Functions and types of nerve fibers. Types of nerve fibers
Nerve fiber structure. Conduction of nerve impulses is a specialized function nerve fibers, i.e. processes nerve cells.
Nerve fibers separate soft, or myelinated, And pulpless, or unmyelinated. Pulp, sensory and motor fibers are part of the nerves supplying the sensory organs and skeletal muscles; they are also present in the autonomic nervous system. Non-pulp fibers in vertebrates belong mainly to the sympathetic nervous system.
Nerves usually consist of both pulpy and non-pulphate fibers, and their ratio in different nerves is different. For example, in many cutaneous nerves the predominant nerve fibers predominate. So, in the autonomic nerves nervous system, for example, in the vagus nerve, the number of pulpless fibers reaches 80-95%. In contrast, the nerves innervating skeletal muscles contain only a relatively small number of non-pulp fibers.
As electron microscopic studies have shown, the myelin sheath is created as a result of the fact that the myelocyte (Schwann cell) repeatedly wraps the axial cylinder (Fig. 2.27"), its layers merge, forming a dense fatty sheath - the myelin sheath. The myelin sheath through gaps equal length is interrupted, leaving open areas of the membrane approximately 1 µm wide. These areas were named Ranvier interceptions.
Rice. 2.27. The role of the myelocyte (Schwann cell) in the formation of the myelin sheath in the pulpy nerve fibers: successive stages of the spiral-shaped twisting of the myelocyte around the axon (I); relative position myelocytes and axons in non-pulp nerve fibers (II)
The length of the interstitial areas covered by the myelin sheath is approximately proportional to the diameter of the fiber. Thus, in nerve fibers with a diameter of 10-20 microns, the length of the gap between the interceptions is 1-2 mm. In the thinnest fibers (diameter
1-2 µm) these areas are about 0.2 mm long.
Non-pulp 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 one pulpless fiber. Often, however, several thin, pulpless fibers appear in the folds of the myelocyte.
The myelin sheath has a dual function: an electrical insulator function and a trophic function. The insulating properties of the myelin sheath are due to the fact that myelin, as a substance of lipid nature, 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 pulpal nerve fibers is not possible throughout the entire length of the axial cylinder, but only in limited areas - the nodes of Ranvier. This is important for distribution nerve impulse along the fiber.
The trophic function of the myelin sheath, apparently, is that it takes part in the processes of regulation of metabolism and growth of the axial cylinder.
Conduction of excitation in unmyelinated and myelinated nerve fibers. In soft nerve fibers, excitation spreads continuously along the entire membrane, from one excited area to another located nearby. In contrast, in myelinated fibers the action potential can propagate only spasmodically, “jumping” through sections of the fiber covered with an insulating myelin sheath. This is called salipatory.
Direct electrophysiological studies carried out by Kato (1924) and then by Tasaki (1953) on single myelinated frog nerve fibers showed that action potentials in these fibers arise only at the nodes, and the myelin-covered areas between the nodes are practically inexcitable.
The density of sodium channels in the interceptions is very high: there are about 10,000 sodium channels per 1 µm2 membrane, which is 200 times higher than their density in the membrane of the giant squid axon. High density sodium channels are the most important condition saltatory conduction of excitation. In Fig. Figure 2.28 shows how a nerve impulse “jumps” from one interception to another.
At rest, the outer surface of the excitable membrane of all nodes of Ranvier is positively charged. There is no potential difference between adjacent interceptions. At the moment of excitation, the surface of the interception membrane WITH becomes charged electronegatively with respect to the membrane surface of the adjacent interception D. This leads to the emergence of local (lo 
Rice. 2.28.
A- unmyelinated fiber; IN- myelinated fiber. The arrows show the direction of the current
cal) electric current that passes through the interstitial fluid surrounding the fiber, membrane and axoplasm in the direction shown in the figure by the arrow. Coming out through interception D the current excites it, causing the membrane to recharge. In interception WITH the excitement still continues, and he becomes refractory for a while. Therefore interception D is capable of bringing into a state of excitation only the next interception, etc.
