CONDUCTING A NERVE IMPULSE

STRUCTURE OF NERVE FIBERS

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

Nerve fibers are divided into pulpy, or myelinated, And 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 vegetative 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 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 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.

Electrical phenomena in living tissues are associated with differences in the 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 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 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).

and from one cell to another. P.n. And. along nerve conductors occurs with the help of electrotonic potentials and action potentials, which propagate along the fiber in both directions, without passing to neighboring fibers (see Bioelectric potentials, Nervous impulse). Transmission of intercellular signals occurs through synapses, most often with the help of mediators that cause the appearance of postsynaptic potentials (See Postsynaptic potentials). Nerve conductors can be considered as cables that have a relatively low axial resistance (axoplasmic resistance - r i) and higher shell resistance (membrane resistance - r m). The nerve impulse propagates along the nerve conductor through the passage of current between the resting and active sections of the nerve (local currents). In a conductor, as the distance from the point of excitation increases, there is a gradual, and in the case of a homogeneous structure of the conductor, an exponential decay of the pulse, which decreases by 2.7 times at a distance λ = r m and r i are in inverse relation to the diameter of the conductor, then the attenuation of the nerve impulse in thin fibers occurs earlier than in thick ones. The imperfection of the cable properties of nerve conductors is compensated by the fact that they have excitability. The main condition for excitation is the presence of a resting potential in the nerves (See Resting potential). If a local current through a resting region causes depolarization (See Depolarization) of the membrane reaching a critical level (threshold), this will lead to the appearance of a propagating action potential (See Action potential) (AP). The ratio of the level of threshold depolarization and AP amplitude, usually at least 1: 5, ensures high reliability of conduction: sections of the conductor that have the ability to generate AP can be separated from each other at such a distance, overcoming which the nerve impulse reduces its amplitude by almost 5 times. This weakened signal will be amplified again to a standard level (AP amplitude) and will be able to continue its path along the nerve.

Speed ​​P. n. And. depends on the speed with which membrane container in the area ahead of the impulse, it is discharged to the level of the AP generation threshold, which, in turn, is determined by the geometric features of the nerves, changes in their diameter, and the presence of branching nodes. In particular, thin fibers have a higher r i, and a larger surface capacity, and therefore the speed of P. n. And. on them below. At the same time, the thickness of the nerve fibers limits the possibility of the existence of a large number of parallel communication channels. Conflict between physical properties nerve conductors and the requirements for the “compactness” of the nervous system was resolved by the appearance during the evolution of vertebrates so-called. pulpy (myelinated) fibers (see Nerves). Speed ​​P. n. And. in myelinated fibers of warm-blooded animals (despite their small diameter - 4-20 µm) reaches 100-120 m/sec. The generation of PD occurs only in limited areas of their surface - the nodes of Ranvier, and along the inter-intercept areas of the P. and. And. carried out electrotonically (see Saltatory conduction). Some medicinal substances, such as anesthetics, greatly slow down the process until it completely blocks P. n. And. This is used in practical medicine for pain relief.

L. G. Magazanik.

Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .

See what “Conduction of nerve impulses” is in other dictionaries:

- (lat. decrementum decrease, from decresco decrease, decrease) P. v. without a significant change in the magnitude of the nerve impulse... Big medical dictionary

- (lat. decrementum decrease from decresco to decrease, decrease) P. v., accompanied by a decrease in the magnitude of the nerve impulse ... Large medical dictionary

CONDUCT- 1. Transmission of a nerve impulse from one place to another. 2. Mechanical transmission sound waves through the eardrum and auditory ossicles...

