Conduction of a nerve impulse along the fiber occurs due to the propagation of a depolarization wave along the sheath of the process. Most peripheral nerves, through their motor and sensory fibers, provide impulse conduction at a speed of up to 50-60 m / s. The actual depolarization process is quite passive, while the restoration of the resting membrane potential and the ability to conduct is carried out by the functioning of the NA / K and Ca pumps. Their work requires ATP, a prerequisite for the formation of which is the presence of segmental blood flow. The cessation of the blood supply to the nerve immediately blocks the conduction of the nerve impulse.

According to the structural features and functions, nerve fibers are divided into two types: unmyelinated and myelinated. Unmyelinated nerve fibers do not have a myelin sheath. Their diameter is 5-7 microns, the speed of impulse conduction is 1-2 m/s. Myelin fibers consist of an axial cylinder covered by a myelin sheath formed by Schwann cells. The axial cylinder has a membrane and oxoplasm. The myelin sheath consists of 80% lipids and 20% protein. The myelin sheath does not completely cover the axial cylinder, but is interrupted and leaves open areas of the axial cylinder, which are called nodal intercepts (Ranvier intercepts). The length of the sections between the intercepts is different and depends on the thickness of the nerve fiber: the thicker it is, the longer the distance between the intercepts.

Depending on the speed of excitation conduction, nerve fibers are divided into three types: A, B, C. Type A fibers have the highest excitation conduction speed, the excitation conduction speed of which reaches 120 m/s, B has a speed of 3 to 14 m/s, C - from 0.5 to 2 m/s.

There are 5 laws of excitation:

• 1. The nerve must maintain physiological and functional continuity.
• 2. Under natural conditions, the propagation of an impulse from the cell to the periphery. There is a 2-sided impulse conduction.
• 3. Conducting an impulse in isolation, i.e. myelinated fibers do not transmit impulses to neighboring nerve fibers, but only along the nerve.
• 4. The relative indefatigability of the nerve, in contrast to the muscles.
• 5. The rate of excitation depends on the presence or absence of myelin and the length of the fiber.
• 3. Classification of peripheral nerve injuries

Damage is:

• A) firearms: -direct (bullet, fragmentation)
• -mediated
• - pneumatic damage
• B) non-firearms: cut, stab, bitten, compression, compression-ischemic

Also in the literature there is a division of injuries into open (cut, stab, torn, chopped, bruised, crushed wounds) and closed (concussion, bruise, squeezing, stretching, rupture and dislocation) injuries of the peripheral nervous system.

CONDUCTION OF A NERVE IMPULSE

STRUCTURE OF NERVE FIBERS

The conduction of nerve impulses is a specialized function of nerve fibers, i.e., processes of nerve cells.

Nerve fibers are divided into pulpy, or myelinated, and fleshless, unmyelinated. Pulp, sensory and motor fibers are part of the nerves that supply the sense organs and skeletal muscles; they are also found in the autonomic nervous system. Non-fleshy fibers in vertebrates belong mainly to the sympathetic nervous system.

Nerves usually consist of both pulpy and non-pulmonic fibers, and the ratio between the number of both in different nerves is different. For example, in many cutaneous nerves, amyopiatic nerve fibers predominate. So, in the nerves of the autonomic nervous system, for example, in the vagus nerve, the number of non-fleshy fibers reaches 80-95%. On the contrary, in the nerves innervating skeletal muscles, there are only a relatively small number of amyopiatic fibers.

On fig. 42 schematically shows the structure of a myelinated nerve fiber. As you can see, it consists of an axial cylinder and a myelin sheath covering it. The surface of the axial cylinder is formed by the plasma membrane, and its content is an axoplasm penetrated by the thinnest (10-40 nm in diameter) neurofibrils (and microtubules), between which there is a large number of mitochondria and microsomes. The diameter of nerve fibers ranges from 0.5 to 25 microns.

As shown by electron microscopic studies, the myelin sheath is created as a result of the fact that the myelocyte (Schwann cell) repeatedly wraps around the axial cylinder (Fig. 43, I), its layers merge, forming a dense fatty case - the myelin sheath. The myelin sheath is interrupted at intervals of equal length, leaving open sections of the membrane with a width of approximately 1 μm. These areas are called intercepts. (interceptions of Ranvier).

The length of the interstitial areas covered with myelin sheath is approximately proportional to the diameter of the fiber. So, in nerve fibers with a diameter of 10-20 microns, the length of the gap between intercepts is 1-2 mm. In the thinnest fibers (1–2 µm in diameter), these sections are about 0.2 mm long.

Amyelinated nerve fibers do not have a myelin sheath, they are isolated from each other only by Schwann cells. In the simplest case, a single myelocyte surrounds a single, amyeloid fiber. Often, however, in the folds of the myelocyte there are several thin non-fleshy fibers (Fig. 43. II).

