Electrical phenomena in living tissues are associated with the difference in concentrations of ions carrying electrical 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 in 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. Membrane potential at the moment the peak is reached it is +40 mV. During the repolarization phase, the membrane potential again reaches the 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 the fibers of the mixed nerve, it varies from 120 m/s (thick, myelinated fibers, up to 20 μm in diameter) to 0.5 m/s (the thinnest, 0.1 μm in diameter, non-myelinated fibers).

CONDUCTING A NERVE IMPULSE

STRUCTURE OF NERVE FIBERS

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

Nerve fibers are divided into pulpy, or myelinated, And pulpless, 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-pulse fibers, and the ratio between the number of both in different nerves is different. For example, in many cutaneous nerves the predominant nerve fibers predominate. Thus, in the nerves of the autonomic nervous system, for example in the vagus nerve, the number of soft fibers reaches 80-95%. On the contrary, in the nerves innervating skeletal muscles there are only relatively few a large number of pulpless fibers.

In Fig. Figure 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 a plasma membrane, and its contents are axoplasm, penetrated by the finest (10-40 nm in diameter) neurofibrils (and microtubules), between which there are a large number of mitochondria and microsomes. The diameter of nerve fibers ranges from 0.5 to 25 microns.

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. 43, I), its layers merge, forming a dense fatty sheath - the myelin sheath. Myelin sheath at intervals equal length is interrupted, leaving open areas of the membrane approximately 1 µm wide. These areas are called interceptions (Ranvier interceptions).

The length of the interstitial areas covered by the myelin sheath is approximately proportional to the diameter of the fiber. Thus, in nerve fibers having a diameter of 10-20 microns, the length of the gap between the interceptions is 1-2 mm. In the thinnest fibers (1-2 µm in diameter), these sections 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 a single pulpless fiber. Often, however, several thin, pulpless fibers appear in the folds of the myelocyte (Fig. 43. II).

Rice. 43. The role of the myelocyte (Schwann cell) in the formation of the myelin sheath in pulpal nerve fibers. The successive stages of spiral-shaped twisting of the myelocyte around the axon are shown (I). Mutual arrangement of myelocytes and axons in non-pulp nerve fibers (II).

PHYSIOLOGICAL ROLE OF STRUCTURAL ELEMENTS OF MYELINIZED NERVE FIBER

It can be considered proven that in the processes of emergence and implementation nerve impulse The main role is played by the surface membrane of the axial cylinder. 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 the node (interception of Ranvier). It has important for the propagation of a 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.

Rice. 44. Hypothetical transport mechanism of nerve fiber.

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

Neurofibrils, microtubules and transport filaments provide transport of various substances and some cellular organelles along nerve fibers from the neuron body to the nerve endings and in the opposite direction. Thus, the following are transported along the axon from the cell body to the periphery: 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 average-diameter axon 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, play the same role in the nerve fiber that myosin plays in the muscle fiber. Transport filaments formed by actin “slide” along microtubules at a speed of 410 µm/day. They bind various substances (for example, protein molecules) or cellular organelles (mitochondria) and transport them along the fiber (Fig. 44).

Just like the muscular contractile apparatus, the nerve fiber transport system uses ATP energy for its work and requires the presence of ions Ca 2+ in cytoplasm.

REBIRTH OF NERVE FIBERS AFTER NERCE CUT

Nerve fibers cannot exist without connection with the body of the nerve cell: cutting the nerve leads to the death of those fibers that are separated from the cell body. In warm-blooded animals, already 2-3 days after transection of the nerve, its peripheral process loses the ability to conduct nerve impulses. Following this, 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 droplets; the disintegrated fibers and their myelin are resorbed and cords formed by lemmocytes (Schwann cells) remain in place of the nerve fibers. All these changes were first described by the English physician Waller and named after him Wallerian degeneration.

Nerve regeneration occurs very slowly. Lemmocytes remaining at the site 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 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 it reaches the corresponding peripheral tissue or organ, where the fibers form nerve endings. From this time on, normal innervation of the organ or tissue is restored.

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

final recovery occurs much later, sometimes after a year.

LAWS OF EXCITATION IN THE NERVES

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

Anatomical and physiological fiber continuity. Conduction of impulses is possible only if the anatomical integrity of the fiber is present, therefore, both ceresis of nerve fibers and any injury to the surface membrane disrupt conduction. Nonconductivity is also observed when the physiological integrity of the fiber is violated (blockade of sodium channels of the excitable membrane by tetrodotoxin or local anesthetics, sudden cooling, etc.). Conduction is also disrupted during 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 partial or complete disruption of conduction function.

