, excitation occurs. 5) synapses have the ability to transform the frequency of incoming impulses: increasing or decreasing it. For example, if a strong suprathreshold stimulation (single) is applied, then the muscle can contract tetanically (repeatedly): that is, 1 stimulus - 100 impulses per muscle. Or, conversely, if the stimulus is 500 Hz, then it cannot cause contractions, or only 100 impulses (depending on the lability of the synapse, it cannot be higher than it). 6) trace phenomena in synapses: since acetylcholine is not instantly destroyed, until it is destroyed, it is recorded along with. peak potential, significant potential fluctuations.

7) high fatigue of synapses, this may be the result of depletion or untimely synthesis of the neurotransmitter with prolonged receipt of impulses that destroy mediators. In the neuromuscular synapse, acetylcholine normally acts on the synaptic membrane for a short time (1-2 ms), since it immediately begins to be destroyed by acetylcholinesterase. In cases where this does not happen and acetylcholine is not destroyed for hundreds of milliseconds, its action on the membrane stops and the membrane does not depolarize, but hyperpolarizes and excitation through this synapse is blocked.

Blockade of neuromuscular transmission can be caused in the following ways:

1) the action of local anesthetic substances that block excitation in the presynaptic part;

2) blockade of mediator release in the presynaptic part (for example, botulinum toxin);

3) violation of the synthesis of the mediator, for example, under the action of hemicholinium;

4) blockade of acetylcholine receptors, for example, under the action of bungarotoxin;

5) displacement of acetylcholine from receptors, for example, the action of curare;

6) inactivation of the postsynaptic membrane by succinylcholine, decamethonium, etc.;

7) inhibition of cholinesterase, which leads to a long-term preservation of acetylcholine and causes deep depolarization and inactivation of synapse receptors. This effect is observed under the action of organophosphorus compounds.

Especially to reduce muscle tone, especially during operations, blockade of neuromuscular transmission with muscle relaxants is used; depolarizing muscle relaxants act on the receptors of the subsynaptic membrane (succinylcholine, etc.), non-depolarizing muscle relaxants that eliminate the effect of acetylcholine on the membrane by competition (drugs of the curare group).

Physiological properties of muscles. Muscles are divided into 3 groups: striated (skeletal), myocardium and smooth muscles. All of them have physical and physiological properties. Physical properties: I) extensibility - the ability of a muscle to change its length under the influence of a load, 2) elasticity - after the load is removed, the muscle is able to take its initial length. 3) viscosity - due to the friction of myofibrils, which are located in the muscle in large numbers. Due to this, resistance to stretching, a change in the length of the muscle, occurs. Physiological properties: I) excitability - the ability to respond to irritation. According to the degree of excitability: skeletal muscles are the most excitable, then the myocardium, then smooth muscles (due to the high relative refractoriness), 2) conductivity - the ability to conduct excitation from one area to another. According to the speed of conduction, the muscles are arranged as follows: skeletal muscles, myocardium (has its own conducting system), smooth muscles (feature - they can conduct excitation in different directions), 3) contractility - the ability of a muscle to change its length under the influence of an impulse, to contract. For smooth muscles and myocardium, there is one more property: 4) automation - myocardium and smooth muscles contract due to impulses that arise in the muscle itself. Types of contraction for skeletal muscles: both ends of the muscle are fixed. This increases the tension. Under natural conditions, this is when trying to lift an unbearable load: the tension increases, but the load does not move, and the length of the muscle does not change. 2) isotonic - when only one end of the muscle is fixed, and the tone is not isometric - while the length does not change. In artificial conditions, this can be obtained if both change, but the length of the muscle changes. Under natural conditions, a mixed contraction occurs in the whole organism - 3) auxotonic - when at any moment there is either an isometric or isotonic contraction. The tongue always contracts according to the principle of isotonic contraction. Especially a lot of energy is spent during isometric contraction, and with isotonic contraction, little energy is spent. The heart beats differently during different periods of work. 4) skeletal muscle under artificial conditions can contract as a single contraction: for one irritation - one contraction. It consists of a latent period, a period of contraction and relaxation. For the myocardium, a single contraction is a physiological contraction. Skeletal muscles under natural conditions are able to contract according to type 5) tetanic contraction. The condition for the occurrence of this contraction is an increase in the frequency of incoming impulses, or the frequency of applied irritations. If, with an increase in frequency, each subsequent irritation will fall for a period of relaxation, then the muscle does not completely relax, and contracts again. It turns out a serrated tetanus. If you increase the frequency of impulses, then these irritations fall for the period of contraction, and the muscle does not have time to relax, a continuous (smooth) tetanus is observed. The value of the serrated tetanus is greater than that of a single contraction, and that of the smooth tetanus is greater than that of the serrated. When excitation falls to the peak of the exaltation phase, there will be a maximum contraction. This happens in natural conditions: if the work is hard, then the frequency of impulses increases, and the muscle contracts more strongly.

Functions and properties of smooth muscles

electrical activity. Visceral smooth muscles have an unstable membrane potential. Regardless of nerve influences, fluctuations in the membrane potential cause irregular contractions, due to which the muscle is constantly in a state of partial contraction - tone. It is clearly expressed in the sphincters of hollow organs: the gallbladder, bladder, at the junction of the stomach into the duodenum and the small intestine into the large intestine, as well as in the smooth muscles of small arteries and arterioles. In a state of relative rest, the value of the membrane potential is on average 50 mV. The value of PD can vary over a wide range. In smooth muscles, the duration of AP is 50-250 ms; There are PDs of various shapes. In some smooth muscles, such as the ureter, stomach, and lymphatics, APs have a prolonged plateau during repolarization, reminiscent of the potential plateau in myocardial cells. Plateau-like APs provide the entry into the cytoplasm of myocytes of a significant amount of extracellular calcium, which activates the contractile proteins of smooth muscle cells. The ionic nature of smooth muscle AP is determined by the features of the channels of the smooth muscle cell membrane. Ca2+ ions play the main role in the mechanism of PD occurrence. Calcium channels of the membrane of smooth muscle cells pass not only Ca 2 + ions, but also other doubly charged ions (Ba, Mg +), as well as Na +. The entry of Ca² into the cell during PD is necessary to maintain tone and develop contraction, therefore blocking the calcium channels of the smooth muscle membrane is widely used in practical medicine to correct the motility of the digestive tract and vascular tone in hypertension.

Automation. APs of smooth muscle cells have an autorhythmic (pacemaker) character, similar to the potentials of the conduction system of the heart. Pacemaker potentials are recorded in various parts of the smooth muscle. This indicates that any visceral smooth muscle cells are capable of spontaneous automatic activity. Smooth muscle automation, i.e. the ability for automatic (spontaneous) activity is inherent in many internal organs and vessels.

Stretch response. A unique feature of smooth muscle is its response to stretch. In response to stretching, the smooth muscle contracts as it at this time, the membrane potential of the cells decreases, the frequency of AP increases, and, ultimately, the tone of smooth muscles increases. In the human body, this property of smooth muscles is one of the ways to regulate the motor activity of internal organs. For example, when the stomach is full, its wall is stretched. An increase in the tone of the stomach wall in response to its stretching contributes to the preservation of the volume of the organ and better contact of its walls with the incoming food. In blood vessels, the stretch created by fluctuations in blood pressure is the main factor in myogenic self-regulation of vascular tone.

Plastic. A specific property of a smooth muscle is the variability of tension without a regular connection with its length. Thus, if a visceral smooth muscle is stretched, its tension will increase, but if the muscle is kept in a state of lengthening for a long time, then the tension will gradually decrease, sometimes not only to the level that existed before the stretch, but even below this level. This property is called smooth muscle plasticity. The plasticity of smooth muscles contributes to the normal functioning of the internal hollow organs.

Excitable tissues are tissues that are able to perceive the action of a stimulus and respond to it by switching to a state of excitation.

Excitable tissues include three types of tissues - nervous, muscular and glandular.

Excitable tissues have a number of general and particular properties.