Rice. 43. The role of the myelocyte (Schwann cell) in the formation of the myelin sheath in the pulpy nerve fibers. The successive stages of the spiraling of the myelocyte around the axon are shown (I). Mutual arrangement of myelocytes and axons in amyeloid nerve fibers (II).

PHYSIOLOGICAL ROLE OF THE STRUCTURAL ELEMENTS OF THE MYELINATED NERVE FIBER

It can be considered proven that the surface membrane of the axial cylinder plays the main role in the processes of occurrence and conduction of a nerve impulse. The myelin sheath performs a dual function: the function of an electrical insulator and a trophic function. The insulating properties of the myelin sheath are due to the fact that myelin, as a lipid substance, prevents the passage of ions and therefore has a very high resistance. Due to the existence of the myelin sheath, the occurrence of excitation in the pulpy nerve fibers is possible not throughout the entire length of the axial cylinder, but only in limited areas - the interceptions of the node (the interception of Ranvier). This is essential for the propagation of the nerve impulse along the fiber.

The trophic function of the myelin sheath, apparently, is that it takes part in the regulation of metabolism and the growth of the axial cylinder.

Rice. 44. Hypothetical transport mechanism of the nerve fiber.

It is assumed that microtubules (MT) and neurofilaments (NF) are formed by myosin, while thin transport filaments are formed by actin. When ATP is cleaved, the transport filaments slide along the microtubules and thus transport mitochondria (M), protein molecules (B), or vesicles (P) with mediator attached to them. ATP is produced by mitochondria as a result of the breakdown of glucose penetrating the fiber. The energy of ATP is also partly used by the sodium pump of the surface membrane.

Neurofibrils, microtubules and transport filaments ensure the transport of various substances and some cell organelles along the nerve fibers from the neuron body to the nerve endings and vice versa. So, along the axon from the cell body to the periphery are transported: proteins that form ion channels and pumps;

excitatory and inhibitory mediators; mitochondria. It is estimated that approximately 1000 mitochondria move through a cross section of an axon of average diameter during the day.

It was found that neurofibrils are formed by the contractile protein actin, and microtubules - by the protein tubulin. It is assumed that microtubules, interacting with neurofibrils, perform the same role in the nerve fiber that myosin plays in the muscle fiber. The transport filaments formed by actin "slide" along the microtubules at a speed of 410 µm/day. They bind various substances (for example, protein molecules) or cell organelles (mitochondria) and carry them along the fiber (Fig. 44).

As well as the muscular contractile apparatus, the transport system of the nerve fiber uses the energy of ATP for its work and needs the presence of ions. Ca2+ in cytoplasm.

REGENERATION OF NERVE FIBERS AFTER NERVE TRANSCTION

Nerve fibers cannot exist outside of connection with the body of the nerve cell: transection of the nerve leads to the death of those fibers that have been separated from the cell body. In warm-blooded animals, already 2-3 days after nerve transection, its peripheral process loses the ability to conduct nerve impulses. Following this, the degeneration of nerve fibers begins, and the myelin sheath undergoes fatty degeneration. This is expressed in the fact that the pulpy membrane loses myelin, which accumulates in the form of drops; the disintegrated fibers and their myelin are resorbed and strands formed by the lemmocyte (Schwann cell) remain in place of the nerve fibers. All these changes were first described by the English physician Waller and named after him the Wallerian rebirth.

Nerve regeneration is very slow. Lemmocytes remaining in place of degenerated nerve fibers begin to grow near the site of transection towards the central segment of the nerve. At the same time, the cut ends of the axons of the central segment form the so-called growth flasks - thickenings that grow in the direction of the peripheral segment. Some of these branches enter the old bed of the cut nerve and continue to grow in this bed at a rate of 0.5-4.5 mm per day until they reach the corresponding peripheral tissue or organ, where the fibers form nerve endings. Since that time, the normal innervation of the organ or tissue is restored.

In various organs, the restoration of function after nerve transection occurs at different times. In muscles, the first signs of functional recovery may appear after 5-6 weeks;

the final restoration occurs much later, sometimes after a year.

LAWS OF CONDUCTION OF EXCITATION IN NERVA

When studying the conduction of excitation along the nerve, several necessary conditions and rules (laws) for the course of this process were established.

Anatomical and physiological continuity of the fiber. Conduction of impulses is possible only under the condition of the anatomical integrity of the fiber, therefore, both the cerebroscissus of nerve fibers and any injury to the surface membrane disrupt conduction. Non-conductivity is also observed when the physiological integrity of the fiber is violated (blockade of the sodium channels of the excitable membrane with tetrodotoxin or local anesthetics, sudden cooling, etc.). Conduction is also disturbed with persistent depolarization of the nerve fiber membrane by K ions, which accumulate during ischemia in the intercellular gaps. Mechanical trauma, compression of the nerve during inflammatory tissue edema may be accompanied by a partial or complete violation of the conduction function.