Bilateral conduction. When a nerve fiber is stimulated, excitation spreads along it in both centrifugal and centripetal directions. This is proven by the following experiment.

Two pairs of electrodes connected to two electrical measuring instruments A and B are applied to a nerve fiber, motor or sensory (Fig. 45). Irritation is applied between these electrodes. As a result of bilateral 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 conduction. IN In the peripheral nerve, impulses propagate along each fiber in isolation, that is, without passing from one fiber to another and having an effect only on those cells with which the endings of a given nerve fiber are in contact. This is very important due to the fact that every peripheral nerve trunk contains a large number of nerve fibers - motor, sensory and autonomic, which innervate different cells and tissues, sometimes far apart and dissimilar in structure and function. For example, the vagus nerve innervates all organs of the chest cavity and a significant part of the organs abdominal cavity, sciatic nerve- all 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 the mixed nerve can be proven simple experience 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 if excitation transfers from one nerve fiber to another, but only those groups of muscle fibers that are innervated by the irritated root. Even more rigorous evidence of isolated conduction of excitation can be obtained by abstracting action potentials from various nerve fibers of the nerve trunk.

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

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

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

Nerve fibers They are processes of nerve cells, among which dendrites and axons are distinguished. One of the most important functions of these fibers is the perception of signals from the external and internal environment, their conversion into nerve impulses and conduction of the latter through dendrites into or along axons from the central nervous system to effector cells.

Nerve fibers (nerve cell processes) carry out nerve impulses. Nerve fibers are divided into myelin(covered with myelin sheath) and unmyelinated. Myelinated fibers predominate in motor nerves, and non-myelinated fibers predominate in the autonomic nervous system.

Fiber structure

The nerve fiber consists of an axial cylinder and a myelin sheath covering it, which is interrupted at certain intervals (nodes of Ranvier). The myelin sheath is formed as a result of the lemmocyte (Schwann cell) repeatedly wrapping itself around the axial cylinder, forming a dense lipid layer. Such fibers are called myelin, or pulpy. Nerve fibers that do not have a myelin sheath are called unmyelinated, or pulpless. The axial cylinder has plasma membrane and axoplasm.

Nerves or nerve trunks are formed from nerve fibers, enclosed in a common connective tissue sheath. The nerve contains both myelinated and unmyelinated fibers.

Rice. Diagram of the structure of nerve fibers

Depending on the function and direction of nerve impulses, fibers are divided into afferent, conducting signals to the central nervous system, and efferent, conducting them from the central nervous system to the executive organs. Nerve fibers form nerves and numerous signal pathways within the nervous system itself.

Types of nerve fibers

Nerve fibers, based on their diameter and speed of excitation, are usually divided into three types: A, B, C. Type A fibers, in turn, are divided into subtypes: A-α, A-β, A-γ, A-δ.

Fibers type A covered with a myelin sheath. The thickest among them (A-a) have a diameter of 12-22 microns and have the highest excitation speed - 70-120 m/s. These fibers carry excitation from the motor nerve centers of the spinal cord to the skeletal muscles and from muscle receptors to the corresponding nerve centers. Other type A fibers have a smaller diameter and lower excitation speed (from 5 to 70 m/s). They relate primarily to sensory fibers that conduct excitation from various receptors (tactile, temperature, etc.) to the central nervous system.

To the fibers type B include myelinated preganglionic fibers of the autonomic nervous system. Their diameter is 1-3.5 microns, and the excitation speed is 3-18 m/s.

To the fibers type C These include thin (diameter 0.5-2 µm) unmyelinated nerve fibers. The speed of excitation through them is 0.5-3.0 m/s. Fibers of this type are part of the postganglionic fibers of the autonomic nervous system. These fibers also conduct excitation from thermoreceptors and pain receptors.

Conducting excitation along nerve fibers

The characteristics of excitation in nerve fibers depend on their structure and properties. Based on these characteristics, nerve fibers are divided into groups A, B and C. Fibers of groups A and B are represented by myelinated fibers. They are covered with a myelin sheath, which is formed by tightly adjacent membranes of glial cells, repeatedly wrapped around the axial cylinder of the nerve fiber. In the central nervous system, the myelin sheath is formed by oligodendrocytes, and the myelin of peripheral nerves is formed by Schwann cells.

