Synapse of a nerve cell and conduction of a nerve impulse. nerve impulses

Lecture No. 3
nervous
momentum
The structure of the synapse

Nerve fibers

Pulp
(myelinated)
Pulpless
(unmyelized)
Sensory and motor
fibers.
They belong mainly
sympathetic n.s.
PD propagates in leaps and bounds
(saltatory conduction).
PD spreads continuously.
in the presence of even weak myelination
with the same fiber diameter - 1520 m/s. More often with a larger diameter of 120
m/sec.
With a fiber diameter of about 2 µm and
lack of myelin sheath
speed will be
~1 m/s

I - unmyelinated fiber II - myelinated fiber

According to the speed of conduction, all nerve fibers are divided into:

Type A fibers - α, β, γ, δ.
Myelinated. The thickest α.
Excitation speed 70-120m/s
Conduct excitation to skeletal muscles.
Fibers β, γ, δ. They have a smaller diameter
speed, longer PD. Mainly
sensory fibers of tactile, pain
temperature receptors, internal
organs.

Type B fibers are covered with myelin
shell. Speed ​​from 3 -18 m/s
- predominantly preganglionic
fiber of the autonomic nervous system.
Type C fibers are pulpless. Highly
small diameter. Carrying out speed
excitation from 0-3 m/sec. This is
postganglionic fibers
sympathetic nervous system and
some sensory fibers
receptors.

Laws of conducting excitation in nerves.

1) The law of anatomical and
physiological continuity
fibers. Any nerve injury
(transection) or its blockade
(novocaine), excitation along the nerve is not
held.

2) The law of 2-sided holding.
Excitation is conducted along the nerve from
sites of irritation in both
sides are the same.
3) The law of isolated conduct
arousal. in the peripheral nerve
impulses propagate through each
fiber in isolation, i.e. without moving from
one fiber to another and render
action only on those cells, endings
nerve fiber which is in contact

The sequence of processes leading to the blockade of the conduction of nerve impulses under the influence of a local anesthetic

1. Diffusion of anesthetic through the nerve sheath and
nerve membrane.
2. Fixation of the anesthetic in the receptor zone in sodium
channel.
3. Sodium channel blockade and inhibition of permeability
membranes for sodium.
4. Decreased rate and degree of depolarization phase
action potential.
5. The impossibility of reaching the threshold level and
action potential development.
6. Conduction blockade.

Synapse.

Synapse - (from the Greek "to connect, connect").
This concept was introduced in 1897 by Sherrington

General plan of the structure of the synapse

The main properties of synapses:

1. Unilateral excitation.
2. Delay in conducting excitation.
3. Summation and transformation. allocated
small doses of the mediator are summed up and
cause arousal.
As a result, the frequency of nerve
impulses coming down the axon
converted to a different frequency.

4. In all synapses of one neuron
one mediator is singled out, or
excitatory or inhibitory action.
5. Synapses are characterized by low lability
and high sensitivity to chemicals
substances.

Synapse classification

By mechanism:
Chemical
Electric
Electrochemical
By location:
1. neuromuscular By sign:
- excitatory
2. Nervous
- axo-somatic - brake
- axo-dendritic
- axo-axonal
- dendro-dendritic

The mechanism of conduction of excitation in the synapse.

Sequencing:

* Receipt of excitation in the form of PD to
end of the nerve fiber.
* presynaptic depolarization
membranes and release of Ca++ ions
from the sarcoplasmic reticulum
membranes.
*Receipt of Ca++ upon admission to
promotes synaptic plaque
release of the mediator from the vesicles.

Nerve fibers 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 their conduction through dendrites into or along axons from the CNS to effector cells.

Nerve fibers (outgrowths of nerve cells) conduct nerve impulses. Nerve fibers are divided into myelin(covered with myelin sheath) and unmyelinated. Myelinated fibers predominate in the motor nerves, and unmyelinated fibers in the autonomic nervous system.