Bilateral holding. When a nerve fiber is irritated, excitation spreads along it in both centrifugal and centripetal directions. This is proved by the following experiment.

Two pairs of electrodes are applied to the nerve fiber, motor or sensory, connected to two electrical measuring instruments A and B (Fig. 45). Irritation is applied between these electrodes. As a result of bilateral conduction of excitation, the devices will register the passage of the pulse both under electrode A and under electrode B.

Bilateral conduction is not just a laboratory phenomenon. Under natural conditions, the action potential of a nerve cell arises in that part of it, where the body passes into its process - the axon (the so-called initial segment). From the initial segment, the action potential propagates bilaterally: in the axon towards the nerve endings and into the cell body towards its dendrites.

Isolated holding. AT In the peripheral nerve, impulses propagate along each fiber in isolation, i.e., without passing from one fiber to another and affecting only those cells with which the endings of this nerve fiber come into contact. This is very important due to the fact that any peripheral nerve trunk contains a large number of nerve fibers - motor, sensory and vegetative, which innervate different, sometimes far from each other and heterogeneous in structure and functions, cells and tissues. For example, the vagus nerve innervates all the organs of the chest cavity and a significant part of the abdominal organs, the sciatic nerve - all the muscles, bone apparatus, blood vessels and skin of the lower limb. If excitation passed inside the nerve trunk from one fiber to another, then in this case the normal functioning of peripheral organs and tissues would be impossible. Isolated conduction in individual fibers of a mixed nerve can be proved by a simple experiment on a skeletal muscle innervated by a mixed nerve, in the formation of which several spinal roots are involved. If one of these roots is irritated, not the entire muscle contracts, as would be the case in the case of a transfer of excitation from one nerve fiber to another, but only those groups of muscle fibers that are innervated by the irritated root. An even more rigorous proof of the isolated conduction of excitation can be obtained by diverting action potentials from various nerve fibers of the nerve trunk.

The isolated conduction of a nerve impulse is due to the fact that the resistance of the fluid filling the intercellular gaps is much lower than the resistance of the membrane.

Rice. 45. Schematic representation of the experiment to prove the bilateral conduction of the impulse in the nerve. Explanation in the text.

branes of nerve fibers. Therefore, the main part of the current that occurs between the excited (depolarized) and resting sections of the excitable membrane passes through the intercellular gaps without entering neighboring fibers.

CONDUCTION OF A NERVE IMPULSE

nerve impulse, the transmission of a signal in the form of a wave of excitation within one neuron and from one cell to another. P. n. and. along the nerve conductors occurs with the help of electrotonic potentials and action potentials that propagate along the fiber in both directions without passing to neighboring fibers (see Bioelectric potentials, Nerve impulse). The transmission of intercellular signals is carried out through synapses most often with the help of mediators that cause the appearance of postsynaptic potentials. Nerve conductors can be considered as cables with relatively low axial resistance (axoplasmic resistance - ri) and higher sheath resistance (membrane resistance - rm). The nerve impulse propagates along the nerve conductor through the passage of current between the resting and active parts of the nerve (local currents). In the conductor, as the distance from the site of excitation increases, a gradual, and in the case of a homogeneous conductor structure, exponential attenuation of the pulse occurs, which decreases by a factor of 2.7 at a distance l (length constant). Since rm and ri are inversely related to the diameter of the conductor, the attenuation of the nerve impulse in thin fibers occurs earlier than in thick ones. The imperfection of the cable properties of the nerve conductors is made up for by the fact that they are excitable. The main condition for excitation is the presence of a resting potential in the nerves. If a local current through the resting region causes membrane depolarization reaching a critical level (threshold), this will lead to the emergence of a propagating action potential (AP). The ratio of the level of threshold depolarization and AP amplitude, usually at least 1:5, ensures high reliability of conduction: sections of the conductor that have the ability to generate AP can be separated from each other at such a distance, overcoming which the nerve impulse reduces its amplitude by almost 5 times. This attenuated signal will be amplified again to the standard level (AP amplitude) and will be able to continue its journey down the nerve.

