Functions and types of nerve fibers. nerve impulses

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.

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).

Conduction of nerve impulses along nerve fibers and through synapses. The high-voltage potential that occurs when a receptor is excited in a nerve fiber is 5-10 times greater than the receptor's irritation threshold. Conducting an excitation wave along the nerve fiber is ensured by the fact that each subsequent section of it is irritated by the high-voltage potential of the previous section. In the fleshy nerve fibers, this potential does not spread continuously, but abruptly; he jumps over one or even several interceptions of Ranvier, in which he strengthens. The duration of the excitation between two adjacent interceptions of Ranvier is equal to 5-10% of the duration of the high-voltage potential.

Conduction of a nerve impulse along a nerve fiber occurs only under the condition of its anatomical continuity and its normal physiological state. Violation of the physiological properties of the nerve fiber by severe cooling or poisoning with poisons and drugs stops the conduction of the nerve impulse even with its anatomical continuity.

Nerve impulses are conducted in isolation along individual motor and sensory nerve fibers that are part of the mixed nerve, which depends on the insulating properties of the myelin sheaths covering them. In non-fleshy nerve fibers, the biocurrent propagates continuously along the fiber and, thanks to the connective tissue sheath, does not pass from one fiber to another. A nerve impulse can propagate along a nerve fiber in two directions: centripetal and centrifugal. Therefore, there are three rules for conducting a nerve impulse in nerve fibers: 1) anatomical continuity and physiological integrity, 2) isolated conduction, and 3) bilateral conduction.

2-3 days after the separation of the nerve fibers from the body of the neuron, they begin to regenerate, or degenerate, and the conduction of nerve impulses stops. Nerve fibers and myelin are destroyed and only the connective tissue sheath is preserved. If the cut ends of the nerve fibers, or nerve, are connected, then after the degeneration of those areas that are separated from the nerve cells, restoration, or regeneration, of the nerve fibers begins from the bodies of the neurons, from which they grow into the preserved connective tissue membranes. Regeneration of nerve fibers leads to the restoration of impulse conduction.

Unlike nerve fibers, nerve impulses are conducted through the neurons of the nervous system in only one direction - from the receptor to the working organ. It depends on the nature of the conduction of the nerve impulse through the synapses. In the nerve fiber above the presynaptic membrane there are many tiny vesicles of acetylcholine. When the biocurrent reaches the presynaptic membrane, some of these vesicles burst, and acetylcholine passes through the smallest holes in the presynaptic membrane into the synaptic cleft.
There are sites in the postsynaptic membrane that have a special affinity for acetylcholine, which causes the temporary appearance of pores in the postsynaptic membrane, which makes it temporarily permeable to ions. As a result, excitation and a high-voltage potential arise in the postsynaptic membrane, which propagates along the next neuron or innervated organ. Therefore, the transmission of excitation through the synapses occurs chemically through the mediator, or mediator, acetylcholine, and the conduction of excitation along the next neuron is again carried out electrically.

The action of acetylcholine on the conduction of a nerve impulse through the synapse is short-lived; it is quickly destroyed, hydrolyzed by the enzyme cholinesterase.

Since the chemical transmission of a nerve impulse in a synapse occurs within a fraction of a millisecond, in each synapse the nerve impulse is delayed for this time.

Unlike nerve fibers, in which information is transmitted according to the “all or nothing” principle, that is, discretely, in synapses, information is transmitted according to the “more or less” principle, that is, gradually. The more the mediator acetylcholine is formed up to a certain limit, the higher the frequency of high-voltage potentials in the subsequent neuron. After this limit, excitation turns into inhibition. Thus, the digital information transmitted along the nerve fibers passes in synapses into measuring information. measuring electronic machines,

in which there are certain relationships between actually measured quantities and the quantities that they represent, are called analog, working on the principle of "more or less"; we can assume that a similar process takes place in synapses and its transition to digital occurs. Consequently, the nervous system functions according to a mixed type: both digital and analog processes are performed in it.

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.

synapses- these are structures designed to transmit an impulse from one neuron to another or to muscle and glandular structures. Synapses provide polarization of impulse conduction along the chain of neurons. Depending on the method of impulse transmission synapses can be chemical or electrical (electrotonic).

Chemical synapses transmit an impulse to another cell with the help of special biologically active substances - neurotransmitters located in synaptic vesicles. The axon terminal is the presynaptic part, and the area of the second neuron, or other innervated cell with which it contacts, is the postsynaptic part. The area of synaptic contact between two neurons consists of the presynaptic membrane, the synaptic cleft, and the postsynaptic membrane.

Electrical or electrotonic synapses in the nervous system of mammals are relatively rare. In the area of such synapses, the cytoplasm of neighboring neurons is connected by slot-like junctions (contacts), which ensure the passage of ions from one cell to another, and, consequently, the electrical interaction of these cells.

The speed of impulse transmission by myelinated fibers is greater than by unmyelinated ones. Thin fibers, poor in myelin, and non-myelinated fibers conduct a nerve impulse at a speed of 1-2 m/s, while thick myelin fibers - at a speed of 5-120 m/s.

In a non-myelinated fiber, the wave of membrane depolarization goes along the entire axolemma without interruption, while in a myelinated fiber it occurs only in the area of interception. Thus, myelin fibers are characterized by saltatory conduction of excitation, i.e. jumping. Between the intercepts there is an electric current, the speed of which is higher than the passage of the depolarization wave along the axolemma.

№ 36 Comparative characteristics of the structural organization of the reflex arcs of the somatic and autonomic nervous system.

reflex arc- this is a chain of nerve cells, necessarily including the first - sensitive and the last - motor (or secretory) neurons. The simplest reflex arcs are two- and three-neuron, closing at the level of one segment of the spinal cord. In a three-neuron reflex arc, the first neuron is represented by a sensitive cell, which moves first along the peripheral process, and then along the central process, heading towards one of the nuclei of the dorsal horn of the spinal cord. Here, the impulse is transmitted to the next neuron, the process of which is directed from the posterior horn to the anterior, to the cells of the nuclei (motor) of the anterior horn. This neuron performs a conductive (conductor) function. It transmits an impulse from a sensitive (afferent) neuron to a motor (efferent) neuron. The body of the third neuron (efferent, effector, motor) lies in the anterior horn of the spinal cord, and its axon is part of the anterior root, and then the spinal nerve extends to the working organ (muscle).

With the development of the spinal cord and the brain, the connections in the nervous system became more complex. formed multineuron complex reflex arcs, the construction and functions of which involve nerve cells located in the overlying segments of the spinal cord, in the nuclei of the brain stem, hemispheres, and even in the cerebral cortex. The processes of nerve cells that conduct nerve impulses from the spinal cord to the nuclei and cortex of the brain and in the opposite direction form bundles, fasciculi.