The general properties of excitable tissues are:

1. Irritability

2. Excitability

Conductivity

Irritability is the ability of a cell, tissue or organ to perceive the action of a stimulus by changing its metabolism, structure and functions.

Irritability is a universal property of all living things and is the basis of adaptive reactions of a living organism to constantly changing conditions of the external and internal environment.

Excitability is the ability of a cell, tissue or organ to respond to the action of a stimulus by switching from a state of functional rest to a state of physiological activity.

Excitability is a new, more perfect property of tissues, into which (in the process of evolution) irritability has been transformed. Different tissues have different excitability: nervous > muscular > glandular

The measure of excitability is the threshold of irritation

The irritation threshold is the minimum strength of the stimulus that can cause spreading excitation.

Excitability and irritation threshold are inversely related (the more excitability, the< поpог pаздpажения)

Excitability depends on:

1. Resting potential values

2. The level of critical depolarization

The resting potential is the potential difference between the inner and outer surfaces of the membrane at rest

The level of critical depolarization is the value of the membrane potential that must be achieved in order for the excitation to have a propagating character

The difference between the values ​​of the resting potential and the level of critical depolarization determines the depolarization threshold (than< поpог деполяpизации, тем >excitability)

Conductivity is the ability to conduct excitation

Conductivity is determined:

1. Fabric structure

2.Functional features of the fabric

excitability

Memory is the ability to record changes in the functional state of a cell, tissue, organ and organism at the molecular level

Determined by the genetic program

Allows you to respond to the action of individual stimuli that are significant for the body ahead of time

Specific properties of excitable tissues include:

1. Contractility

2. Secretory activity

Automation

Contractility - the ability of muscle structures to change length or tension in response to stimulation

Depends on the type of muscle tissue

Secretory activity is the ability to secrete a mediator or secretion in response to stimulation.

Neuron terminals secrete neurotransmitters

Glandular cells excrete sweat, saliva, gastric and intestinal juice, bile, and also increte hormones and biologically active substances

Automation is the ability to be independently excited, that is, to be excited without the action of a stimulus or an incoming nerve impulse.

Characteristic for cardiac muscle, smooth muscle, individual nerve cells of the central nervous system

Excitable tissues are characterized by 2 types of functional activity

Physiological rest - a state without manifestations of specific activity (in the absence of the action of a stimulus)

Excitation - an active state, which manifests itself by structural and physico-chemical changes (a specific form of response in response to the action of a stimulus or an incoming nerve impulse)

Different types of functional activity are determined by the structure, properties and state of plasma membranes

No. 9 Functions: 1. Barrier - the membrane, with the help of appropriate mechanisms, participates in the creation concentration gradients, preventing free diffusion.

2. The regulatory function of the cell membrane consists in the fine regulation of intracellular contents and intracellular reactions due to the reception of extracellular biologically active substances, which leads to a change in the activity of membrane enzyme systems and the launch of mechanisms of secondary “messengers” (“mediators”).

3. Converting external stimuli of a non-electrical nature into electrical signals (in receptors).

4. Release of neurotransmitters in synaptic endings.

Fluid mosaic model by Singer and Nicholson:

In the phospholipid bilayer, globular proteins are integrated, the polar regions of which form a hydrophilic surface in the aqueous phase. These integrated proteins perform various functions, including receptor, enzymatic, form ion channels, are membrane pumps and carriers of ions and molecules.

Some protein molecules diffuse freely in the plane of the lipid layer; in the normal state, parts of protein molecules that emerge on opposite sides of the cell membrane do not change their position.

The special morphology of cell membranes determines their electrical characteristics, among which the most important are capacitance and conductivity.

Capacitance properties are mainly determined by the phospholipid bilayer, which is impermeable to hydrated ions and at the same time thin enough (about 5 nm) to provide efficient separation and accumulation of charges and electrostatic interaction of cations and anions. active transport- transport of substances across the membrane, which is carried out against a concentration gradient and requires a significant amount of energy. One third of the basal metabolic rate is spent on active transport.

Active transport is:

1. primary active- such transport, for which the energy of macroergs is used - ATP, GTP, creatine phosphate. for example: Potassium-sodium pump - an important role in the processes of excitability in the cell. It is embedded in the membrane.

Potassium sodium pump- enzyme potassium-sodium ATPase. This enzyme is a protein. It exists in the membrane in the form of 2 forms:

E 1, E 2

In enzymes, there active site, which interacts with potassium and sodium. When the enzyme is in form E 1, its active site faces the inside of the cell and has a high affinity for sodium , and therefore contributes to its addition (3 Na atoms). As soon as sodium is added, the conformation of this protein occurs, which moves 3 sodium atoms through the membrane and sodium is detached from the outer surface of the membrane. In this case, the transition of the enzyme from form E 1 to E 2. E 2 has an active site facing to the outer surface of the cell, has a high affinity for potassium . At the same time, 2 K atoms are attached to the active site of the enzyme, the conformation of the protein changes and potassium moves inside the cell . It comes with a lot of energy, since the enzyme ATPase constantly breaks down the energy of ATP.

2. secondary active- this is transport, which is also carried out against the concentration gradient, but not the energy of macroergs is spent on this movement, but the energy of electrochemical processes that occurs when any substances move through the membrane during primary active transport.

for example: Conjugated transport of sodium and glucose, energy - due to the movement of sodium in the potassium-sodium pump.

A classic example of secondary active transport is sodium - H (ash) exchanger - when sodium and hydrogen are exchanged (this is also secondary active transport).

Methods of transport through the membrane:

1. Uniport- this is a type of transport of substances across the membrane, when one substance (Na-channels) is transported by a carrier or channel

2. Symport- this is a type of transport when 2 or more substances in their transport through the membrane are interconnected and transported together in the same direction. (Na and glucose - into the cell) This is a type of coupled transport

3. Antiport- such an associated mode of transport, when its participants cannot be transported without each other, but the flows go towards each other (K-Na-pump-active mode of transport).

Endocytosis, exocytosis - as forms of transport of substances through the membrane.

1. General properties of excitable tissues.

Excitability - the ability of a tissue to respond to irritation by changing a number of its properties. excitability index - irritation threshold . This is the smallest irritation that can cause a visible tissue response.

Conductivity - the ability of the tissue to conduct excitation along its entire length. Conductivity index - excitation conduction rate . Conductivity directly depends on the excitability of the tissue: the higher the excitability, the higher the conductivity, since the adjacent tissue area is excited faster.

refractoriness - the ability of the tissue to lose or reduce excitability in the process of excitation. In this case, during the response, the tissue ceases to perceive the stimulus. Refractoriness is absolute (no response to any stimulus) and relative (excitability is restored, and the tissue responds to a subthreshold or suprathreshold stimulus). Refractory index ( refractory period) is the time during which tissue excitability is reduced. The refractory period is the shorter, the higher the excitability of the tissue

Lability - the ability of excitable tissue to respond to irritation with a certain speed. Lability is characterized by the maximum number of excitation waves that occur in the tissue per unit time (1 s) in exact accordance with the rhythm of the applied stimuli without the phenomenon of transformation. Lability is determined by the duration of the refractory period (the shorter the refractory period, the greater the lability).

Muscle tissue is also characterized contractility. Contractility - the ability of a muscle to respond by contraction to stimulation.

2. Classification of irritants

Stimulus - a factor that can cause a response of excitable tissues.

1) natural (nerve impulses that occur in nerve cells and various receptors);

2) artificial: physical (mechanical - impact, injection; temperature - heat, cold; electric current - alternating or constant), chemical (acids, bases, ethers, etc.), physico-chemical (osmotic - sodium chloride crystal) .

In its own waynatureirritants are:

1. chemical;
2. physical;
3. mechanical;
4. thermal;
5. biological.

Bybiological conformity , that is, how much the stimulus corresponds to this tissue.

1) adequate- irritants that correspond given fabric. For example, for the retina of the eye, light - all other stimuli do not correspond to the retina, for muscle tissue- nerve impulse, etc.;

2) inadequate- irritants that do not correspond given fabric. For the retina of the eye, all stimuli except light will be inadequate, and for muscle tissue all stimuli except nerve impulses.