Speed ​​P. n. and. depends on the speed with which the membrane capacitance in the area ahead of the pulse is discharged to the level of the AP generation threshold, which, in turn, is determined by the geometric features of the nerves, changes in their diameter, and the presence of branch nodes. In particular, thin fibers have a higher ri and a greater surface capacitance, and therefore the speed of P. n. and. on them below. At the same time, the thickness of nerve fibers limits the existence of a large number of parallel communication channels. The conflict between the physical properties of the nerve conductors and the requirements for the "compactness" of the nervous system was resolved by the appearance in the course of the evolution of vertebrates of the so-called. pulpy (myelinated) fibers (see Nerves). Speed ​​P. n. and. in myelinated fibers of warm-blooded animals (despite their small diameter - 4-20 microns) reaches 100-120 m/sec. The generation of AP occurs only in limited areas of their surface - the intercepts of Ranvier, and along the inter-intercept areas P. and. and. it is carried out electrotonic (see. Saltatorny carrying out). Some medicinal substances, for example anesthetics, strongly slow down P.'s n to the full block. and. This is used in practical medicine for pain relief.

Lit. see under the articles Excitation, Synapses.

L. G. Magazanik.

Great Soviet Encyclopedia, TSB. 2012

See also interpretations, synonyms, meanings of the word and what is NERVE PULSE CONDUCTION in Russian in dictionaries, encyclopedias and reference books:

• CARRYING OUT in the Encyclopedic Dictionary of Brockhaus and Euphron:
  in a broad sense, the use of musical thought in a composition in which it constantly takes place in different voices, in its present form or ...
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• CARRYING OUT in the Full accentuated paradigm according to Zaliznyak:
  conduction, conduction, conduction, conduction, conduction, conduction, conduction, conduction, conduction, conduction, conduction, conduction, ...
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  cf. The process of action by value. verb: to conduct (1 *), ...
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  holding, pl. no, cf. Action on verb. hold in 1, 2, 4, 5, 6 and 7 digits. - spend 1 ...
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• SALTATOR CONDUCTION
  conduction (lat. saltatorius, from salto - I jump, jump), spasmodic conduction of a nerve impulse along the pulpy (myelinated) nerves, the sheath of which has relatively ...
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  The incidence of breast cancer has increased significantly over the past 10 years: the disease occurs in 1 in 9 women. The most common location...
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1. Physiology of nerves and nerve fibers. Types of nerve fibers

Physiological properties of nerve fibers:

1) excitability- the ability to come into a state of excitement in response to irritation;

2) conductivity- the ability to transmit nerve excitation in the form of an action potential from the site of irritation along the entire length;

3) refractoriness(stability) - the property of temporarily sharply reducing excitability in the process of excitation.

Nervous tissue has the shortest refractory period. The value of refractoriness is to protect the tissue from overexcitation, to carry out a response to a biologically significant stimulus;

4) lability- the ability to respond to irritation with a certain speed. Lability is characterized by the maximum number of excitation impulses for a certain period of time (1 s) in exact accordance with the rhythm of the applied stimuli.

Nerve fibers are not independent structural elements of the nervous tissue, they are a complex formation, including the following elements:

1) processes of nerve cells - axial cylinders;

2) glial cells;

3) connective tissue (basal) plate.

The main function of nerve fibers is to conduct nerve impulses. The processes of nerve cells conduct the nerve impulses themselves, and glial cells contribute to this conduction. According to the structural features and functions, nerve fibers are divided into two types: unmyelinated and myelinated.

Unmyelinated nerve fibers do not have a myelin sheath. Their diameter is 5–7 µm, the pulse conduction velocity is 1–2 m/s. Myelin fibers consist of an axial cylinder covered by a myelin sheath formed by Schwann cells. The axial cylinder has a membrane and oxoplasm. The myelin sheath consists of 80% lipids with high ohmic resistance and 20% protein. The myelin sheath does not completely cover the axial cylinder, but is interrupted and leaves open areas of the axial cylinder, which are called nodal intercepts (Ranvier intercepts). The length of the sections between the intercepts is different and depends on the thickness of the nerve fiber: the thicker it is, the longer the distance between the intercepts. With a diameter of 12–20 µm, the excitation velocity is 70–120 m/s.

Depending on the speed of conduction of excitation, nerve fibers are divided into three types: A, B, C.

Type A fibers have the highest excitation conduction speed, the excitation conduction speed of which reaches 120 m / s, B has a speed of 3 to 14 m / s, C - from 0.5 to 2 m / s.

The concepts of "nerve fiber" and "nerve" should not be confused. Nerve- a complex formation consisting of a nerve fiber (myelinated or unmyelinated), loose fibrous connective tissue that forms the nerve sheath.

2. Mechanisms of conducting excitation along the nerve fiber. Laws of conduction of excitation along the nerve fiber

The mechanism of conduction of excitation along the nerve fibers depends on their type. There are two types of nerve fibers: myelinated and unmyelinated.