Myelin is multilayer membrane, consisting of phospholipids, cholesterol, myelin basic protein and small amounts of other substances. The myelin sheath is interrupted through approximately equal sections (0.5-2 mm), and the nerve fiber membrane remains uncovered by myelin. These areas are called nodes of Ranvier. In the nerve fiber membrane in the area of ​​interceptions there is a high density of voltage-gated sodium and potassium channels. The length of the interceptions is 0.3-14 microns. The larger the diameter of the myelinated fiber, the longer its sections are covered with myelin and the fewer nodes of Ranvier there are per unit length of such fiber.

Group A fibers are divided into 4 subgroups: a, β, y, δ (Table 1).

Table 1. Properties of various nerve fibers of warm-blooded animals

Aa fibers- the largest in diameter (12-20 microns) - have an excitation speed of 70-120 m/s. They perform the functions of afferent fibers that conduct excitation from tactile receptors of the skin, muscle and tendon receptors, and are also efferent fibers that transmit excitation from spinal a-motoneurons to extrafusal contractile fibers. The information transmitted through them is necessary for the implementation of fast reflex and voluntary movements. Nerve fibers carry out excitation from spinal γ-motoneurons to the contractile cells of muscle spindles. Having a diameter of 3-6 microns, Ay fibers excite at a speed of 15-30 m/s. The information transmitted along these fibers is not used directly to initiate movements, but rather to coordinate them.

From the table 1 shows that thick myelinated fibers are used in those sensory and motor nerves through which information must be transmitted most quickly to carry out urgent reactions.

Processes controlled by the autonomic nervous system are carried out at lower speeds than the motor reactions of skeletal muscles. The information necessary for their implementation is perceived by sensory receptors and transmitted to the central nervous system along the thinnest afferent myelinated Aδ-, B- and unmyelinated C- fibers. Efferent fibers of type B and C are part of the nerves of the autonomic nervous system.

The mechanism of excitation along nerve fibers

To date, it has been proven that the conduction of excitation along myelinated and non-myelinated nerve fibers is carried out on the basis of ionic mechanisms of action potential generation. But the mechanism of excitation through both types of fibers has certain features.

Thus, when excitation propagates along an unmyelinated nerve fiber, local currents that arise between its excited and unexcited sections cause depolarization of the membrane and the generation of an action potential. Then local currents arise between the excited section of the membrane and the nearest unexcited section. Repeated repetition of this process promotes the spread of excitation along the nerve fiber. Since all sections of the fiber membrane are sequentially involved in the excitation process, this mechanism of excitation is called continuous. Continuous conduction of the action potential occurs in muscle fibers and in unmyelinated type C nerve fibers.

The presence of areas in myelinated nerve fibers without this myelin sheath (nodes of Ranvier) determines a specific type of conduction of excitation. In these fibers, local electrical currents arise between adjacent nodes of Ranvier, separated by a portion of the fiber with a myelin sheath. And the excitement “jumps” through the areas covered with the myelin sheath, from one interception to another. This mechanism of excitation propagation is called saltatory(spitty), or intermittent. The speed of saltatory conduction of excitation is much higher than in unmyelinated fibers, since not the entire membrane is involved in the excitation process, but only part of it large plots in the field of interceptions.

“Jumping” of the action potential through the myelin region is possible because its amplitude is 5-6 times greater than the value required to excite the adjacent node of Ranvier. Sometimes the action potential can “jump” even over several inter-intercept intervals.

Transport function of nerve fibers

The implementation by the membrane of nerve fibers of one of their main functions - the conduction of nerve impulses - is inextricably linked with the transformation of electrical potentials into the release of signal molecules - neurotransmitters - from the nerve endings. In many cases, their synthesis occurs in the nucleus of the nerve cell body, and the axons of the nerve cell, which can reach a length of 1 m, deliver neurotransmitters to the nerve endings through special transport mechanisms called axonal transport of substances. With their help, not only neurotransmitters move along nerve fibers, but also enzymes, plastic and other substances necessary for growth, maintenance of the structure and function of nerve fibers, synapses and postsynaptic cells.

Axon transport is divided into fast and slow.