Bystrength:

1) subthreshold stimuli- this is the strength of the stimulus at which no response occurs;

2) threshold stimulus- this is the minimum force that causes a response with an infinite duration of action. This force is also called rheobase- it is unique for each tissue;

3) suprathreshold, or submaximal;

4)maximum stimulus is the minimum force at which the maximum response occurs tissue reaction;

5) supramaximal stimuli- with these stimuli, the tissue reaction is either maximum, or decreases, or temporarily disappears.

Thus, for each tissue there is one threshold stimulus, one maximum and many subthreshold, suprathreshold and supermaximal.

3. Physiology of cell membranes. Mechanisms of transmembrane transport.

— Border function. The membrane delimits the cytoplasm from the intercellular fluid, and most of the intracellular structures: mitochondria, nucleus, endoplasmic reticulum - from the cytoplasm.

- Biotransforming function. Any substance passing through the membrane enters into a complex interaction with it and undergoes a series of biochemical transformations. As a result of biotransformation, the medicinal substance, as a rule, passes into a form that is easily absorbed by the cell.

— Transport function. The transfer of substances through biological membranes is associated with metabolic processes, maintaining the constancy of the internal environment of the cell, excitation and conduction of a nerve impulse.

There are two main types of transfer: passive(filtration, diffusion, facilitated diffusion, osmosis) and active(the work of membrane protein "pumps")

Passive transport. Filtration carried out through membrane protein channels - pores, depends on the pressure difference outside and inside the cell and the permeability of the membrane for liquid and low molecular weight substances. The pore diameter is extremely small, so only low molecular weight substances, water and some ions are filtered.

Diffusion - passive movement of molecules or ions along a concentration gradient (from an area of ​​high concentration to an area of ​​low). Osmosis is a special case of solvent diffusion through a semi-permeable membrane that does not allow solutes to pass through.

Passive transport does not require energy.

active transport. This is a transfer of substances, universal for all types of membranes, against concentration or electrochemical gradients (from a region of low concentration to a region of high). With the help of active transport, hydrophilic polymer molecules, inorganic ions (Na, Ca, K), hydrogen, sugars, amino acids, vitamins, hormones and medicinal substances are transferred. Active transport is carried out with the obligatory expenditure of energy generated during the breakdown (oxidative phosphorylation) of adenosine triphosphoric acid (ATP).

A type of active transport associated with the activity of the cell itself is microvesicular transport (pinocytosis, exocytosis and phagocytosis). At pinocytosis there is an active absorption of fluid from the environment by the cell with the formation of bubbles and their subsequent transfer through the cytoplasm. The process of fusion of vesicles with the cell membrane and the secretion of a substance by the cell in the form of secretory granules or vacuoles is called exocytosis. Phenomenon phagocytosis is the ability of cells to actively capture and absorb microorganisms, destroyed cells and foreign particles.

- Receptor function. Biological membranes have a large number of receptors - sites, the molecular structure of which is characterized by selective affinity for certain physiologically active substances: hormones, mediators, antigens.

— Formation of intercellular contacts.

— Generation of bioelectric potentials. In the course of evolution, the glandular epithelium, muscle and nervous tissues acquired the property of excitability - the ability to respond to environmental influences with excitation. An external manifestation of excitation is the emergence of a bioelectric potential.

4. Ionic mechanisms of the resting membrane potential

About the state of rest in excitable tissues they say in the case when the tissue is not affected by an irritant from the external or internal environment. At the same time, a relatively constant level of metabolism is observed, there is no visible functional tissue administration.

Membrane potential (or resting potential)- this is the potential difference between the outer and inner surface of the membrane in a state of relative physiological rest. The resting potential arises as a result of two reasons:

1) uneven distribution of ions on both sides of the membrane. Inside the cell there is most of the K ions, outside it is little. There are more Na ions and Cl ions outside than inside. This distribution of ions is called ionic asymmetry;

2) selective permeability of the membrane for ions. At rest, the membrane is not equally permeable to different ions. The cell membrane is permeable to K ions, slightly permeable to Na ions, and impermeable to organic substances.

These two factors create conditions for the movement of ions. This movement is carried out without energy expenditure by passive transport - diffusion as a result of the difference in ion concentration. K ions leave the cell and increase the positive charge on the outer surface of the membrane, Cl ions passively pass into the cell, which leads to an increase in the positive charge on the outer surface of the cell. Na ions accumulate on the outer surface of the membrane and increase its positive charge. Organic compounds remain inside the cell. As a result of this movement, the outer surface of the membrane is charged positively, while the inner surface is negatively charged. The inner surface of the membrane may not be absolutely negatively charged, but it is always negatively charged with respect to the outer one. This state of the cell membrane is called the state of polarization. The movement of ions continues until the potential difference across the membrane is balanced, i.e., electrochemical equilibrium occurs. The moment of equilibrium depends on two forces:

1) diffusion forces;

2) forces of electrostatic interaction.

The value of electrochemical equilibrium:

1) maintenance of ionic asymmetry;

2) maintaining the value of the membrane potential at a constant level.

The diffusion force (difference in ion concentration) and the force of electrostatic interaction are involved in the occurrence of the membrane potential, therefore the membrane potential is called concentration-electrochemical.

To maintain ionic asymmetry, electrochemical equilibrium is not enough. The cell has another mechanism - the sodium-potassium pump. The sodium-potassium pump is a mechanism for ensuring active transport of ions. The cell membrane has a system of carriers, each of which binds the three Na ions that are inside the cell and brings them out. From the outside, the carrier binds to two K ions located outside the cell and transfers them to the cytoplasm. Energy is taken from the breakdown of ATP. The operation of the sodium-potassium pump provides:

1) a high concentration of K ions inside the cell, i.e., a constant value of the resting potential;

2) a low concentration of Na ions inside the cell, i.e., it maintains normal osmolarity and cell volume, creates the basis for generating an action potential;

3) a stable concentration gradient of Na ions, facilitating the transport of amino acids and sugars.

5. Membrane action potential: phases, ionic mechanisms.

action potential- this is an abrupt change in the constant membrane potential from negative to positive polarization and vice versa.

Under the action of a threshold or suprathreshold stimulus, the permeability of the cell membrane for ions changes to varying degrees. For Na ions, it increases by a factor of 400–500, and the gradient increases rapidly; for K ions, it increases by a factor of 10–15, and the gradient develops slowly. As a result, the movement of Na ions occurs inside the cell, K ions move out of the cell, which leads to a recharge of the cell membrane. The outer surface of the membrane is negatively charged, while the inner surface is positive.

Excitation of a nerve cell under the action of a chemical signal (less often an electrical impulse) leads to the appearance action potential. This means that the resting potential of -60 mV jumps to +30 mV and after 1 ms returns to its original value. The process begins with the opening of the Na+ channel (1). Na+ ions rush into the cell (along the concentration gradient), which causes a local reversal of the sign of the membrane potential (2). In this case, the Na+ channels are immediately closed, i.e., the flow of Na+ ions into the cell lasts a very short time (3). In connection with a change in the membrane potential, voltage-gated K+ channels (2) open (for a few ms) and K+ ions rush in the opposite direction, out of the cell. As a result, the membrane potential assumes the initial value (3), and even exceeds for a short time resting potential(4). After this, the nerve cell becomes excitable again.

In one pulse, a small part of the Na+ and K+ ions pass through the membrane, and the concentration gradients of both ions are preserved (the level of K+ is higher in the cell, and the level of Na+ is higher outside the cell). Therefore, as cellular impulses are received, the process of local reversal of the sign of the membrane potential can be repeated many times. The propagation of an action potential over the surface of a nerve cell is based on the fact that the local reversal of the membrane potential stimulates the opening of neighboring voltage-gated ion channels, as a result of which the excitation propagates in the form of a depolarization wave to the entire cell.