The structure of the fibers

The nerve fiber consists of an axial cylinder and a myelin sheath covering it, interrupted at certain intervals (Ranvier's intercepts). The myelin sheath is formed as a result of the fact that the lemmocyte (Schwann cell) repeatedly wraps 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 a plasma membrane and an axoplasm.

From nerve fibers, nerves or nerve trunks are formed, 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, the fibers are divided into afferent, which conduct signals to the CNS, and efferent, conducting them from the central nervous system to the executive organs. Nerve fibers form the nerves and numerous signaling pathways within the nervous system itself.

Types of nerve fibers

Nerve fibers are usually divided into three types according to their diameter and speed of excitation: 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 myelin sheath. The thickest among them (A-a) have a diameter of 12-22 microns and have the highest speed of excitation - 70-120 m / s. Through these fibers, excitation is carried 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 a lower speed of excitation (from 5 to 70 m/s). They refer mainly to sensitive fibers that conduct excitation from various receptors (tactile, temperature, etc.) in the central nervous system.

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

To fibers type C include thin (diameter 0.5-2 microns) non-myelinated 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.

Conduction of excitation along nerve fibers

Features of the conduction of excitation in nerve fibers depend on their structure and properties. According to these features, nerve fibers are divided into groups A, B and C. The fibers of groups A and B are represented by myelinated fibers. They are covered by a myelin sheath, which is formed by densely attached glial cell membranes repeatedly wrapped around the axial cylinder of the nerve fiber. In the CNS, the myelin sheath is formed by oligodendrocytes, and the myelin of peripheral nerves is formed by Schwann cells.

Myelin is a multilayer membrane composed of phospholipids, cholesterol, myelin basic protein, and a small amount of other substances. The myelin sheath is interrupted through approximately equal sections (0.5-2 mm), and the nerve fiber membrane remains uncovered with myelin. These sections are called intercepts of Ranvier. There is a high density of voltage-gated sodium and potassium channels in the nerve fiber membrane in the area of ​​intercepts. The length of the intercepts 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 are present per unit length of such a fiber.

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

Table 1. Properties of various warm-blooded nerve fibers

Fiber type	Fiber diameter, µm	Conduction speed, m/s	Function	Action potential peak duration, ms	Duration of trace depolarization, ms	Duration of trace hyperpolarization, ms
			proprioception function Motor fibers of skeletal muscles, afferent fibers from muscle receptors
			Tactile function Afferent fibers from touch receptors
			motor function Afferent fibers from touch and pressure receptors, afferent fibers to muscle spindles
			Pain, temperature and tactile functions Afferent fibers from some receptors for heat, pressure, pain
			Preganglionic autonomic fibers		Is absent
			Sympathetic function Postganglionic autonomic fibers, afferent fibers from some receptors for heat, pressure, pain

Aa fibers- the largest in diameter (12-20 microns) - have a speed of excitation of 70-120 m / s. They perform the functions of afferent fibers that conduct excitation from skin tactile receptors, 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 the spinal y-motor neurons to the contractile cells of the muscle spindles. Having a diameter of 3-6 µm, Ay-fibers carry out excitation at a speed of 15-30 m/s. The information transmitted through these fibers is not used directly to initiate movements, but rather to coordinate them.

From Table. Figure 1 shows that thick myelinated fibers are used in those sensory and motor nerves that must be used to transmit information most quickly for immediate responses.

The processes controlled by the autonomic nervous system are carried out at lower rates 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 through 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 conduction of excitation along nerve fibers

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

Thus, when excitation spreads along an unmyelinated nerve fiber, local currents that arise between its excited and unexcited sections cause membrane depolarization and the generation of an action potential. Then local currents arise already between the excited area of ​​the membrane and the nearest unexcited area. The repeated repetition of this process contributes to the spread of excitation along the nerve fiber. Since all sections of the fiber membrane are sequentially involved in the process of excitation, such a mechanism for conducting excitation is called continuous. Continuous conduction of the action potential occurs in muscle fibers and in unmyelinated type C nerve fibers.