Metabolic processes in unmyelinated fibers do not provide a quick compensation for energy expenditure. The spread of excitation will go with a gradual attenuation - with a decrement. The decremental behavior of excitation is characteristic of a low-organized nervous system. The excitation is propagated by small circular currents that occur inside the fiber or in the liquid surrounding it. A potential difference arises between the excited and unexcited areas, which contributes to the occurrence of circular currents. The current will spread from the "+" charge to "-". At the exit point of the circular current, the permeability of the plasma membrane for Na ions increases, resulting in membrane depolarization. Between the newly excited area and the adjacent unexcited potential difference again arises, which leads to the occurrence of circular currents. The excitation gradually covers the neighboring sections of the axial cylinder and thus spreads to the end of the axon.

In myelin fibers, thanks to the perfection of metabolism, excitation passes without fading, without decrement. Due to the large radius of the nerve fiber, due to the myelin sheath, the electric current can enter and leave the fiber only in the area of ​​interception. When irritation is applied, depolarization occurs in the area of ​​​​intercept A, the adjacent intercept B is polarized at this time. Between the interceptions, a potential difference arises, and circular currents appear. Due to the circular currents, other interceptions are excited, while the excitation spreads in a saltatory way, abruptly from one interception to another. The saltatory method of excitation propagation is economical, and the speed of excitation propagation is much higher (70–120 m/s) than along unmyelinated nerve fibers (0.5–2 m/s).

There are three laws of conduction of irritation along the nerve fiber.

The law of anatomical and physiological integrity.

Conduction of impulses along the nerve fiber is possible only if its integrity is not violated. If the physiological properties of the nerve fiber are violated by cooling, the use of various drugs, squeezing, as well as cuts and damage to the anatomical integrity, it will be impossible to conduct a nerve impulse through it.

The law of isolated conduction of excitation.

There are a number of features of the spread of excitation in the peripheral, pulpy and non-pulmonic nerve fibers.

In peripheral nerve fibers, excitation is transmitted only along the nerve fiber, but is not transmitted to neighboring nerve fibers that are in the same nerve trunk.

In the pulpy nerve fibers, the role of an insulator is performed by the myelin sheath. Due to myelin, the resistivity increases and the electrical capacitance of the shell decreases.

In the non-fleshy nerve fibers, excitation is transmitted in isolation. This is due to the fact that the resistance of the fluid that fills the intercellular gaps is much lower than the resistance of the nerve fiber membrane. Therefore, the current that occurs between the depolarized area and the non-polarized one passes through the intercellular gaps and does not enter the adjacent nerve fibers.

The law of bilateral excitation.

The nerve fiber conducts nerve impulses in two directions - centripetally and centrifugally.

In a living organism, excitation is carried out in only one direction. The two-way conduction of a nerve fiber is limited in the body by the place of origin of the impulse and by the valvular property of the synapses, which consists in the possibility of conducting excitation in only one direction.

The essence of the concept of "Excitement"

The emergence and conduction of nervous excitation

Excitation is the response of a tissue to irritation, which manifests itself in addition to non-specific reactions (generation of an action potential, metabolic changes) in the performance of a function specific to this tissue; excitable are nervous (conduction of excitation), muscle (contraction) and glandular (secretion) tissues.

Excitability is the property of cells to respond to irritation with excitation.

When excited, a living system passes from a state of relative physiological rest to a state of physiological activity. Excitation is based on complex physical and chemical processes. A measure of excitation is the strength of the stimulus that causes excitation.

Excitable tissues are highly sensitive to the action of a weak electric current (electrical excitability), which was first demonstrated by L. Galvani.

action potential.

An action potential is a wave of excitation that moves along the membrane of a living cell in the process of transmitting a nerve signal. In essence, it represents an electrical discharge - a quick short-term change in potential in a small section of the membrane of an excitable cell (neuron, muscle fiber or glandular cell), as a result of which the outer surface of this section becomes negatively charged with respect to neighboring sections of the membrane, while its inner surface becomes positively charged with respect to neighboring regions of the membrane. The action potential is the physical basis of a nerve or muscle impulse that plays a signal (regulatory) role. Action potentials can differ in their parameters depending on the type of cell and even on different parts of the membrane of the same cell. The most characteristic example of differences is the action potential of the heart muscle and the action potential of most neurons. However, the following phenomena underlie any action potential:

1. The membrane of a living cell is polarized - its inner surface is negatively charged with respect to the outer one due to the fact that in the solution near its outer surface there are a larger number of positively charged particles (cations), and near the inner surface - a larger number of negatively charged particles (anions). ).

2. The membrane has selective permeability - its permeability for various particles (atoms or molecules) depends on their size, electric charge and chemical properties.

3. The membrane of an excitable cell is able to quickly change its permeability for a certain type of cations, causing the transition of a positive charge from the outside to the inside (Fig. 1).

The first two properties are characteristic of all living cells. The third is a feature of the cells of excitable tissues and the reason why their membranes are able to generate and conduct action potentials.