Ascending branch of the chart:

1. resting potential– initial ordinary polarized electronegative state of the membrane (–70 mV);
2. increasing local potential - depolarization proportional to the stimulus;
3. critical level of depolarization (-50 mV) - sharp acceleration depolarization(due to self-opening of sodium channels), from this point the spike begins - the high-amplitude part of the action potential;
4. self-reinforcing steeply increasing depolarization;
5. transition of the zero mark (0 mV) - change of the polarity of the membrane;
6. "overshoot" - positive polarization ( inversion, or reversion, of the membrane charge);
7. peak (+30 mV) – the top of the process of changing the polarity of the membrane, the top of the action potential.

Descending branch of the chart:

1. repolarization– restoration of the former electronegativity of the membrane;
2. transition of the zero mark (0 mV) - reverse change of the polarity of the membrane to the previous, negative one;
3. transition of the critical level of depolarization (-50 mV) - the termination of the phase of relative refractoriness (non-excitability) and the return of excitability;
4. trace processes (trace depolarization or trace hyperpolarization);
5. restoration of the resting potential - the norm (-70 mV).

6. Classification of nerve fibers.

BUT- nerve fibers with the thickest myelin sheath. The highest speed of transmission of a nerve impulse.

AT- the myelin sheath is thinner, the speed of excitation is lower

With- unmyelinated fibers with a relatively low speed of impulse transmission.

Type fibers	Diameter fibers (mk)	Speed holding (m/s)	Duration capacity actions (ms)	Duration negative trace capacity (ms)	Duration positive trace capacity (ms)	Function
A (α)	12-22	70-120	0,4-0,5	12-20	40-60	motor fibers skeletal muscles, afferent muscle receptor fibers
A (β)	8-12	40-70	0,4-0,6	—	—	afferent fibers from touch receptors
A (γ)	4-8	15-40	0,5-0,7	—	—	afferent fibers from touch receptors and pressure, efferent fibers to muscle spindles
A (Δ)	1-4	5-15	0,6-1,0	—	—	afferent fibers from some receptors heat, pressure, pain
AT	1-3	3-14	1-2	Absent- there is	100-300	preganglionic vegetative fibers
With	0,5-1,0	0,5-2	2,0	50-80	300-1000	Preganglionic autonomic fibers, afferent fibers from some receptors pain, pressure, heat

Excitation. Excitability. Conductivity. refractoriness and lability. Physiological properties of nerve fibers (non-myelinated and myelinated). Nerve fatigue. Physiological properties of synapses.

"Everything is regulated, flows along cleared channels, makes its circuit in accordance with the law and under its protection."

I. Ilf and E. Petrov "The Golden Calf"

All cells and tissues of a living organism, under the influence of stimuli, pass from a state of relative physiological rest to a state of activity (excitation). The highest degree of activity is observed in the nervous and muscle tissue.

The main properties of excitable tissues are: I. excitability, II conductivity, III refractoriness and lability, which are associated with one of the most common properties of living things - irritability.

Changes in the environment or organism are called irritants, and their action is called irritation.

By nature, stimuli are: mechanical, chemical, electrical, temperature.

On a biological basis, stimuli are divided into:

adequate, which are perceived by the corresponding specialized receptors (eyes - light, ear - sound, skin - pain, temperature, touch, pressure, vibration);

2. inadequate, to which specialized receptors are not adapted, but perceive them with excessive strength and duration (strike - eye - light).

The most common, adequate and natural stimulus for all cells and tissues of the body is a nerve impulse.

The main physiological properties of the nervous tissue (excitability, conductivity, refractoriness and lability) characterize the functional state of the human nervous system and determine its mental processes.

I. Excitability - the ability of living tissue to respond to the action of a stimulus by the occurrence of an excitation process with a change in physiological properties.

A quantitative measure of excitability is the threshold of excitation, i.e. the smallest amount of stimulus that can elicit a tissue response.

A stimulus of lesser strength is called subthreshold, and a greater one is called suprathreshold.

Excitability is, first of all, a change in the metabolism in tissue cells. The change in metabolism is accompanied by the passage of negatively and positively charged ions through the cell membrane, which change the electrical activity of the cell. The potential difference at rest between the internal contents of the cell and the cell membrane, which is 50-70 mV (millivolts), is called the resting membrane potential.

The basis of this state of the cell is the selective permeability of the membrane with respect to K+ and Na+ ions. Na+ ions, located in the extracellular environment, through the membrane into the cells, the path is closed, and K+ freely penetrates through the pores of the cell membrane from the cytoplasm of the cell into the tissue fluid. As a result, negatively charged ions remain in the cytoplasm, and positively charged K+ and Na+ ions accumulate on the membrane surface.

When the cell is excited, the permeability of Na + ions increases sharply, and they rush into the cytoplasm, reducing the resting potential to zero, and then increasing the potential difference of the opposite value to 80-110 mV. Such a short-term (0.004-0.005 sec) change in the potential difference is called an action potential (spike); English spike - a point.

Following this, the disturbed equilibrium of the ions is restored again. For this, there is a special cellular mechanism - the "sodium-potassium pump", which provides active "pumping out" of Na + from the cell and "forcing" K + into it. Thus, there are 2 types of movement of ions through the cell membrane:

1 - passive ion transport along the ion concentration gradient;

2 - active ion transport against the concentration gradient, carried out by the "sodium-potassium pump" with the expenditure of ATP energy.

Conclusion: the excitation of a nerve cell is associated with a change in metabolism and is accompanied by the appearance of electrical potentials (nerve impulses).

Conductivity - the ability of living tissue to conduct excitation waves - bioelectric impulses.

To ensure homeostatic unity, all body structures (cells, tissues, organs, etc.) must be able to interact spatially. The spread of excitation from the place of its origin to the executive organs is one of the main ways of such interaction. The action potential that has arisen at the site of application of irritation is the cause of irritation of neighboring, unexcited sections of the nerve (or muscle) fiber. Due to this phenomenon, the action potential wave creates an action current that propagates along the entire length of the nerve fiber. In non-myelinated nerve fibers, excitation is carried out with some attenuation - decrement, and in myelinated nerve fibers - without attenuation. Conducting excitation is also accompanied by a change in metabolism and energy.

III. Refractoriness is a temporary decrease in tissue excitability that occurs when an action potential appears. At this point, repeated irritations do not cause a response (absolute refractoriness). It lasts no more than 0.4 milliseconds, and then comes the phase of relative refractoriness, when irritation can cause a weak reaction. This phase is replaced by a phase of increased excitability - supernormality.

Such dynamics of excitability is due to the processes of changing and restoring the equilibrium of ions on the cell membrane.

Professor N.E. Vvedensky studied the features of these processes and found that excitable tissues can respond with a different number of action potentials to a certain frequency of stimulation. He called this phenomenon lability (functional mobility).

Lability is the property of an excitable tissue to reproduce the maximum number of action potentials per unit time.

The maximum lability is in the nervous tissue. The frequency of stimuli that causes the maximum reaction is called optimal (lat. optimum - the best), and the frequency that causes inhibition of the reaction is called pessimal (lat. pessimum - the worst).

*Nerve fiber - up to 1000 pulses/sec, muscle - 200-250 pulses/sec, synapse - up to 100-125 pulses/sec.

Pessimum is an active tissue reaction aimed at protecting it from excessive irritation. This is one of the manifestations of inhibition. Excitation and inhibition are self-regulating processes opposite in meaning, which establish the "golden mean" of the level of relations between the organism and the environment.

Nerve fibers (outgrowths of nerve cells) have all the properties of excitable tissues, and the conduction of nerve impulses is their special function. The speed of excitation depends on:

1 - fiber diameter (thicker ® ​​faster),

2 - structures of their shell.

Myelin-free (non-fleshy) fibers are covered only with lemmocytes (Schwann cells). Between them and the axial cylinder (neuronal axon) there is a gap with intercellular fluid, therefore, the cell membrane remains uninsulated. The impulse propagates along the fiber at a speed of only 1-3 m/s.