“Jumping” of the action potential across the interinterceptor region is possible only because the amplitude of the action potential in each interception is 5-6 times higher than the threshold value required to excite the neighboring interception. Under certain conditions, the action potential can “jump” not only through one, but also through two interinterceptor sections - in particular, if the excitability of the adjacent interception is reduced by some pharmacological agent, for example, novocaine, cocaine, etc.
The assumption about the spasmodic propagation of excitation in nerve fibers was first expressed by B.F. Verigo (1899). This method of conduction has a number of advantages compared to continuous conduction in non-pulp fibers: firstly, by “jumping” over relatively large sections of the fiber, excitation can spread at a much higher speed than with continuous conduction along a non-pulp fiber of the same diameter; secondly, discontinuous propagation is energetically more economical, since not the entire membrane comes into a state of activity, but only its small sections in the interception region, having a width of less than 1 μm. The losses of ions (per unit length of fiber) accompanying the occurrence of an action potential in such limited areas of the membrane are very small, and therefore the energy costs for work are also low. sodium potassium pump, necessary to restore altered ionic ratios between the internal contents of the nerve fiber and tissue fluid.
- See: Human Physiology / Ed. A. Kositsky.
1. Physiology of nerves and nerve fibers. Types of nerve fibers
Physiological properties of nerve fibers:
1) excitability– the ability to become excited in response to stimulation;
2) conductivity– the ability to transmit nerve excitation in the form of an action potential from the site of irritation along its entire length;
3) refractoriness(stability) – the property of temporarily sharply reducing excitability during the process of excitation.
Nervous tissue has the shortest refractory period. The meaning of refractoriness is to protect the tissue from overexcitation and respond to a biologically significant stimulus;
4) lability– the ability to respond to stimulation at a certain speed. Lability is characterized by the maximum number of excitation impulses per certain period time (1 s) in exact accordance with the rhythm of applied stimulation.
Nerve fibers are not independent structural elements nerve tissue, they represent comprehensive education, 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 facilitate this conduction. Based on their 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 speed of impulse transmission is 1–2 m/s. Myelin fibers consist of an axial cylinder covered with 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 nodes of Ranvier. The length of the sections between the interceptions is different and depends on the thickness of the nerve fiber: the thicker it is, the longer the distance between the interceptions. With a diameter of 12–20 microns, the excitation speed is 70–120 m/s.
Depending on the speed of excitation, nerve fibers are divided into three types: A, B, C.
Type A fibers have the highest excitation speed, the excitation speed of which reaches 120 m/s, B has a speed from 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 for conducting excitation along the nerve fiber. Laws for the conduction of excitation along nerve fibers
The mechanism for conducting excitation along nerve fibers depends on their type. There are two types of nerve fibers: myelinated and unmyelinated.
Metabolic processes in unmyelinated fibers do not provide rapid compensation for energy expenditure. The spread of excitation will occur with gradual attenuation - with decrement. Decremental arousal behavior is characteristic of a low-organized nervous system. Excitation propagates due to small circular currents that arise into the fiber or into the surrounding liquid. A potential difference arises between excited and unexcited areas, which contributes to the emergence of circular currents. The current will spread from the “+” charge to the “-”. At the point where the circular current exits, permeability increases plasma membrane for Na ions, resulting in depolarization of the membrane. A potential difference again arises between the newly excited area and the neighboring unexcited one, which leads to the emergence of circular currents. The excitation gradually covers neighboring areas of the axial cylinder and thus spreads to the end of the axon.
In myelin fibers, due 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, electric current can enter and exit the fiber only in the interception area. When stimulation is applied, depolarization occurs in the area of interception A, the neighboring interception B is polarized at this time. A potential difference arises between interceptions, and circular currents. Due to circular currents, other interceptions are excited, while the excitation spreads saltatory, jumpwise from one interception to another. The saltatory method of propagation of excitation is economical, and the speed of propagation of excitation is much higher (70-120 m/s) than along unmyelinated nerve fibers (0.5–2 m/s).
There are three laws for the conduction of stimulation along a nerve fiber.
Law of anatomical and physiological integrity.
Conduction of impulses along a nerve fiber is possible only if its integrity is not compromised. In case of violation physiological properties nerve fiber through cooling, the use of various narcotic drugs, compression, as well as cuts and damage to the anatomical integrity, it will be impossible to conduct a nerve impulse through it.