The presence in myelinated nerve fibers of areas without this myelin sheath (intercepts of Ranvier) determines the specific type of conduction of excitation. In these fibers, local electrical currents occur between adjacent nodes of Ranvier, separated by a section of fiber with a myelin sheath. And the excitation "jumps" over the areas covered with myelin sheath, from one intercept to another. This propagation mechanism is called saltatory(jumping) or intermittent. The speed of saltatory conduction of excitation is much higher than in non-myelinated fibers, since not the entire membrane is involved in the process of excitation, but only its small sections in the area of ​​intercepts.

"Jumping" of the action potential through the myelin area is possible because its amplitude is 5-6 times greater than the value necessary to excite the adjacent node of Ranvier. Sometimes the action potential is able to "jump" even through several interceptive gaps.

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 is carried out in the nucleus of the body of the nerve cell, 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 the nerve fibers, but also enzymes, plastic and other substances necessary for the growth, maintenance of the structure and function of nerve fibers, synapses and postsynaptic cells.

Axon transport is divided into fast and slow.

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

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

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

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

Conducting a nerve impulse

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

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

The structure of the nerve fiber

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

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

Rice. 1. The role of the myelocyte (Schwann cell) in the formation of the myelin sheath in the pulpy nerve fibers: the successive stages of the spiral twisting of the myelocyte around the axon (I); mutual arrangement of myelocytes and axons in amyeloid nerve fibers (II)

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

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

The myelin sheath performs a dual function: the function of an electrical insulator and a trophic function. The insulating properties of the myelin sheath are due to the fact that myelin, as a lipid substance, prevents the passage of ions and therefore has a very high resistance. Due to the existence of the myelin sheath, the occurrence of excitation in the pulpy nerve fibers is possible not throughout the entire length of the axial cylinder, but only in limited areas - the nodes of Ranvier. This is essential for the propagation of the nerve impulse along the fiber.

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

Conduction of excitation in unmyelinated and myelinated nerve fibers

In amyospinous 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 only propagate in jumps, "jumping" over sections of the fiber covered with an insulating myelin sheath. Such conduct is called saltatory.

Direct electrophysiological studies carried out by Kago (1924) and later by Tasaki (1953) on single myelinated frog nerve fibers showed that action potentials in these fibers arise only at nodes, and the myelin-covered areas between nodes are practically non-excitable.

The density of sodium channels in the intercepts is very high: there are about 10,000 sodium channels per 1 μm 2 of the membrane, which is 200 times higher than their density in the membrane of the giant squid axon. The high density of sodium channels is the most important condition for the saltatory conduction of excitation. On fig. 2 shows how the "jumping" of the nerve impulse from one intercept to another occurs.

At rest, the outer surface of the excitable membrane of all nodes of Ranvier is positively charged. There is no potential difference between adjacent intercepts. At the moment of excitation, the surface of the interception membrane With becomes charged electronegatively with respect to the membrane surface of the adjacent node D. This leads to the appearance of a local (local) electric current, which goes through the interstitial fluid surrounding the fiber, the membrane and the axoplasm in the direction shown by the arrow in the figure. Coming out through the interception D the current excites it, causing the membrane to recharge. In intercept C, the excitation still continues, and it becomes refractory for a while. Therefore interception D is able to bring into a state of excitation only the next interception, etc.

"Jumping" of the action potential through the inter-nodal area is possible only because the amplitude of the action potential in each intercept is 5-6 times higher than the threshold value required to excite the adjacent intercept. Under certain conditions, the action potential can "jump" not only through one, but also through two interceptive sites - 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 pulpy 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 put forward by B.F. Verigo (1899). This method of conduction has a number of advantages compared to continuous conduction in non-fleshy fibers: firstly, by “jumping” over relatively large sections of the fiber, excitation can propagate at a much higher speed than during continuous conduction through a non-fleshy fiber of the same diameter; secondly, spasmodic propagation is energetically more economical, since not the entire membrane enters the active state, but only its small sections in the region of intercepts, which have a width of less than 1 μm. Losses of ions (per unit length of the fiber) accompanying the occurrence of an action potential in such limited areas of the membrane are very small, and, consequently, the energy costs for the operation of the sodium-potassium pump necessary to restore the changed ionic ratios between the internal contents of the nerve fiber and tissue fluid.