Myelin fibers are covered with spiral layers of Schwann cells with a layer of myelin, a fat-like substance with high resistivity. The myelin sheath is interrupted at intervals of equal length, leaving bare sections of the axial cylinder with a length of » 1 µm.

Due to this structure, electric currents can enter and leave the fibers only in the region of non-isolated sections - the intercepts of Ranvier. When irritation is applied, depolarization occurs in the nearest intercept, and neighboring intercepts are polarized. A potential difference arises between them, which leads to the appearance of circular action currents.

Thus, the impulse in the myelin fiber passes spasmodically (saltatorically) from interception to interception. In this case, the excitation propagates without attenuation, and the speed of the impulse conduction reaches 120-130 m/s.

When irritation is applied to a nerve fiber, the excitation spreads both ways - in the centripetal and centrifugal directions. This does not contradict the principle of one-way conduction of impulses, and is explained by the primacy of the appearance of excitation in receptors or nerve centers, as well as the presence of synapses. The neurotransmitter (transmitter) is contained only in the presynaptic apparatus and carries the potential only in one direction (see lecture on anatomy No. 2).

Excitation is carried out not only in the right direction, but also along one isolated fiber, without spreading to neighboring fibers. This causes a strictly coordinated reflex activity. For example, the sciatic nerve up to 12 mm in diameter carries thousands of nerve fibers (myelinated and unmyelinated, sensory and motor, somatic and autonomic). In the case of non-isolated excitation, a chaotic response would be observed.

The isolated conduction of excitation in myelinated fibers is provided by the myelin sheath, and in non-myelinated ones, by the high resistivity of the surrounding intercellular fluid (hence the potential damping).

NOT. Vvedensky in 1883 for the first time established that the nerve is not easily fatiguable. The low fatigue of nerve fibers is explained by the fact that the energy costs in them during excitation are insignificant, and the recovery processes proceed quickly. In the body, nerve fibers also work with underload. For example, a motor fiber is highly labile and can conduct up to 2500 pulses/sec. No more than 50-40 pulses per second come from the nerve centers.

Conclusion: the practical indefatigability of nerve fibers is associated with low energy costs, with high lability of nerve fibers, with constant underloading of fibers.

Synapses (see structure in lecture on anatomy No. 2) have the following physiological properties:

1 - unilateral conduction of excitation, which is associated with structural features of the synapse itself,

Lecture Search

Physiological properties of skeletal muscles. Phase changes in the excitability of nervous and muscle tissue. Methods for measuring excitability

Physiological properties of muscles

Excitability— the ability to come into a state of excitation under the action of stimuli.

Conductivity — ability to conduct excitation.

Contractility — the ability of a muscle to change its length or tension in response to a stimulus.

Lability - according to N.E. Vvedensky, the largest number of action potentials that an excitable tissue is able to reproduce per unit time (1 sec.) under the influence of frequent applications of stimuli to it (muscle fiber lability is 20-30 impulses per second, nervous about 1000).

§ Automation- the ability to generate impulses without external irritation (the property is characteristic of the heart muscle and smooth muscles).

Skeletal (striated) muscles in the body play the role of a kind of "machines" that convert chemical energy directly into mechanical and thermal energy. Muscle contraction occurs in response to electrical impulses coming to them from alpha motor neurons - nerve cells that lie in the anterior horns of the spinal cord.

Muscles and the motor neurons that innervate them make up the human neuromuscular apparatus.

The human body has a pronounced ability to adapt to constantly changing environmental conditions. The adaptive reactions of the body are based on the universal property of living tissue - irritability - the ability to respond to the action of irritating factors by changing the structural and functional properties. All tissues of animal and plant organisms have irritability. In the process of evolution, there was a gradual differentiation of tissues that carry out the adaptive activity of the organism. The irritability of these tissues reached its highest development and was transformed into a new property - excitability. This term is understood as the ability of a tissue to respond to irritation with a specialized reaction - excitation. Excitation - this is a complex biological process, which is characterized by a specific change in the processes of metabolism, heat generation, temporary depolarization of the cell membrane and manifested by a specialized tissue reaction (muscle contraction, secretion by the gland, etc.).

excitability nervous, muscular and secretory tissues, they are combined in the concept of "excitable tissues".

The excitability of different tissues is not the same. The measure of excitability is irritation threshold - the minimum strength of the stimulus that can cause arousal. Less powerful stimuli are called subthreshold, and the stronger ones superthreshold. An irritant of a living cell can be any change in the external or internal environment, if it is large enough, has arisen quickly enough and lasts long enough.

The nature of arousal

The first attempts to consistently develop the doctrine of "animal electricity" are associated with the name of L. Galvani. E. Dubois-Reymond was the first to show that the outer surface of a muscle is positively charged with respect to its inner content. Therefore, at rest, there is a potential difference between the outer and inner surfaces of the cell membrane, which was then called resting membrane potential or membrane potential. Its value in different cells ranges from 60 to 90 mV.

A. Hodgkin, A. Huxley and B. Katz in the 50s of the 20th century explained the causes of the occurrence of the resting membrane potential, for which they significantly reworked pre-existing ideas and created membrane ion theory. According to their views, the resting membrane potential (RMP) is due to the unequal concentration of sodium, potassium, calcium, chlorine ions inside the cell and in the extracellular fluid, as well as the unequal permeability of the cell surface membrane for these ions (Fig. 2.4). The cytoplasm of nerve and muscle cells contains 30-50 times more potassium ions, 8-10 times less sodium ions and 50 times less chloride ions than the extracellular fluid. Therefore, at rest, there is an asymmetry in the concentration of ions inside the cell and in its environment.

Rice. 2.4. Registration of the resting potential

The membrane has ion channels, formed by protein macromolecules penetrating the lipid layer. Membrane channels are divided into non-specific (leak channels) and specific (selective, having the ability to pass only certain ions). Non-specific channels allow various ions to pass through and are constantly open. Specific channels open and close in response to changes in the MTP.

shiz. 1. General properties of excitable tissues. Excitability

These channels are called voltage-dependent.

In a state of physiological rest, the membrane of nerve fibers is 25 times more permeable to K+ than for Na+.

The release of positively charged potassium ions leads to the appearance of a positive charge on the outer surface of the membrane. Organic anions - large molecular compounds that carry a negative charge, and for which the cell membrane is impermeable, give a negative charge under these conditions to the inner surface of the membrane (Fig. 2.5).

Fig.2.5. The concentration of major ions inside and outside the cell.

At rest, there are small movements K+ and Na+ through the membrane along their concentration gradient (Table 2.2), K+ more than Na+.

Tab. 2.2.

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II. The concept of irritability, excitability and arousal. Classification of stimuli

Irritability- this is the ability of cells, tissues, the body as a whole to move under the influence of external or internal environmental factors from a state of physiological rest to a state of activity. The state of activity is manifested by a change in the physiological parameters of a cell, tissue, organism, for example, a change in metabolism.

Excitability- this is the ability of living tissue to respond to irritation with an active specific reaction - excitation, i.e. generation of a nerve impulse, contraction, secretion. Thus, excitability characterizes specialized tissues - nervous, muscular, glandular, which are called excitable.

Excitation is a complex of processes of response of an excitable tissue to the action of an irritant, manifested by a change in the membrane potential, metabolism, etc. Excitable tissues are conductive. This is the ability of the tissue to conduct excitation. Nerves and skeletal muscles have the highest conductivity.

Stimulus is a factor of the external or internal environment acting on living tissue.

The process of exposure of an irritant to a cell, tissue, organism is called irritation.

All stimuli are divided into the following groups:

1.By nature : a) physical (electricity, light, sound, mechanical effects, etc.); b) chemical (acids, alkalis, hormones, etc.); c) physical and chemical (osmotic pressure, partial pressure of gases, etc.); d) biological (food for an animal, an individual of the opposite sex); e) social (a word for a person).

2. By place of impact : a) external (exogenous); b) internal (endogenous).

3. By strength : a) subthreshold; b) threshold (minimum stimuli, the force at which excitation occurs); c) superthreshold (strength above the threshold).