Fast axon transport ensures the movement of mediators, some intracellular organelles, and enzymes in the direction from the neuron body to the presynaptic terminals of the axon. This type of transport is called antegrade. It is carried out with the participation of the actin protein, Ca 2+ ions and microtubules and microfilaments running along the axon. Its speed is 25-40 cm/day. Transport consumes the energy of cellular metabolism.

Slow axon transport occurs at a speed of 1-2 mm/day in the direction from the neuron body to the nerve endings. Slow antegrade transport is the movement of axoplasm together with the organelles, RNA, proteins and biologically active substances it contains from the body of the neuron to its endings. The speed of their movement determines the growth rate of the axon when it restores its length (regenerates) after damage.

Also distinguished retrograde axon transport in the direction from the nerve ending to the neuron body. With the help of this type of transport, the enzyme acetylcholinesterase, fragments of destroyed organelles, and some biological substances regulating protein synthesis in the neuron. Transport speed reaches 30 cm/day. Taking into account the presence of retrograde transport is also important because with its help pathogenic agents can penetrate into the nervous system: polio viruses, herpes, rabies, tetanus toxin.

Axon transport is necessary to maintain the normal structure and function of nerve fibers, the delivery of energy substances, mediators and neuropeptides to presynaptic terminals. It is important for exerting a trophic effect on innervated tissues and for restoring damaged nerve fibers. If a nerve fiber is crossed, then its peripheral section, deprived of the ability to exchange various substances with the body of the nerve cell using axonal transport, degenerates. The central section of the nerve fiber, which has retained its connection with the nerve cell body, regenerates.

Conduction of nerve impulses

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

Nerve fibers are divided into pulpy, 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.

Nerve fiber structure

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. Thus, in the nerves of the autonomic nervous system, for example in the vagus nerve, the number of soft 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. 1), its layers merge, forming a dense fatty sheath - the myelin sheath. The myelin sheath is interrupted at intervals of equal length, leaving exposed areas of the membrane approximately 1 µm wide. These areas were named Ranvier interceptions.

Rice. 1. 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); mutual arrangement 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 (1-2 µm in diameter), these sections 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 a single 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 the propagation of the 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 saltatory.

Direct electrophysiological studies carried out by Kago (1924) and then by Tasaki (1953) on single myelinated frog nerve fibers showed that action potentials in these fibers arise only in the nodes, and the areas between the nodes, covered with myelin, 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 of sodium channels is the most important condition saltatory conduction of excitation. In Fig. Figure 2 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 a local (local) electric current, which 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 C, excitation still continues, and it temporarily becomes refractory. 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.

Rice. 2. Saltatory spread of excitation in the pulpal nerve fiber from interception to interception: A - unmyelinated fiber; B - myelinated fiber. The arrows show the direction of the current

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, abrupt propagation is energetically more economical, since not the entire membrane comes into a state of activity, but only its small sections in the interception area, having a width of less than 1 μm. The losses of ions (per unit fiber length) accompanying the occurrence of an action potential in such limited areas of the membrane are very small, and therefore the energy costs for the operation of the sodium-potassium pump, necessary to restore the altered ionic ratios between the internal contents of the nerve fiber and tissue fluid.

Laws of conduction of excitation in nerves

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

Anatomical and physiological fiber continuity. A prerequisite for excitation is the morphological and functional integrity of the membrane. Any strong impact on the fiber - ligation, compression, stretching, the action of various chemical agents, excessive cold or heat - causes its damage and cessation of excitation.

Bilateral conduction of excitation. Excitation is carried out along nerve fibers in both the afferent and efferent directions. This feature of nerve fibers was proven by the experiments of A.I. Babukhin (1847) on the electric organ of the Nile catfish. The electrical organ of the catfish consists of separate plates innervated by branches of a single axon. A.I. Babukhin removed the middle plates to avoid conducting excitation through the electrical organ, and cut one of the branches of the nerve. By irritating the central end of the cut nerve, he observed a response in all segments of the electrical organ. Consequently, excitation along the nerve fibers took place in different directions- centripetal and centrifugal.

Bilateral conduction is not only 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 conduction. In the peripheral nerve, impulses propagate along each fiber separately, i.e. without moving from one fiber to another and exerting an effect only on those cells with which the endings of a given nerve fiber are in contact. This is due to the characteristics of the myelin sheath. Possessing high resistance, it is an insulator that prevents the spread of excitation to adjacent fibers. This is very important due to the fact that every peripheral nerve trunk contains a large number of nerve fibers - motor, sensory and autonomic, which innervate different cells and tissues, sometimes far apart and heterogeneous in structure and function. For example, the vagus nerve innervates all the organs of the thoracic cavity and a significant part of the abdominal organs, the sciatic nerve innervates all the muscles, bone apparatus, blood vessels and skin of the lower limb. If excitation passed within the nerve trunk from one fiber to another, then in this case the normal isolated functioning of peripheral organs and tissues would be impossible.