Laws of conducting excitation in nerves

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

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

Bilateral excitation. Along the nerve fibers, excitation is carried out both in the afferent and in the efferent direction. This feature of nerve fibers was proved by the experiments of A.I. Babukhin (1847) on the electric organ of the Nile catfish. The electric organ of the catfish consists of separate plates innervated by branches of a single axon. A.I. Babukhin removed the middle plates in order to avoid conducting excitation through the electrical organ, and cut one of the branches of the nerve. 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 holding. In the peripheral nerve, impulses propagate along each fiber in isolation, i.e. without passing from one fiber to another and exerting an effect only on those cells with which the endings of this nerve fiber come into contact. This is due to the characteristics of the myelin sheath. Possessing high resistance, it is an insulator that prevents the propagation of excitation to neighboring fibers. This is very important due to the fact that any peripheral nerve trunk contains a large number of nerve fibers - motor, sensory and vegetative, which innervate different, sometimes far from each other and heterogeneous in structure and function, cells and tissues. For example, the vagus nerve innervates all the organs of the chest cavity and a significant part of the abdominal organs, the sciatic nerve - all the muscles, bone apparatus, blood vessels and skin of the lower limb. If excitation passed inside the nerve trunk from one fiber to another, then in this case the normal isolated functioning of peripheral organs and tissues would be impossible.

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

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

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

Nerve fiber properties

The 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 the nerve fiber, a very small amount of ATP is expended to restore ionic 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 found that when a damaging agent (alteration), such as a chemical, is exposed to a nerve site, the lability of this site decreases. This is due to the blockade of sodium and potassium permeability of the membrane. Such a state of reduced lability N.E. Vvedensky named parabiosis. Parabiosis is divided into three successive phases: equalizing, paradoxical and inhibitory.

AT equalization phase the same value of the 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 - more.

Paradoxical phase It is characterized by the fact that a reaction of a greater magnitude is noted to weak stimuli than to strong ones.

AT braking phase fiber lability is reduced 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 a short-term effect on the nerve of a damaging substance, after the termination of its action, the nerve leaves the state of parabiosis and goes through similar phases, but in reverse order.

nerve fatigue

Nerve fatigue was first shown by N.E. Vvedensky (1883), who observed the preservation of the working capacity of the nerve after continuous 8-hour stimulation. Vvedensky conducted an experiment on two neuromuscular preparations of the legs of a frog. Both nerves were irritated for a long time by a rhythmic induction current of the same strength. But on one of the nerves, closer to the muscle, DC electrodes were additionally installed, with the help of which the conduction of excitation to the muscles was blocked. Thus, both nerves were irritated for 8 hours, but the excitation passed only to the muscles of one leg. After an 8-hour irritation, when the muscles of the working drug stopped contracting, the block was removed from the nerve of another drug. At the same time, his muscles contracted in response to nerve irritation. Consequently, the nerve conducting excitation to the blocked paw did not get tired, despite prolonged irritation.

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

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 not high. Therefore, the nerve conducts a smaller number of impulses per unit time than it could reproduce. This ensures its relative tirelessness.

Electrical phenomena in living tissues are associated with the difference in the concentrations of ions that carry electrical charges.

According to the generally accepted membrane theory of the origin of biopotentials, the potential difference in a living cell arises because the ions carrying electric charges are distributed on both sides of the semi-permeable cell membrane, depending on its selective permeability to different ions. The active transport of ions against the concentration gradient is carried out using the so-called ion pumps, which are a system of carrier enzymes. For this, the energy of ATP is used.