4. By physiological nature : a) adequate (physiological for a given cell or receptor, which have adapted to it in the process of evolution, for example, light for eye photoreceptors); b) inadequate.

5. If the reaction to the stimulus is reflex , then they also distinguish:

a) unconditioned reflex stimuli; b) conditioned reflex.

III. Resting potential (MPP)

Resting potential - a relatively stable difference in electrical potentials between the outer and inner sides of the cell membrane. Its value usually varies within 30-90 mV (in skeletal muscle fibers - 60-90 mV, in nerve cells - 50-80 mV, in smooth muscles - 30-70 mV, in the heart muscle - 80 -90 mV).

PP plays an extremely important role in the life of the cell itself and the organism as a whole, since it is the basis for the emergence of excitation (action potential), with the help of which the nervous system perceives and processes information, regulates the activity of internal organs and the musculoskeletal system by starting the processes of excitation and contraction in the muscle. Violation of excitation processes in cardiomyocytes leads to cardiac arrest.

According to the membrane-ion theory (Bernstein, Hodgkin, Huxley, Katz, 1902-1952), the direct cause of the formation of PP is the unequal concentration of anions and cations inside and outside the cell.

Various ions are distributed unevenly on both sides of the cell membrane, firstly, due to the unequal permeability of the cell membrane for various ions, and secondly, as a result of the operation of ion pumps that transport ions into the cell and from the cell contrary to the concentration and electrical gradients.

The role of cell membrane permeability in the formation of PP. The permeability of a cell membrane is its ability to pass water, uncharged and charged particles (ions) according to the laws of diffusion and filtration. The term "conductivity" should be used only in relation to charged particles. Therefore, conductivity is the ability of charged particles (ions) to pass through the cell membrane according to the electrochemical gradient.

Na + and K + in a resting cell move through the membrane according to the laws of diffusion, while K + leaves the cell in a much larger amount than Na + enters the cell, since the permeability of the cell membrane for K + is approximately 25 times greater than the permeability for Na +.

Organic anions, due to their large size, cannot leave the cell, so there are more negative ions inside the cell at rest than positive ones. For this reason, the cell from the inside has a negative charge. It is interesting that at all points of the cell the negative charge is almost the same. This is evidenced by the same RI value when the microelectrode is introduced to different depths inside the cell, as was the case in the experiments of Hodgkin, Huxley and Katz. The giant squid axon (its diameter is about 1 mm) was placed in sea water in this experiment, one electrode was inserted into the axon, the other was placed in sea water. The charge inside the cell is negative both absolutely (the cell hyaloplasm contains more anions than cations) and relative to the outer surface of the cell membrane. However, the excess of the absolute number of anions over the number of cations in a cell is extremely small. But this difference is enough to create a difference in electrical potentials inside and outside the cell.

The main ion providing the formation of PP is the K+ ion. This is evidenced by the results of the experiment with the perfusion of the internal contents of the giant squid axon with saline solutions. With a decrease in the concentration of K + in the perfusate, PP decreases, with an increase in the concentration of K +, PP increases. In a resting cell, a dynamic balance is established between the number of K+ ions leaving the cell and entering the cell. Electric and concentration gradients counteract each other: according to the concentration gradient, K + tends to leave the cell, the negative charge inside the cell and the positive charge of the outer surface of the cell membrane prevent this. When the concentration and electrical gradients are balanced, the number of K+ ions leaving the cell is compared with the number of K+ ions entering the cell. In this case, the so-called equilibrium potassium potential is established on the cell membrane. The equilibrium potential for any ion can be calculated using the Nernst formula, and for several using the Goldman-Hodgkin-Katz formula

In general, PP is a derivative of the equilibrium potentials of all ions inside and outside the cell and the surface charges of the cell membrane.

The contribution of Na+ and Cl- to the creation of PP. The permeability of the cell membrane at rest for Na + is very low - much lower than for K +, however, it takes place, therefore, Na + ions, according to the concentration and electrical gradients, tend to and pass into the cell in a small amount. This leads to a decrease in PP, since the total number of positively charged ions on the outer surface of the cell membrane decreases, although slightly, and part of the negative ions inside the cell are neutralized by the positively charged Na+ ions entering the cell. The entry of Na+ into the cell reduces PP. As for CL — , its effect on the PP value is opposite to that of Na+ and depends on the permeability of the cell membrane for Cl — (it is 2 times lower than for K+). The point is that CL — , according to the concentration gradient, tends and passes into the cell. Concentrations of K+ and Cl ions — are close to each other. But Cl — is located mainly outside the cell, and K + - inside the cell. Prevents the entry of Cl — the electrical gradient into the cell, since the charge inside the cell is negative, as is the charge of Cl — . There comes an equilibrium of forces of the concentration gradient, which contributes to the entry of Cl — into the cell, and an electric gradient that prevents the entry of Cl — in a cell. Therefore, the intracellular concentration of Cl — is only 5-10 mmol / l, and outside the cell - 120-130 mmol / l. Upon receipt of Cl — inside the cell, the number of negative charges outside the cell decreases somewhat, and inside the cell increases: Сl — is added to large protein anions located inside the cell. These anions, due to their large size, cannot pass through the channels of the cell membrane to the outside of the cell - into the interstitium. Thus, Cl-, penetrating into the cell, increases PP. Partially, as well as outside the cell, Na + and Cl — neutralize each other inside the cell. As a result, the combined supply of Na+ and Cl — inside the cell does not significantly affect the value of PP.

The role of surface charges of the cell membrane and Ca2+ ions in the formation of PP. The outer and inner surfaces of the cell membrane carry their own electrical charges, mostly with a negative sign. These are polar molecules of the cell membrane - glycolipids, phospholipids, glycoproteins. Fixed external negative charges, neutralizing the positive charges of the outer surface of the membrane, reduce the PP. Fixed internal negative charges of the cell membrane, on the contrary, adding up with anions inside the cell, increase the PP. The role of Ca2+ ions in the formation of PP is that they interact with the external negative fixed charges of the cell membrane and the negative carboxyl groups of the interstitium and neutralize them, which leads to an increase and stabilization of PP.

Thus, PP is the algebraic sum not only of all ion charges outside and inside the cell, but also the algebraic sum of the negative external and internal surface charges of the membrane itself.

The role of ion pumps in the formation of PP. As a result of the continuous movement of various ions through the cell membrane, their concentration inside and outside the cell should gradually equalize. However, despite the constant diffusion of ions (leakage of ions), the PP of the cells remains at the same level. Therefore, in addition to the intrinsic ionic mechanisms of PP formation associated with different permeability of the cell membrane, there is an active mechanism for maintaining concentration gradients of various ions inside and outside the cell. They are ion pumps, in particular Na / K-pump (pump). As a result of conjugated transport of Na + and K +, a constant difference in the concentrations of these ions is maintained inside and outside the cell. One ATP molecule provides one cycle of the Na / K-pump - the transfer of three Na + ions outside the cell and two K + ions inside the cell.

Excitability and conductivity - properties characteristic of tissue

The asymmetric transfer of Na/K-pump ions maintains an excess of positively charged particles on the outer surface of the cell membrane and negative charges inside the cell, which allows us to consider the Na/K-pump as an electrogenic structure, additionally increasing the PP by about 5 10 mV (on average, about 10% in different excitable cells - some more, others less). This fact indicates that the decisive factor in the formation of PP is the selective permeability of the cell membrane for different ions. If we equalize the permeability of the cell membrane for all ions, then the PP will be only 5-10 mV - due to the operation of the N / K-pump.

The normal value of the PP is a necessary condition for the onset of the cell excitation process, i.e. the emergence and propagation of an action potential that initiates specific cell activity.

III. Electrotonic and local potentials(fig.6)

If a cell is exposed to an irritant in the amount of 1-50% of the threshold, the cell will respond with an electrotonic potential - a shift in the cell's MP. This is a passive reaction of the cell to an electrical stimulus; the state of ion channels and ion transport does not change, or changes very slightly for fractions of milliseconds. EP is not a physiological reaction of the cell, and so. is not arousal.