- (lat. saltatorius, from salto I gallop, I jump) spasmodic conduction of a nerve impulse along the pulpy (myelinated) nerves, the sheath of which has a relatively high resistance to electric current. Along the length of the nerve regularly... ... Great Soviet Encyclopedia

- (lat. saltatorius, from salto I gallop, jump), spasmodic conduction of a nerve impulse from one node of Ranvier to another along the pulpy (myelinated) axon. S. is characterized by a combination of electrotonic. spread across... ... Biological encyclopedic dictionary

Continuous conduction- is a term that denotes the characteristic of the conduction of a nerve impulse along an axon, which occurs in the “all or nothing” mode... encyclopedic Dictionary in psychology and pedagogy

UNDAMPED CONDUCT- A phrase used to characterize the conduction of a nerve impulse along an axon, which occurs in an all-or-nothing mode... Dictionary in psychology

A wave of excitation propagating along a nerve fiber in response to irritation of neurons. Provides the transmission of information from receptors to the central nervous system and from it to the executive organs (muscles, glands). Conducting nervous... encyclopedic Dictionary

Nerve fibers are processes of neurons covered with glial membranes. In different parts of the nervous system, the sheaths of nerve fibers differ significantly in their structure, which underlies the division of all fibers into myelinated and non-myelinated... Wikipedia

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... ... Wikipedia

In addition to excitability, the main property of a nerve is the ability to conduct excitation - conduction. The action current is 5-10 times greater than the stimulation threshold, which creates a “reliability factor” for the conduction of excitation along the nerve. Excitation impulses are transmitted along the surface of the membrane of the axial cylinder of the nerve fiber, and the neurofibrils of which it consists carry physiologically active substances.

When excitation spreads to one of the nerve fibers that make up a mixed nerve, it is not transmitted to neighboring fibers. Consequently, there is isolated conduction in afferent and motor fibers (necessary for obtaining coordinated movements), as well as in vascular, secretory and other nerve fibers that are part of the common nerve trunk.

It is very likely that the Schwann and myelin sheaths of nerve fibers act as an insulator, preventing the conduction of excitation to adjacent nerve fibers. The myelin sheath also functions as a current capacitor. It has a very high resistance to electric current, since myelin, consisting of lipids, does not allow ions to pass through. Therefore, impulses are not conducted along the membrane between the nodes of Ranvier; action potentials in the pulp fibers arise only between the nodes and jump over them. This conduction of impulses jumping through interceptions is called saltatory. In contrast to pulpy fibers, excitation in non-pulp fibers spreads along the membrane along its entire length.

At the nodes of Ranvier, the voltage of action potentials that transmit excitation impulses along the nerve increases. This increase prevents a significant loss of voltage along the nerve due to its resistance as a conductor. The loss of voltage potentials would lead to a large decrease in excitation and a slowdown in its conduction along the nerve.

Along the human motor nerve fiber from the spinal cord to the muscles of the fingers, there are about 800 nodes of Ranvier or “stations” for increasing the voltage of action potentials.

Thanks to the “reliability factor”, the action potential can jump through one node of Ranvier, and possibly through several nodes, since the distance between them is 1-2.5 mm. The fact of excitation jump is denied by some authors. The nerve fiber sheath is involved in its metabolism, in the growth of the axial cylinder and in the formation of the mediator (trophic function). The main way to study the conduction of excitation in nerves is to record potentials, which makes it possible to judge the physiological processes occurring in a nerve separated from an organ - a muscle or a gland. Under natural conditions, an indicator of the conduction of excitation along the motor nerve is muscle contraction. In secretory nerves, the indicator of excitation is the secretion of the gland.

Excitation is carried along the nerve only if it is anatomically continuous, but this is still not enough to transmit excitation. Bandaging and compression, which do not violate the anatomical continuity, stop the conduction of excitation along the nerve, as they disrupt its physiological properties. Some poisons and drugs, extreme cold or exposure and other influences also disrupt or stop the conduction of excitation along the nerve. Nerves conduct excitation in both directions from the irritated area, which is proven by the occurrence of potentials at both ends of the nerve; thus, excitation within a neuron can propagate both centripetally and centrifugally.

The rule of bilateral conduction does not contradict the rule of isolated conduction, since excitation is carried out in both directions in the branches of the same isolated nerve fiber.

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) are released during the reaction process. chemical composition). 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 occurrence electric field, so a potential arises that irritates the membrane neighboring plot 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.