As a result of the work of ion pumps, the concentration of K + ions inside the cell is 40-50 times higher, 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. Thus it is created resting potential, at which the membrane inside the cell is negatively charged 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 the membrane potential. It includes two phases:

• Depolarization phase corresponds to a rapid change in membrane potential of about 110 mV. This is explained by the fact that at the site of excitation, the permeability of the membrane for Na + ions increases sharply, since 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 time of reaching the peak is +40 mV. During the repolarization phase, the membrane potential again reaches the resting level (the membrane repolarizes), after which hyperpolarization occurs to a value of approximately -80 mV.
• Repolarization phase potential is associated with the closing of sodium and the opening of potassium channels. Since positive charges are removed as K+ is pushed out, the membrane repolarizes. Hyperpolarization of the membrane to a level greater (more negative) than the resting potential is due to high potassium permeability in the repolarization phase. Closing of potassium channels leads to the restoration of the initial level of the membrane potential; the permeability values ​​for K + and Na + also return to the previous ones.

Conducting a nerve impulse

The potential difference that occurs between the excited (depolarized) and resting (normally polarized) sections of the fiber propagates along its entire length. In unmyelinated nerve fibers, excitation is transmitted at a speed of up to 3 m/s. On axons covered with a myelin sheath, the speed of excitation reaches 30-120 m/s. This high speed is due to the fact that the depolarizing current does not flow through the areas covered with an insulating myelin sheath (areas between nodes). The action potential here is distributed spasmodically.

The rate of conduction of an action potential along an axon is proportional to its diameter. In the fibers of the mixed nerve, it varies from 120 m/s (thick, up to 20 µm in diameter, myelinated fibers) to 0.5 m/s (the thinnest, 0.1 µm in diameter, amyelinated fibers).

Action potential or nerve impulse, a specific reaction 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 transfer data from the receptor to the nervous system, and after that it directs this information to the right 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, sequentially transmitted electrical changes are called nerve impulses.

Excitatory action and its distribution is subject to physico-chemical nature. The energy for this process is generated directly in the nerve itself. This is due to the fact that the passage of the pulse entails the formation of heat. As soon as it has passed, the fading or referential state begins. In which only a fraction of a second the nerve can not conduct a stimulus. The speed at which an impulse can arrive ranges from 3 m/s to 120 m/s.

The fibers through which the excitation passes have a specific sheath. Roughly speaking, this system resembles an electrical cable. In its composition, the sheath can be myelinated and unmyelinated. The most important component of the myelin sheath is myelin, which plays the role of an insulator.

The pulse propagation speed depends on several factors, for example, on the thickness of the fibers, and the thicker it is, the faster the speed develops. Another factor in speeding up conduction is myelin itself. But at the same time, it is not located over the entire surface, but in sections, as if strung. Accordingly, between these areas there are those that remain "naked". They carry current from the axon.

An axon is a process, with the help of which data is transmitted from one cell to the rest. This process is regulated with the help of a synapse - a direct connection between neurons or a neuron and a cell. There is also the so-called synaptic space or gap. When an irritant impulse arrives at a neuron, neurotransmitters (molecules of chemical composition) are released during the reaction. They pass through the synaptic opening, eventually falling on 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 there is no release of the neurotransmitter.

The autonomic system is provided mainly by non-myelinated tissues. Through them, excitement spreads constantly and continuously.

The principle of transmission is based on the appearance of an electric field, therefore, a potential arises that irritates the membrane of the neighboring 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 on 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 taking, then the conduction of a nerve impulse is impossible.
• Isolated holding of irritation. Excitation can be transmitted along, in no way, without spreading to neighboring ones.
• Bilateral holding. 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.
• Decrementless execution. The impulses do not subside, in other words, they are conducted without a decrement.

Chemistry of impulse conduction

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

Irritation of a neuron entails the opening of sodium channels at the site of irritation. This can facilitate the entry of positively charged particles into the interior of the cell. Accordingly, the negative charge decreases and an action potential occurs or a nerve impulse occurs. After that, the sodium channels close again.

It is often found that it is the weakening of polarization that contributes to 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 the potassium-sodium pumps are turned on, with the help of which sodium ions leave the cell, and potassium enters it.

As a result, it can be said that when electrochemical processes are resumed, impulses occur, striving along the fibers.

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

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

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

There are 5 laws of excitation:

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

Damage is:

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

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