If the cell is affected by a subthreshold current (50-99% of the threshold value), a prolonged shift of the MP develops - a local response. This is an active reaction of the cell to the stimulus, however, the state of ionic and ion transport changes slightly. LO is called local excitation, because. it does not propagate across the membranes of excitable cells, nor is it a propagating depolarization of the membrane. It is mainly due to the movement of Na + ions into the cell. As a result, the level of membrane polarization decreases.

LO properties:

• spreads with decay
• obeys the law of gradualness (gradual rise or fall)
• can be summed up
• no refractory period
• has a phase of depolarization and repolarization

rice. 6

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Excitable tissues are tissues that are able to perceive the action of a stimulus and respond to it by switching to a state of excitation.

Excitable tissues include three types of tissues - nervous, muscular and glandular.

Excitable tissues have a number of general and particular properties.

The general properties of excitable tissues are:

1. Irritability

2. Excitability

Conductivity

Irritability is the ability of a cell, tissue or organ to perceive the action of a stimulus by changing metabolism, structure and functions.

Irritability is a universal property of all living things and is the basis of adaptive reactions of a living organism to constantly changing conditions of the external and internal environment.

Excitability is the ability of a cell, tissue or organ to respond to the action of a stimulus by switching from a state of functional rest to a state of physiological activity.

Excitability is a new, more perfect property of tissues, into which (in the process of evolution) irritability has been transformed. Different tissues have different excitability: nervous > muscular > glandular

The measure of excitability is the threshold of irritation

The threshold of irritation is the minimum strength of the stimulus that can cause spreading excitation.

Excitability and irritation threshold are inversely related (the more excitability, the< поpог pаздpажения)

Excitability depends on:

1. Resting potential values

2. The level of critical depolarization

The resting potential is the potential difference between the inner and outer surfaces of the membrane at rest

The level of critical depolarization is the value of the membrane potential that must be reached in order for the excitation to be of a propagating nature.

The difference between the values ​​of the resting potential and the level of critical depolarization determines the depolarization threshold (than< поpог деполяpизации, тем >excitability)

Conductivity is the ability to conduct excitation

Conductivity is determined:

1. Fabric structure

2.Functional features of the fabric

excitability

Memory is the ability to fix changes in the functional state of a cell, tissue, organ and organism at the molecular level

Determined by the genetic program

Allows you to respond to the action of individual stimuli that are significant for the body ahead of time

Specific properties of excitable tissues include:

1. Contractility

2. Secretory activity

Automation

Contractility - the ability of muscle structures to change length or tension in response to stimulation

Depends on the type of muscle tissue

Secretory activity is the ability to secrete a mediator or secretion in response to stimulation.

Neuron terminals secrete neurotransmitters

Glandular cells excrete sweat, saliva, gastric and intestinal juice, bile, and also increte hormones and biologically active substances

Automation is the ability to be independently excited, that is, to be excited without the action of a stimulus or an incoming nerve impulse.

Characteristic for cardiac muscle, smooth muscle, individual nerve cells of the central nervous system

Excitable tissues are characterized by 2 types of functional activity

Physiological rest - a state without manifestations of specific activity (in the absence of the action of a stimulus)

Excitation is an active state, which is manifested by structural and physico-chemical changes (a specific form of reaction in response to the action of a stimulus or an incoming nerve impulse)

Different types of functional activity are determined by the structure, properties and state of plasma membranes

No. 9 Functions: 1. Barrier - the membrane, with the help of appropriate mechanisms, participates in the creation concentration gradients, preventing free diffusion.

2. The regulatory function of the cell membrane consists in the fine regulation of intracellular contents and intracellular reactions due to the reception of extracellular biologically active substances, which leads to a change in the activity of membrane enzyme systems and the launch of mechanisms of secondary “messengers” (“mediators”).

3. Converting external stimuli of a non-electrical nature into electrical signals (in receptors).

4. Release of neurotransmitters in synaptic endings.

Fluid mosaic model by Singer and Nicholson:

In the phospholipid bilayer, globular proteins are integrated, the polar regions of which form a hydrophilic surface in the aqueous phase. These integrated proteins perform various functions, including receptor, enzymatic, form ion channels, are membrane pumps and carriers of ions and molecules.

Some protein molecules diffuse freely in the plane of the lipid layer; in the normal state, parts of protein molecules that emerge on opposite sides of the cell membrane do not change their position.

The special morphology of cell membranes determines their electrical characteristics, among which the most important are capacitance and conductivity.

Capacitance properties are mainly determined by the phospholipid bilayer, which is impermeable to hydrated ions and at the same time thin enough (about 5 nm) to provide efficient separation and accumulation of charges and electrostatic interaction of cations and anions. active transport- transport of substances across the membrane, which is carried out against a concentration gradient and requires a significant amount of energy. One third of the basal metabolic rate is spent on active transport.

Active transport is:

1. primary active- such transport, for which the energy of macroergs is used - ATP, GTP, creatine phosphate. for example: Potassium-sodium pump - an important role in the processes of excitability in the cell. It is embedded in the membrane.

Potassium sodium pump- enzyme potassium-sodium ATPase.

Excitable tissues and their main properties

This enzyme is a protein. It exists in the membrane in the form of 2 forms:

E 1, E 2

In enzymes, there active site, which interacts with potassium and sodium. When the enzyme is in form E 1, its active site faces the inside of the cell and has a high affinity for sodium , and therefore contributes to its addition (3 Na atoms). As soon as sodium is added, the conformation of this protein occurs, which moves 3 sodium atoms through the membrane and sodium is detached from the outer surface of the membrane. In this case, the transition of the enzyme from form E 1 to E 2. E 2 has an active site facing to the outer surface of the cell, has a high affinity for potassium . At the same time, 2 K atoms are attached to the active site of the enzyme, the conformation of the protein changes and potassium moves inside the cell . It comes with a lot of energy, since the enzyme ATPase constantly breaks down the energy of ATP.

2. secondary active- this is transport, which is also carried out against the concentration gradient, but not the energy of macroergs is spent on this movement, but the energy of electrochemical processes that occurs when any substances move through the membrane during primary active transport.

for example: Conjugated transport of sodium and glucose, energy - due to the movement of sodium in the potassium-sodium pump.

A classic example of secondary active transport is sodium - H (ash) exchanger - when sodium and hydrogen are exchanged (this is also secondary active transport).

Methods of transport through the membrane:

1. Uniport- this is a type of transport of substances across the membrane, when one substance (Na-channels) is transported by a carrier or channel

2. Symport- this is a type of transport when 2 or more substances in their transport through the membrane are interconnected and transported together in the same direction. (Na and glucose - into the cell) This is a type of coupled transport

3. Antiport- such an associated mode of transport, when its participants cannot be transported without each other, but the flows go towards each other (K-Na-pump-active mode of transport).

Endocytosis, exocytosis - as forms of transport of substances through the membrane.

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What is excitation, what tissues are excitable?

Excitation

What phenomenon reflects the state of muscle cell excitation?

Cell membrane charge

What is excitability?

Ability to excite.

5. How can you evaluate the excitability of various cells, give an example?

According to the threshold force parameter. The lower the threshold force, the greater the excitability. The most excitable tissue is nervous.

Cell A has an ARC of 60 mV, the membrane potential is 80 mV, cell B has an ARC of 60 mV, a membrane potential of 90 mV, which cell is more excitable, why?

Cell A has greater excitability, since excitation is estimated by the threshold strength of the stimulus (its minimum strength at which the cell is excited).

Cell A has an ARC of 60 mV, membrane potential (MP) of 80 mV, cell B has an ARC of 70 mV, MP of 90 mV, which cell is more excitable, prove the answer?

Cells have the same excitability, since excitation is estimated by the threshold strength of the stimulus (its minimum strength at which the cell is excited), and they are the same for cells.

8. What electrophysiological characteristics of the cell membrane predetermine the excitability of cells? Give an example.