Law of isolated conduction of excitation.
There are a number of features of the spread of excitation in peripheral, pulpal and non-pulpate nerve fibers.
In peripheral nerve fibers, excitation is transmitted only along the nerve fiber, but is not transmitted to neighboring ones, which are located in the same nerve trunk.
In the pulpy nerve fibers, the myelin sheath plays the role of an insulator. Due to myelin it increases resistivity and the electrical capacitance of the shell decreases.
In the soft nerve fibers, excitation is transmitted in isolation. This is explained by the fact that the resistance of the fluid that fills the intercellular gaps is significantly lower than the resistance of the nerve fiber membrane. Therefore, the current that arises between the depolarized area and the unpolarized one passes through the intercellular gaps and does not enter neighboring nerve fibers.
The law of two-way conduction of excitation.
The nerve fiber conducts nerve impulses in two directions - centripetal and centrifugal.
In a living organism, excitation is carried out only in one direction. Bilateral conductivity of the nerve fiber is limited in the body by the place where the impulse originates and the valve property of synapses, which consists in the possibility of excitation in only one direction.
Electrical phenomena in living tissues are associated with differences in the concentrations of ions carrying electric charges.
According to generally accepted membrane theory of the origin of biopotentials, the potential difference in a living cell arises because ions carrying electrical charges are distributed on both sides of the semipermeable cell membrane depending on its selective permeability to different ions. Active transport of ions against a concentration gradient is carried out using so-called ion pumps, which are a system of transport enzymes. The energy of ATP is used for this.
As a result of the operation of ion pumps, the concentration of K + ions inside the cell is 40-50 times greater, and Na + ions - 9 times less than in the intercellular fluid. Ions come to the surface of the cell, anions remain inside it, imparting a negative charge to the membrane. This creates resting potential, in which the membrane inside the cell is charged negatively with respect to the extracellular environment (its charge is conventionally taken as zero). In different cells, the membrane potential varies from -50 to -90 mV.
Action potential occurs as a result of short-term fluctuations membrane potential. It includes two phases:
- Depolarization phase corresponds to a rapid change in membrane potential of approximately 110 mV. This is explained by the fact that at the site of excitation the permeability of the membrane for Na + ions sharply increases, as sodium channels open. The flow of Na + ions rushes into the cell, creating a potential difference with a positive charge on the inner and negative on the outer surface of the membrane. The membrane potential at the moment of reaching the peak is +40 mV. During the repolarization phase, the membrane potential again reaches its resting level (the membrane is repolarized), after which hyperpolarization occurs to a value of approximately -80 mV.
- Repolarization phase potential is associated with the closing of sodium and opening of potassium channels. Since positive charges are removed as K+ falls out, the membrane is repolarized. Hyperpolarization of the membrane to a level greater (more negative) than the resting potential is due to high potassium permeability during the repolarization phase. Closure of potassium channels leads to restoration of the original level of membrane potential; the permeability values for K + and Na + also return to their previous values.
Conduction of nerve impulses
The potential difference that arises between the excited (depolarized) and resting (normally polarized) sections of the fiber spreads along its entire length. In unmyelinated nerve fibers, excitation is transmitted at speeds of up to 3 m/s. Along axons covered with a myelin sheath, the speed of excitation reaches 30-120 m/s. This high speed is explained by the fact that the depolarizing current does not flow through the areas covered by the insulating myelin sheath (the areas between the nodes). The action potential here spreads spasmodically.
The speed of action potential along an axon is proportional to its diameter. In mixed nerve fibers it varies from 120 m/s (thick, up to 20 µm in diameter, myelinated fibers) up to 0.5 m/s (the thinnest, with a diameter of 0.1 microns, non-pulp fibers).
The conduction of a nerve impulse along the fiber occurs due to the propagation of a depolarization wave along the sheath of the process. Majority peripheral nerves through their motor and sensory fibers they ensure impulse conduction at speeds of up to 50-60 m/sec. The depolarization process itself is quite passive, while the restoration of the resting membrane potential and conductivity is carried out through the functioning of NA/K and Ca pumps. For their work, ATP is required, a prerequisite for the formation of which is the presence of segmental blood flow. Cutting off the blood supply to the nerve immediately blocks the conduction of the nerve impulse.