Degeneration of nerve fibers after nerve transection. Nerve fibers cannot exist without connection with the body of the nerve cell: cutting the nerve leads to the death of those fibers that are separated from the cell body. In warm-blooded animals, already two or three days after cutting the nerve, its peripheral process loses the ability to conduct nerve impulses. Following this, degeneration of the nerve fibers begins, and the myelin sheath undergoes fatty degeneration: the myelin sheath loses myelin, which accumulates in the form of droplets; the disintegrated fibers and their myelin are resorbed and cords formed by lemmocytes (Schwann cells) remain in place of the nerve fibers. All these changes were first described by the English physician Waller and named after him Wallerian degeneration.

Nerve regeneration occurs very slowly. Lemmocytes remaining at the site 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 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 it reaches the corresponding peripheral tissue or organ, where the fibers form nerve endings. From this time on, normal innervation of the organ or tissue is restored.

In different organs, restoration of function after nerve transection occurs at different times. In the muscles, the first signs of functional restoration may appear after five to six weeks; final recovery occurs much later, sometimes after a year.

Properties of nerve fiber

Nerve fiber has certain physiological properties: excitability, conductivity and lability.

The nerve fiber is characterized by very low fatigue. This is due to the fact that when conducting one action potential along a nerve fiber, a very small amount of ATP is spent to restore ion gradients.

Lability and parabiosis of nerve fibers

Nerve fibers have lability. Lability (instability) is the ability of a nerve fiber to reproduce a certain number of excitation cycles per unit time. A measure of the lability of a nerve fiber is the maximum number of excitation cycles that it can reproduce per unit time without changing the rhythm of stimulation. The nerve fiber is capable of reproducing up to 1000 impulses per second.

Academician N.E. Vvedensky discovered that when a nerve area is exposed to a damaging agent (alteration), for example chemical substance, the lability of this area decreases. This is due to the blockade of sodium and potassium permeability of the membrane. This state of reduced lability N.E. Vvedensky called parabiosis. Parabiosis is divided into three successive phases: equalizing, paradoxical and inhibitory.

IN equalization phase the same magnitude of response to the action of strong and weak stimuli is established. Under normal conditions, the magnitude of the response of the muscle fibers innervated by this nerve obeys the law of force: the response to weak stimuli is less, and to strong stimuli it is greater.

Paradoxical phase characterized by the fact that a reaction of a larger magnitude is observed to weak stimuli than to strong ones.

IN braking phase the lability of the fiber decreases to such an extent that stimuli of any strength are not able to cause a response. In this case, the fiber membrane is in a state of prolonged depolarization.

Parabiosis is reversible. In the case of short-term exposure to a damaging substance on the nerve, after its action ceases, the nerve leaves the state of parabiosis and goes through similar phases, but in the reverse order.

Nerve fatigue

The indefatigability of the nerve was first shown by N.E. Vvedensky (1883), who observed the preservation of nerve functionality after continuous 8-hour stimulation. Vvedensky conducted an experiment on two neuromuscular preparations of frog legs. Both nerves were stimulated for a long time by a rhythmic induction current of equal strength. But on one of the nerves, closer to the muscle, additional electrodes were installed direct current, with the help of which the conduction of excitation to the muscles was blocked. Thus, both nerves were stimulated for 8 hours, but the excitation passed only to the muscles of one paw. After 8 hours of irritation, when the muscles of the working drug stopped contracting, the nerve block of another drug was removed. At the same time, a contraction of his muscles occurred in response to nerve irritation. Consequently, the nerve conducting excitation to the blocked paw did not become fatigued, despite prolonged irritation.

Thin fibers fatigue faster than thick ones. The relative fatigue resistance of the nerve fiber is associated primarily with the level of metabolism. Since nerve fibers are excited only at the nodes of Ranvier (which is a relatively small surface area) during activity, the amount of energy expended is small. Therefore, resynthesis processes easily cover these costs, even if the excitation lasts several hours. In addition, under natural conditions of functioning of the body, the nerve does not get tired due to the fact that it carries less load than its capacity.