Action potential phases:

Prespike is the process of slow depolarization of the membrane to a critical level of depolarization (local excitation, local response).

Peak potential, or spike, consisting of an ascending part (membrane depolarization) and a descending part (membrane repolarization).

Negative trace potential - from the critical level of depolarization to the initial level of membrane polarization (trace depolarization).

Positive trace potential - an increase in the membrane potential and its gradual return to its original value (trace hyperpolarization).

General provisions.

The polarization of the membrane of a living cell is due to the difference in the ionic composition of its inner and outer sides. When the cell is in a calm (unexcited) state, ions on opposite sides of the membrane create a relatively stable potential difference, called the resting potential. If you introduce an electrode inside a living cell and measure the resting membrane potential, it will have a negative value (of the order of? 70 -? 90 mV). This is explained by the fact that the total charge on the inner side of the membrane is significantly less than on the outer one, although both sides contain both cations and anions. Outside - an order of magnitude more sodium, calcium and chlorine ions, inside - potassium ions and negatively charged protein molecules, amino acids, organic acids, phosphates, sulfates.

It must be understood that we are talking about the charge of the membrane surface as a whole, the environment both inside and outside the cell is neutrally charged. The membrane potential can change under the influence of various stimuli. An artificial stimulus can be an electric current applied to the outer or inner side of the membrane through the electrode.

Under natural conditions, the stimulus is often a chemical signal from neighboring cells, coming through the synapse or by diffuse transmission through the intercellular medium. The shift of the membrane potential can occur in a negative (hyperpolarization) or positive (depolarization) direction. In nervous tissue, an action potential, as a rule, occurs during depolarization - if the depolarization of the neuron membrane reaches or exceeds a certain threshold level, the cell is excited, and a wave of electrical signal propagates from its body to the axons and dendrites. (In real conditions, postsynaptic potentials usually arise on the body of a neuron, which are very different from the action potential in nature - for example, they do not obey the “all or nothing” principle. These potentials are converted into an action potential at a special section of the membrane - the axon hillock, so the action potential does not propagate to the dendrites).

Most of the channels are ion-specific - the sodium channel passes practically only sodium ions and does not pass others (this phenomenon is called selectivity). The cell membrane of excitable tissues (nerve and muscle) contains a large number of voltage-gated ion channels that can quickly respond to a shift in the membrane potential. Membrane depolarization primarily causes voltage-gated sodium channels to open. When enough sodium channels open at the same time, positively charged sodium ions rush through them to the inside of the membrane. The driving force in this case is provided by a concentration gradient (there are many more positively charged sodium ions on the outside of the membrane than inside the cell) and a negative charge on the inside of the membrane. The flow of sodium ions causes an even larger and very rapid change in the membrane potential, which is called the action potential ( in the specialized literature is designated PD).

According to the “all-or-nothing” law, the cell membrane of an excitable tissue either does not respond to the stimulus at all, or responds with the maximum possible force for it at the moment. That is, if the stimulus is too weak and the threshold is not reached, the action potential does not arise at all; at the same time, a threshold stimulus will elicit an action potential of the same amplitude as a stimulus above the threshold. This does not mean that the amplitude of the action potential is always the same - the same section of the membrane, being in different states, can generate action potentials of different amplitudes.

After excitation, the neuron for some time finds itself in a state of absolute refractoriness, when no signals can excite it again, then it enters the phase of relative refractoriness, when exceptionally strong signals can excite it (in this case, the AP amplitude will be lower than usual). The refractory period occurs due to the inactivation of the fast sodium current, i.e. the inactivation of the sodium channels (see below).

Action potential propagation

Action potential propagation along unmyelinated fibers.

AP propagates continuously along the unmyelinated fiber. Conduction of a nerve impulse begins with the propagation of an electric field. The resulting AP due to the electric field is able to depolarize the membrane of the neighboring area to a critical level, as a result of which new APs are generated in the neighboring area. PD themselves do not move, they disappear in the same place where they arise. The main role in the emergence of a new PD is played by the previous one. If an axon in the middle is irritated with an intracellular electrode, then AP will propagate in both directions. Usually, AP propagates along the axon in one direction (from the body of the neuron to the nerve endings), although membrane depolarization occurs on both sides of the site where the AP occurred at the moment. Unilateral conduction of AP is provided by the properties of sodium channels - after opening, they are inactivated for some time and cannot open at any values ​​of the membrane potential (refractoriness property). Therefore, in the area closest to the cell body, where AP has already “passed through”, it does not occur. Ceteris paribus, the propagation of AP along the axon occurs the faster, the larger the fiber diameter. Along the giant axons of the squid, AP can propagate at almost the same speed as along the myelinated fibers of vertebrates (about 100 m/s).

Propagation of the action potential along myelinated fibers.