Based on their 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 transmission is 1-2 m/s. Myelin fibers consist of an axial cylinder covered with a myelin sheath formed by Schwann cells. The axial cylinder has a membrane and oxoplasm. The myelin sheath is composed 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 nodes of Ranvier. The length of the sections between the interceptions is different and depends on the thickness of the nerve fiber: the thicker it is, the longer the distance between the interceptions.
Depending on the speed of excitation, nerve fibers are divided into three types: A, B, C. The highest speed of excitation is possessed by fibers of type A, the 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 2-way conduction of the impulse.
- 3. Conducting the impulse in isolation, i.e. fibers covered with myelin do not transmit excitation to neighboring nerve fibers, but only along the nerve.
- 4. The relative tirelessness of the nerve, unlike the muscles.
- 5. The speed of excitation depends on the presence or absence of myelin and the length of the fiber.
- 3. Classification of peripheral nerve injuries
Damage occurs:
- A) firearms: - direct (bullet, fragmentation)
- -mediated
- -pneumatic injuries
- B) non-gunshot: cut, stab, bitten, compression, compression-ischemic
Also in the literature there is a division of injuries into open (cut, punctured, torn, chopped, bruised, crushed wounds) and closed (concussion, contusion, compression, sprain, rupture and dislocation) injuries of the peripheral nervous system.
319. Conduction of nerve impulses
A nerve impulse is an excitation wave propagating along a nerve fiber that occurs when a neuron is stimulated and carries a signal about a change in the environment (centripetal impulse) or a command signal in response to a change that has occurred (centrifugal impulse).
Resting potential. The occurrence and conduction of an impulse is associated with a change in the state of some structural elements of the neuron. These structures include a sodium pump, including Na^1^-ATPase, and two types of ion-conducting channels - sodium and potassium. Their interaction gives, in a state of rest, a potential difference across different sides plasma membrane of axons (resting potential). The existence of a potential difference is associated" 1) with the high concentration of potassium ions in the cell (20-50 times higher than in the environment); 2) with the fact that intracellular anions (proteins and nucleic acids) cannot leave the cell; 3) with the fact that the permeability of the membrane for sodium ions is 20 times lower than for potassium ions. The potential exists ultimately because potassium ions tend to leave the cell to equalize external and internal concentrations. But potassium ions cannot leave the cell, and this leads to the appearance of a negative charge, which inhibits further equalization of the concentrations of potassium ions. Chlorine ions must remain outside to compensate for the charge of poorly penetrating sodium, but tend to leave the cell along the concentration gradient.
To maintain the membrane potential (about 75 mV), it is necessary to maintain a difference in the concentrations of sodium and potassium ions so that sodium ions entering the cell are removed back from it in exchange for potassium ions. "This is achieved due to the action of membrane Na +, g^-ATPase, which, using the energy of ATP, transfers sodium ions from the cell in exchange for two potassium ions taken into the cell. When abnormal high concentration sodium ions in external environment the pump increases the Na + /K + ratio. Thus, at rest, potassium ions move outward along the gradient. At the same time, a certain amount of potassium is returned by diffusion. The difference between these processes is compensated by the action of the K" 1 ", N8"" pump. Sodium ions enter along a gradient at a rate limited by the permeability of the membrane for them. At the same time, sodium ions are pumped out by a pump against the concentration gradient due to the energy of ATP.
Action potential - sequence of processes caused in a nerve by a stimulus. Irritation of the nerve entails local depolarization of the membrane, a decrease in membrane potential. This occurs due to the entry of a certain amount of sodium ions into the cell. When the potential difference drops to threshold level(about 50 mV), the permeability of the membrane to sodium increases approximately 100 times. Sodium rushes along a gradient into the cell, extinguishing the negative charge on inner surface membranes. The potential value can change from -75 at rest to +50. Not only will the negative charge on the inner surface of the membrane be extinguished, but a positive charge will appear (polarity inversion). This charge prevents further sodium from entering the cell, and the conductivity for sodium drops. The pump restores the original state. The immediate cause of these transformations is discussed below.