Of all the links in the reflex arc, the nerve has the highest lability. Meanwhile, in the whole organism, the frequency of impulses traveling along the efferent nerve is determined by the lability of the nerve centers, which is low. Therefore, the nerve conducts fewer impulses per unit time than it could reproduce. This ensures its relative fatigue-free performance.

The essence of the concept of “Excitement”

The emergence and conduction of nervous excitation

Excitation is a tissue response to irritation, manifested in addition to nonspecific reactions (generation of action potential, metabolic changes) in the performance of a function specific to this tissue; excitable are nervous (conducting excitation), muscle (contraction) and glandular (secretion) tissue.

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

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

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

Action potential.

Action potential is an excitation wave that moves across the membrane of a living cell during the transmission of a nerve signal. Essentially it represents electrical discharge-- rapid short-term change in potential by small area membrane of an excitable cell (neuron, muscle fiber or glandular cell), as a result of which the outer surface of this region becomes negatively charged in relation to neighboring regions of the membrane, while its inner surface becomes positively charged in relation to neighboring regions of the membrane. The action potential is the physical basis of a nerve or muscle impulse that plays a signaling (regulatory) role. Action potentials can vary in their parameters depending on the type of cell and even on various areas membranes of the same cell. The most typical example of differences is the action potential of the heart muscle and the action potential of most neurons. However, the basis of any action potential is the following:

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

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

3. The membrane of an excitable cell is capable of quickly changing its permeability to a certain type of cations, causing a transition of positive charge from outside to the inner one (Fig. 1).

The first two properties are characteristic of all living cells. The third is a feature of excitable tissue cells 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).

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

General provisions.

Polarization of the membrane of a living cell is due to the difference in ionic composition from its internal and outside. When the cell is in a quiet (unexcited) state, ions on opposite sides of the membrane create a relatively stable potential difference, called the resting potential. If you insert an electrode into a living cell and measure the resting membrane potential, it will have a negative value (about 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 side, although both sides contain cations and anions. Outside there are an order of magnitude more sodium, calcium and chlorine ions, inside there are potassium ions and negatively charged protein molecules, amino acids, organic acids, phosphates, sulfates.

We must understand that we are talking specifically about the charge of the surface of the membrane; in general, 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 an electrode.

Under natural conditions, the stimulus is often a chemical signal from neighboring cells, arriving through a synapse or through diffuse transmission through the intercellular medium. The shift in membrane potential can occur in a negative (hyperpolarization) or positive (depolarization) direction. In nervous tissue, an action potential usually 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 appear 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 part of the membrane - the axon hillock, so the action potential does not propagate to the dendrites).

Most channels are ion-specific - the sodium channel allows almost only sodium ions to pass through and does not allow others to pass through (this phenomenon is called selectivity). The cell membrane of excitable tissues (nervous and muscle) contains a large number of voltage-dependent ion channels that can quickly respond to a shift in membrane potential. Membrane depolarization primarily causes the opening of voltage-gated sodium channels. 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 the concentration gradient (there are many more positively charged sodium ions on the outside of the membrane than inside the cell) and the negative charge inside membranes 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 it is designated AP).

According to the all-or-nothing law, the cell membrane excitable tissue either does not respond to the stimulus at all, or responds with the maximum possible response for it. this moment by force. That is, if the stimulus is too weak and the threshold is not reached, the action potential does not occur at all; at the same time, a threshold stimulus will cause an action potential of the same amplitude as a stimulus exceeding 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 finds itself in a state of absolute refractoriness for some time, when no signals can excite it again, then enters a phase of relative refractoriness, when it can only be excited by strong signals (in this case, the AP amplitude will be lower than usual). The refractory period occurs due to inactivation of the fast sodium current, that is, inactivation of sodium channels (see below).

Propagation of action potential

Action potential propagation along unmyelinated fibers.