PD spreads spasmodically along the myelinated fiber (saltatory conduction). Myelinated fibers are characterized by a concentration of voltage-gated ion channels only in the areas of Ranvier intercepts; here their density is 100 times greater than in the membranes of unmyelinated fibers. There are almost no voltage-gated channels in the area of ​​myelin couplings. The AP that has arisen in one node of Ranvier, due to the electric field, depolarizes the membrane of neighboring nodes to a critical level, which leads to the emergence of new AP in them, that is, excitation passes abruptly, from one node to another. In the event of damage to one node of Ranvier, the PD excites the 2nd, 3rd, 4th, and even 5th, since the electrical insulation created by the myelin sleeves reduces the dissipation of the electric field. This increases the rate of propagation of AP along myelinated fibers compared to unmyelinated ones. In addition, myelinated fibers are thicker, and the electrical resistance of thicker fibers is less, which also increases the speed of impulse conduction along myelinated fibers. Another advantage of saltatory conduction is its energy efficiency, since only nodes of Ranvier are excited, the area of ​​which is less than 1% of the membrane, and, therefore, much less energy is needed to restore the transmembrane gradients of Na + and K +, which are consumed as a result of the occurrence of AP, which may have value at a high frequency of discharges going along the nerve fiber. To imagine how effectively the speed of conduction can be increased due to the myelin sheath, it is enough to compare the speed of impulse propagation through unmyelinated and myelinated parts of the human nervous system. With a fiber diameter of about 2 µm and the absence of a myelin sheath, the conduction velocity will be ~1 m/s, and in the presence of even weak myelination with the same fiber diameter, it will be 15–20 m/s. In larger diameter fibers with a thick myelin sheath, the conduction velocity can reach 120 m/s. The rate of propagation of the action potential along the membrane of a single nerve fiber is by no means a constant value - depending on various conditions, this rate can decrease very significantly and, accordingly, increase, returning to a certain initial level.

active properties of the membrane.

The active properties of the membrane, providing the occurrence of an action potential, are based mainly on the behavior of voltage-gated sodium (Na+) and potassium (K+) channels. The initial phase of AP is formed by the incoming sodium current, later potassium channels open and the outgoing K+ current returns the membrane potential to the initial level. The initial concentration of ions is then restored by the sodium-potassium pump. In the course of PD, the channels pass from state to state: Na+ channels have three basic states - closed, open, and inactivated (in reality, the matter is more complicated, but these three are enough for a description), K+ channels have two - closed and open. The behavior of the channels involved in the formation of TP is described in terms of conductance and calculated in terms of transfer (transfer) coefficients. The transfer coefficients were derived by Hodgkin and Huxley.

Resting potential and the mechanism of its formation.

Ion-membrane theory of rest potential and action potential.

Membrane potential / resting potential - the potential difference between the outer and inner sides of this membrane (comparison of the content of potassium and sodium in the internal and external environment of the cell).

In this case, the outer membrane carries a positive charge with respect to its inner side.

Transmembrane distribution of ions.

The concentrations of the main monovalent ions - chlorine, potassium and sodium - inside the cell differ significantly from their content in the extracellular fluid surrounding the cell.

The main intracellular cation (positively charged ion) is potassium;

Intracellular anions (negatively charged ions) are represented mainly by residues of amino acids and other organic molecules.

The main extracellular cation is sodium;

The extracellular anion is chlorine.

This distribution of ions is created as a result of two factors:

1. The presence of negatively charged organic molecules inside the cell.

2. The existence of active transport systems in the cell membrane that “pump” sodium out of the cell, and potassium into the cell.

If such small ions as potassium, sodium and chlorine easily pass through the cell membrane, then organic anions, for example, amino acids and organic acids of the cytoplasm, are too large and cannot pass through the membrane. In this regard, a significant excess of negative charges (organic anions) accumulates in the cell. These charges prevent the penetration of negative ions (chlorine) into the cell, but attract positively charged cations (sodium, potassium) into it; however, most of the sodium entering the cell is immediately removed by the sodium-potassium pump.

The rapid removal of sodium leads to the fact that only potassium accumulates in the cell, which is attracted by the negative charges of organic anions and pumped by the sodium-potassium pump.

Selective permeability of cell membranes.

The membranes have ion channels. Ion (selective) channels allow certain ions to pass through. Depending on the situation, certain channels are open.

At rest, potassium is open, and sodium is almost all closed.

Nerve cells always have pumping mechanisms that carry ions against a concentration gradient.

Concentration gradient - the difference between the concentration from the smallest to the largest.

Measurement of cellular potentials.

There is a potential difference between the outer and inner surfaces of all cells.

The resting potential varies from -40 mV to -95 mV depending on the characteristics of a particular cell.

The resting potential of nerve cells is usually between -30 mV and -70 mV.