The duration of the action potential is less than 1 ms and covers (unlike the resting potential) only a small area of the axon. In myelinated fibers, this is the area between adjacent nodes of Ranve. If the resting potential has changed to a degree that does not reach the threshold, then the action potential does not arise, but if the threshold value is reached, then in each case the same action potential develops (again, “all or nothing”).
The movement of potential in unmyelinated axons occurs as follows. The diffusion of ions from an area with inverted polarity to neighboring ones causes the development of an action potential in them. In this regard, having arisen in one place, the potential spreads along the entire length of the axon.
The movement of an action potential is a nerve impulse, or a propagating wave of excitation, or conduction.
Changes in the concentration of calcium ions inside the axons may be associated with the movement of the action potential and its conduction. All intracellular calcium, except for a small fraction, is bound to protein (the concentration of free calcium is about 0.3 mM), while around the cell its concentration reaches 2 mM. Therefore, there is a gradient that tends to push calcium ions into the cell. The nature of the calcium extrusion pump is unclear. It is known, however, that each calcium ion is exchanged for 3 sodium ions, which penetrate the cell at the moment the action potential increases.
Sodium channel structure has not been sufficiently studied, although a number of facts are known: 1) an essential structural element of the channel is an integral membrane protein; 2) for every square micrometer of the Ranvier interception surface there are about 500 channels; 3) during the rising phase of the action potential, approximately 50,000 sodium ions pass through the channel; 4) rapid removal of ions is possible due to the fact that for each channel in the membrane there are from 5 to 10 Na+,\K^-ATPase molecules.
Each ATPase molecule must push 5-10 thousand sodium ions out of the cell so that the next excitation cycle can begin.
Comparison of the speed of passage of molecules of different sizes made it possible to establish the diameter of the channels - approximately 0.5 nm. The diameter can increase by 0.1 nm. The rate of passage of sodium ions through the channel under real conditions is 500 times higher than the rate of passage of potassium ions and remains 12 times higher even at the same concentrations of these ions.
Spontaneous release of potassium from the cell occurs through independent channels, the diameter of which is about
The threshold level of membrane potential at which its permeability to sodium increases depends on the calcium concentration outside the cell; its decrease during hypocalcemia causes convulsions.
The occurrence of an action potential and the propagation of an impulse in an unmyelinated nerve occurs due to the opening of a sodium channel. The channel is formed by integral protein molecules, its conformation changes in response to an increase in positive charge environment. The increase in charge is associated with the entry of sodium through the adjacent channel.
Depolarization caused by channel opening effectively affects the adjacent channel
In the myelinated nerve, sodium channels are concentrated in the unmyelinated nodes of Ranvier (more than ten thousand per 1 micron). In this regard, in the interception zone, the sodium flow is 10-100 times greater than on the conducting surface of the unmyelinated nerve. Na^K^-ATPase molecules in large quantities are located on adjacent sections of the nerve. Depolarization of one of the nodes causes a potential gradient between the nodes, so the current quickly flows through the axoplasm to the adjacent node, reducing the potential difference there to a threshold level. This ensures a high speed of impulse transmission along the nerve - no less than 2 times faster than through a non-myelinated nerve (up to 50 m/s in a non-myelinated nerve and up to 100 m/s in a myelinated one).
320.Transmission of nerve impulses , those. its spread to another cell is carried out using special structures - synapses , connecting nerve ending and the neighboring cell The synaptic cleft separates the cells. If the gap width is below 2 nm, signal transmission occurs by current propagation, as along the axon. In most synapses, the gap width approaches 20 nm. In these synapses, the arrival of an action potential leads to the release of a transmitter substance from the presynaptic membrane, which diffuses through the synaptic gap and binds to a specific receptor on the postsynaptic membrane, transmitting a signal to it.
Mediator substances(neurotransmitters) - compounds that are in the presynaptic structure in sufficient concentration are released when impulse transmission, cause an electrical impulse after binding to the postsynaptic membrane. An essential feature of a neurotransmitter is the presence of a transport system for its removal from the synapse. Moreover, this transport system must have a high affinity for the mediator.
Depending on the nature of the mediator providing synaptic transmission, synapses are distinguished as cholinergic (transmitter - acetylcholine), and adrenergic (transmitters - catecholamine norepinephrine, dopamine and, possibly, adrenaline)
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