The AP propagates continuously along the unmyelinated fiber. The conduction of a nerve impulse begins with the propagation electric field. The resulting AP due to the electric field is capable of depolarizing the membrane neighboring plot to a critical level, as a result of which new PDs are generated in the neighboring area. The PDs themselves do not move; they disappear in the same place where they appear. The main role in the emergence of a new PD is played by the previous one. If an intracellular electrode is used to stimulate the axon in the middle, the AP will propagate in both directions. Typically, the AP propagates along the axon in one direction (from the body of the neuron to the nerve endings), although depolarization of the membrane occurs on both sides of the area where the AP currently occurs. Unilateral conduction of AP is ensured by the properties of sodium channels - after opening, they are inactivated for some time and cannot open at any value of the membrane potential (refractory property). Therefore, in the area closest to the cell body, where the AP has already “passed” before, it does not occur. All other things being equal, the propagation of APs along the axon occurs the faster, the larger the fiber diameter. The AP can propagate along the giant axons of the squid at almost the same speed as along the myelinated fibers of vertebrates (about 100 m/s).

Action potential propagation along myelinated fibers.

Along the myelinated fiber, the AP propagates in a spasmodic manner (saltatory conduction). Myelinated fibers are characterized by a concentration of voltage-gated ion channels only in the areas of nodes of Ranvier; 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. An AP that arises 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 APs in them, that is, the excitation passes spasmodically, from one node to another. If one node of Ranvier is damaged, the PD excites the 2nd, 3rd, 4th and even the 5th, since the electrical insulation created by the myelin sleeves reduces the dissipation of the electric field. This increases the speed of AP propagation along myelinated fibers compared to unmyelinated fibers. In addition, myelinated fibers are thicker and electrical resistance There are fewer thicker fibers, which also increases the speed of impulse transmission 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, significantly less energy is needed to restore transmembrane gradients of Na+ and K+, consumed as a result of the occurrence of AP, which can have value at a high frequency of discharges traveling along the nerve fiber. To imagine how effectively the conduction speed can be increased due to the myelin sheath, it is enough to compare the speed of impulse propagation through the unmyelinated and myelinated areas 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 - 15-20 m/s. In fibers of larger diameter, which have a thick myelin sheath, the conduction speed can reach 120 m/s. The speed 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 speed can decrease very significantly and, accordingly, increase, returning to a certain initial level.

Active properties of the membrane.

The active properties of the membrane, which ensure 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 ion concentration is then restored by the sodium-potassium pump. During the PD, channels move from state to state: Na+ channels have three main states - closed, open and inactivated (in reality the matter is more complicated, but these three are enough for description), K+ channels have two - closed and open. The behavior of the channels involved in the formation of PD is described in terms of conductivity and calculated through transfer coefficients. Transfer coefficients were derived by Hodgkin and Huxley.

Resting potential and the mechanism of its formation.

Ion-membrane theory of resting potential and action potential.

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

In this case, the outer membrane carries a positive charge in relation 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 washing the cells.

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 small ions such as potassium, sodium and chlorine are easily passed through the cell membrane, then organic anions, such as 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 negative ions (chlorine) from entering the cell, but attract positively charged cations (sodium, potassium) into it; however, most sodium entering the cell is immediately removed by the sodium-potassium pump.

The rapid removal of sodium leads to the accumulation of only potassium 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.

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

At rest, the potassium cells are open, and the sodium cells are almost all closed.

Nerve cells always operate pumping mechanisms that transport ions against a concentration gradient.

Concentration gradient is the difference between concentrations from lower to higher.

Measuring cell 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 liquid washing it. The electrodes are connected to an amplifier, which increases the amplitude of the recorded potential; this amplitude is determined using a voltage meter such as an oscilloscope.

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

A 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 cross-section and the other to the surface, connecting them to a galvanometer, the galvanometer will show the current that always flows from the normal, undamaged surface to the cross-section.

The quiescent current and membrane potential are manifestations of the same property of the membrane; the reason for the appearance of a 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.

IN ideal conditions in case of damage, a potential difference should be recorded = membrane potential. 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, 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]in/[K]ex)

Em = -59*ln([K]in/[K]ex)

R - gas constant.

T - absolute temperature.

F - Faraday number.

[K]in: [K]nar - the ratio of potassium concentrations inside and outside the cell.

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

In experiment, slightly lower values ​​are obtained (-60, -70 mV) than theoretical ones (-80 mV), because the membrane is not a device that perfectly discriminates between ions.

Sodium ions in small quantities 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.

Resting potential is the charge on the membrane at rest.

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

The reason for the occurrence of membrane potential:

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

As each positive potassium ion leaves the cell, it leaves behind an unbalanced negative charge (organic anions). These charges cause 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 disturbance, energy is required, the expenditure of which would counteract the thermal equalization of 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 metabolic energy is spent to maintain the uneven distribution of ions between the cytoplasm and the environment.