1. The membrane potential is quickly determined by measuring the potential difference between two identical electrodes, one of which is inserted into the cell, the other is placed in the fluid surrounding it. The electrodes are connected to an amplifier that increases the amplitude of the recorded potential; this amplitude is determined using an oscilloscope-type voltage meter.

2. The existence of an electric charge on the surface membrane in physiology has been known for a very long time, but it was only discovered in a different way - in the form of the so-called resting current.

The quiescent current occurs in any living structure between its damaged area and an undamaged surface.

If a nerve or muscle is cut, and one electrode is applied to the transverse cut, and the other to the surface, connecting them to a galvanometer, the galvanometer will show a current that always flows from a normal, undamaged surface to the transverse cut.

The quiescent current and the membrane potential are manifestations of the same property of the membrane; the reason for the appearance of the quiescent current is that when the cell is damaged, it actually becomes possible to connect one electrode to the inner side of the membrane, and the other to its outer surface.

Under ideal conditions, in case of damage, a potential difference = membrane potential should be recorded. This, as a rule, does not happen, because part of the current does not go through the galvanometer, but is shunted through the intercellular spaces, the surrounding fluid, etc.

The magnitude of the transmembrane potential difference that can be created by such a process is predicted by the Nernst equation:

Em = ((R*T)/F)*ln([K]ext/[K]ext)

Em \u003d -59 * ln ([K] ext / [K] ext)

R is the gas constant.

T is the absolute temperature.

F is the Faraday number.

[K]ext:[K]nar - the ratio of potassium concentration inside and outside the cell.

The concentration of potassium outside - in the intercellular fluid - is approximately that in the blood. The intracellular concentration can be approximately determined using some analytical techniques or measurements using potassium selective electrodes.

In the experiment, slightly smaller values ​​\u200b\u200bare obtained (-60, -70 mV) than theoretical ones (-80 mV), since the membrane is not a perfect ion discriminator.

Sodium ions in a small amount penetrate into the cell and charge the inner surface of the membrane positively, creating a counter potential difference. Although this difference is small, it can reduce the true value of the membrane potential.

Conditions for the formation of PP.

The resting potential is the charge on the membrane at rest.

One of the main properties of a nerve cell is the presence of a constant electrical polarization of its membrane - the membrane potential. The membrane potential is maintained on the membrane as long as the cell is alive, and disappears only with its death.

Cause of the membrane potential:

1. The resting potential arises primarily in connection with the asymmetric distribution of potassium (ionic asymmetry) on both sides of the membrane. Since its concentration in the cell is about 30 times higher than in the extracellular environment, there is a transmembrane concentration gradient that promotes the diffusion of potassium from the cell.

The release of each positive potassium ion from the cell leads to the fact that an unbalanced negative charge (organic anions) remains in it. These charges cause the negative potential inside the cell.

2. Ionic asymmetry is a violation of thermodynamic equilibrium, and potassium ions should gradually leave the cell, and sodium ions should enter it. To maintain such a violation, energy is needed, the expenditure of which would counteract the thermal equalization of the concentration.

Because ionic asymmetry is associated with the living state and disappears with death, this means that this energy is supplied by the life process itself, i.e. metabolism. A significant part of the metabolic energy is spent on maintaining the uneven distribution of ions between the cytoplasm and the environment.

Active ion transport / ion pump - a mechanism that can transport ions from the cell or into the cell against concentration gradients (located in the surface membrane of the cell and is a complex of enzymes that use the energy released during ATP hydrolysis to transfer).

The asymmetry of chloride ions can also be maintained by the active transport process.

The uneven distribution of ions leads to the appearance of concentration gradients between the cytoplasm of the cell and the external environment: the potassium gradient is directed from the inside to the outside, and sodium and chloride - from the outside to the inside.

The membrane is not completely impermeable and is capable of passing ions through it to a certain extent. This ability is not the same for different ions in the resting state of the cell - it is much higher for potassium ions than for sodium ions. Therefore, the main ion, which at rest can diffuse to a certain extent through the cell membrane, is the potassium ion. In such a situation, the presence of a potassium gradient will lead to a small but noticeable flow of potassium ions from the cell to the outside. At rest, a constant electrical polarization of the cell membrane is created mainly due to the diffusion current of potassium ions through the cell membrane.

The value of the resting potential.

1. The use of microelectrode technology made it possible to determine the basic properties of nerve cells in all parts of the brain, to elucidate the nature of the active processes arising in them, and to establish the patterns of synaptic connections that unite these cells.

2. The presence of ionic gradients and constant electrical polarization of the membrane is the main condition that ensures cell excitability. The electrochemical gradient created by these two factors is a store of potential energy, which is always at the disposal of the cell and which can be immediately used to create active cellular reactions.