Membrane potential and KUD.

9. Give an example of the response of an excitable tissue to threshold and suprathreshold stimuli according to the law of "force ratios". Explain the reason for this response.

Excitable tissue reacts to threshold and suprathreshold stimuli according to the principle that the greater the current, the greater the response. Individual cells, for various reasons, have different excitability thresholds, therefore, at the beginning, the most excitable cells contract in the muscle (threshold contraction), and then, as the strength of the stimulus increases, more and more less excitable cells are involved in the contraction. When all cells are excited, an increase in the amplitude of the stimulus does not cause an increase in the response (reduction

10. Give an example of the response of an excitable tissue according to the law "all or nothing". Explain the reason for this response.

The tissue reacts to threshold and suprathreshold stimuli by contraction of the same force. This is typical for homogeneous systems (1 cell, also cardiac. The heart muscle is the so-called functional syncytium, and the skeletal muscle is a symplast.

What reflects the MPP of a neuron, what is it equal to, how can its value be determined.

The potential difference between the outer and inner surfaces of the cell membrane at rest.

Describe the ionic mechanisms that ensure the occurrence of membrane potential in nerve and muscle cells?

The potassium mechanism is the main mechanism that underlies the polarization of the cell membrane. Also plays the role of K-Na pump and the flow of Na from the cell.

13. How will the MPP change with an increase in the extracellular concentration of K + ions, how will this affect the excitability of the cell?

MPP will decrease, as the charge difference will decrease.

Excitability and conductivity properties characteristic of tissue

Excitability will increase due to changes in MPP.

How will IVD change after cell treatment with a blocker of voltage-gated sodium channels of the membrane?

decrease

Solve the problem - the intracellular potential of the muscle cell is -80 mV. What is the resting membrane potential?

16. Decipher the concepts - polarization, depolarization, repolarization, hyperpolarization.

Polarization is an asymmetric distribution of charges relative to the cell membrane.

Depolarization is a decrease in voltage across the membrane.

Hyperpolarization is an increase in membrane voltage.

Repolarization is the restoration of membrane potential after de- and hyperpolarization.

Draw an electrogram of IVD initiation during threshold and over threshold stimulation of a muscle cell.

1. local response (threshold depolarization)
2. rapid depolarization phase
3. jump (overshoot) - reload phase
4. repolarization phase
5. trace negative potential
6. trace positive potential

Explain the ionic mechanism of IVD formation.

There is an opening of controlled sodium channels under the influence of concentration and electrical gradients. The number of controlled sodium channels is greater than uncontrolled potassium channels. As a result, the membrane is recharged. In the jump phase, the current K from the cell begins to prevail over the current Na into the cell, and the charge begins to fall.

What is useful time, what is chronaxy.

The shortest duration of stimulation with a force of one rheobase necessary for the onset of excitation is called good time. Rheobase is the minimum strength of irritation, to which, with a practically unlimited long duration of its action, a minimal response will occur.

Chronaxia- the minimum time during which a stimulus with a force equal to two rheobases causes excitation

What is the "guarantee factor" of excitation?

Reliability factor (guarantee factor) \u003d PD: excitability of the nerve fiber. Normally 5-6 units

What is excitation, what tissues are excitable?

Excitation- this is the process of generating an action potential under the influence of threshold and suprathreshold stimuli. Excitable tissues: muscular, nervous and glandular.

2. What function does excitation perform. Give examples.

Excitation in excitable tissues triggers special reactions. Muscles - contraction, nerves - impulse, release of the mediator, iron - secretion.

Excitable tissues are nervous, muscular and glandular structures that are capable of being excited spontaneously or in response to the action of an irritant. Excitation is the generation of an action potential (AP) + the spread of AP + a specific tissue response to this potential, for example, contraction, release of a secret, release of a mediator quantum.

Properties of excitable tissues and indicators characterizing them: Properties

1. Excitability - the ability to be excited

2. Conductivity - the ability to conduct excitation, i.e. conduct PD

3. Contractility - the ability to develop force or tension when excited

4. Lability - or functional mobility - the ability to rhythmic activity

5. The ability to secrete a secret (secretory activity), mediator

More details - see below.

Indicators

Threshold of irritation, rheobase, chronaxy, duration of the absolute refractory phase, rate of accommodation.

The speed of AP conduction, for example, in a nerve, it can reach 120 m/s (about 600 km/h).

The maximum value of force (voltage) developed during excitation.

The maximum number of excitations per unit of time, for example, a nerve is capable of generating 1000 APs in 1 s

Quantum yield value, secret volume

ELECTRICAL PHENOMENA IN EXCITABLE TISSUES

Classification:

Biopotentials- the general name of all types of electrical processes in living systems.

Potential damage - historically the first concept of the electrical activity of the living (demarcation potential). This is the potential difference between intact and damaged surfaces of living excitable tissues (muscles, nerves). The clue to its nature led to the creation of the membrane theory of biopotentials.

Membrane potential (MP) is the potential difference between the outer and inner surfaces of the cell (muscle fiber) at rest. Typically, the MP, or resting potential, is 50-80 mV, with a "-" sign inside the cell. When a cell is excited, an action potential is recorded (its phases: peak, trace negativity, trace positivity) - a rapid change in the membrane potential during excitation.

Extracellularly registered action potential. Intracellularly-registered action potential - these are variants of action potentials, the form of which depends on the method of assignment (see below).

Receptor (generator) potential- change in the MP of receptor cells during their excitation.

Postsynaptic potentials(options: excitatory postsynaptic potential - EPSP, inhibitory postsynaptic potential - IPSP, a special case of excitatory postsynaptic potential - PKP - end plate potential).

Evoked Potential- this is the action potential of a neuron that occurs in response to the excitation of a receptor that carries information to this neuron.

ECG (gram), EEG, EMG (myogram) - respectively - the total electrical activity of the heart, brain, skeletal muscles during them excited.

History is Galvani, Matteuci, Dubois-Reymond, Bernstein, Hodgkin, Huxley, Katz. All types of bioelectrical activity will be described in more detail in the following.

EXPERIMENTAL METHODS FOR INVESTIGATION OF BIOELECTRIC PHENOMENA

L. Galvani was the first to be convinced of the existence of "living electricity". His first (balcony) experience was that the preparation of the hind legs of frogs on a copper hook was suspended from an iron balcony. From the wind, he touched the balcony railing, and this caused muscle contraction. According to Galvani, this was the result of closing the current circuit, as a result of which "living electricity" caused contraction. Volta (Italian physicist) refuted this explanation. He believed that the reduction was due to the presence of a "galvanic pair" - iron-copper. In response, Galvani set up a second experiment (experiment without metal), which proved the author's idea: a nerve was thrown between the damaged and undamaged muscle surfaces and, in response, the intact muscle contracted.

There are currently two main registration methods

Rice. 2. The speed of propagation of excitation along nerve fibers of various types.

I - scheme of the experience of stimulating the nerve trunk with a stimulator (St) and diverting the biocurrent from the near point (a) "and remote (b) using installations that include an electrode, an amplifier, an oscilloscope (Us and Os, respectively), M - muscle.

II - a nerve consisting of fibers of types A, B, C. Little people - impulses running through the fibers at different speeds. The velocity dissociation is especially noticeable on the oscilloscope screen. The graph shows the ratios of the action potentials of fibers A (o, (3, y), B, C.

biopotentials: extracellular and intracellular. The extracellular method is the removal of the potential difference between two points of the tissue, organ. Options - monopolar lead (one electrode is grounded), bipolar lead (both electrodes are active). The contact method - the electrodes are in direct contact with the object of study, the distant one (for example, with ECG-graphy) - there is a medium between the object of study and the electrodes. In general, with the extracellular method, only a part of the potential is allocated. Membrane potential cannot be measured.

Intracellular way; one electrode - in the medium, the second (glass pipette) - is introduced into the cell. The potential difference between the outer and inner surfaces of the membrane is recorded. The pipette is pre-filled with potassium chloride solution.