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

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

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

The membrane is not completely impermeable and is capable of allowing ions to pass through it to a certain extent. This ability is not the same for different ions in the resting state of the cell - it is significantly higher for potassium ions than for sodium ions. Therefore, the main ion that can diffuse across the cell membrane to a certain extent at rest is 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, the constant electrical polarization of the cell membrane is created mainly by the diffusion current of potassium ions across 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 clarify the nature of the active processes that occur 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 for cell excitability. The electrochemical gradient created by these two factors represents a reserve of potential energy that is always available to the cell and which can be immediately used to create active cellular reactions.

Action potential or nerve impulse, a specific response that occurs in the form of an excitatory wave and flows along the entire nerve pathway. This reaction is a response to a stimulus. The main task is to transmit data from the receptor to the nervous system, and after that it directs this information to the desired muscles, glands and tissues. After the passage of the pulse, the surface part of the membrane becomes negatively charged, while its inner part remains positive. Thus, a nerve impulse is a sequentially transmitted electrical change.

The exciting effect and its distribution are subject to physico-chemical nature. The energy for this process is generated directly in the nerve itself. This happens due to the fact that the passage of an impulse leads to the formation of heat. Once it has passed, the attenuation or reference state begins. In which only a fraction of a second the nerve cannot conduct a stimulus. The speed at which the pulse can be delivered ranges from 3 m/s to 120 m/s.

The fibers through which excitation passes have a specific sheath. Roughly speaking, this system resembles electrical cable. The composition of the membrane can be myelin or non-myelin. The most important component of the myelin sheath is myelin, which plays the role of a dielectric.

The speed of the pulse depends on several factors, for example, on the thickness of the fibers; the thicker it is, the faster the speed develops. Another factor in increasing conduction speed is the myelin itself. But at the same time, it is not located over the entire surface, but in sections, as if strung together. Accordingly, between these areas there are those that remain “bare”. They cause current leakage from the axon.

An axon is a process that is used to transmit data from one cell to the rest. This process is regulated by a synapse - a direct connection between neurons or a neuron and a cell. There is also a so-called synaptic space or cleft. When an irritating impulse arrives at a neuron, neurotransmitters (molecules of a chemical composition) are released during the reaction. They pass through the synaptic opening, eventually reaching the receptors of the neuron or cell to which the data needs to be conveyed. Calcium ions are necessary for the conduction of a nerve impulse, since without this the neurotransmitter cannot be released.

The autonomic system is provided mainly by non-myelinated tissues. Excitement spreads through them constantly and continuously.

The transmission principle is based on the appearance of an electric field, so a potential arises that irritates the membrane of the adjacent section and so on throughout the fiber.

In this case, the action potential does not move, but appears and disappears in one place. The transmission speed through such fibers is 1-2 m/s.

Laws of conduct

There are four basic laws in medicine:

• Anatomical and physiological value. Excitation is carried out only if there is no violation in the integrity of the fiber itself. If unity is not ensured, for example, due to infringement, drug use, then the conduction of a nerve impulse is impossible.
• Isolated conduction of irritation. Excitation can be transmitted along, in no way, without spreading to neighboring ones.
• Bilateral conduction. The path of impulse conduction can be of only two types - centrifugal and centripetal. But in reality, the direction occurs in one of the options.
• Non-decremental implementation. The impulses do not subside, in other words, they are carried out without decrement.

Chemistry of impulse conduction

The irritation process is also controlled by ions, mainly potassium, sodium and some organic compounds. The concentration of these substances is different, the cell is negatively charged inside itself, and positively charged on the surface. This process will be called potential difference. When a negative charge oscillates, for example, when it decreases, a potential difference is provoked and this process is called depolarization.

Stimulation of a neuron entails the opening of sodium channels at the site of stimulation. This may facilitate the entry of positively charged particles into the cell. Accordingly, the negative charge is reduced and an action potential or nerve impulse occurs. After this, the sodium channels close again.

It is often found that it is the weakening of polarization that promotes the opening of potassium channels, which provokes the release of positively charged potassium ions. This action reduces the negative charge on the cell surface.

The resting potential or electrochemical state is restored when potassium-sodium pumps are activated, with the help of which sodium ions leave the cell and potassium ions enter it.

As a result, we can say that when electrochemical processes are resumed, impulses occur that travel along the fibers.