Action potential or nerve impulse specific reaction, flowing in the form of an exciting wave and flowing throughout neural pathway. This reaction is a response to a stimulus. The main task is the transmission of data from the receptor to the nervous system, and after that it sends 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 stimulating effect and its spread is 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. For nerve impulse Calcium ions are necessary, since without this the release of the neurotransmitter does not occur.

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, therefore, 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 hesitating negative charge, for example, its decrease provokes a potential difference 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.

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 a speed of up to 3 m/s. Along axons covered with a myelin sheath, the speed of excitation reaches 30-120 m/s. This high speed is explained by the fact that the depolarizing current does not flow through the areas covered by the insulating myelin sheath (the areas between the nodes). The action potential here spreads spasmodically.

The speed of action potential along an axon is proportional to its diameter. In mixed nerve fibers it varies from 120 m/s (thick, up to 20 µm in diameter, myelinated fibers) up to 0.5 m/s (the thinnest, with a diameter of 0.1 microns, non-pulp fibers).

Neurotransmitters– these are substances that are characterized by the following characteristics:

Accumulate in the presynaptic membrane in sufficient concentration;

Released upon impulse transmission;

Cause a change in speed after binding to the postsynaptic membrane metabolic processes and the occurrence of an electrical impulse;

They have an inactivation system or a transport system for removing hydrolysis products from the synapse.

Neurotransmitters play important role in functioning nerve tissue, providing synaptic transmission nerve impulse. Their synthesis occurs in the body of neurons, and accumulation in special vesicles, which gradually move with the participation of systems of neurofilaments and neurotubules to the tips of axons.

Neurotransmitters include amino acid derivatives: taurine, norepinephrine, dopamine, GABA, glycine, acetylcholine, homocysteine ​​and some others (adrenaline, serotonin, histamine), as well as neuropeptides.

Cholinergic synapses

Acetylcholine synthesized from choline and acetyl-CoA. Choline synthesis requires the amino acids serine and methionine. But, as a rule, ready-made choline enters the nervous tissue from the blood. Acetylcholine is involved in synaptic transmission of nerve impulses. It accumulates in synaptic vesicles, forming complexes with the negatively charged protein vesiculin (Fig. 22). The transfer of excitation from one cell to another is carried out using a special synaptic mechanism.

Rice. 22. Cholinergic synapse

A synapse is a functional contact between specialized areas of the plasma membranes of two excitable cells. A synapse consists of a presynaptic membrane, a synaptic cleft, and a postsynaptic membrane. The membranes at the point of contact have thickenings in the form of plaques - nerve endings. A nerve impulse that reaches a nerve ending is unable to overcome the obstacle that has arisen in front of it - the synaptic cleft. After this, the electrical signal is converted into a chemical signal.

The presynaptic membrane contains special channel proteins similar to the proteins that form the sodium channel in the axon membrane. They also respond to the membrane potential by changing their conformation and forming a channel. As a result, Ca 2+ ions pass through the presynaptic membrane along a concentration gradient in nerve ending. The Ca 2+ concentration gradient is created by the work of the Ca 2+ -dependent ATPase. An increase in the concentration of Ca 2+ inside the nerve ending causes the fusion of acetylcholine-filled vesicles present there. Acetylcholine is then secreted into the synaptic cleft by exocytosis and binds to receptor proteins located on the surface of the postsynaptic membrane.

The acetylcholine receptor is a transmembrane oligomeric glycoprotein complex consisting of 6 subunits. The density of receptor proteins in the postsynaptic membrane is very high - about 20,000 molecules per 1 µm 2. The spatial structure of the receptor strictly corresponds to the conformation of the mediator. When interacting with acetylcholine, the receptor protein changes its conformation so that a sodium channel is formed inside it. The cation selectivity of the channel is ensured by the fact that the gate of the channel is formed by negatively charged amino acids. That. The permeability of the postsynaptic membrane to sodium increases and an impulse (or contraction of the muscle fiber) occurs. Depolarization of the postsynaptic membrane causes dissociation of the acetylcholine-protein-receptor complex, and acetylcholine is released into the synaptic cleft. Once acetylcholine is in the synaptic cleft, it undergoes rapid hydrolysis within 40 μs by the enzyme acetylcholinesterase into choline and acetyl-CoA.

Irreversible inhibition of acetylcholinesterase causes death. Enzyme inhibitors are organophosphorus compounds. Death occurs as a result of respiratory arrest. Reversible acetylcholinesterase inhibitors are used as therapeutic drugs, for example, in the treatment of glaucoma and intestinal atony.

Adrenergic synapses(Fig. 23) found in postganglionic fibers, in sympathetic fibers nervous system, in various parts of the brain. They serve as mediators catecholamines: norepinephrine and dopamine. Catecholamines in nervous tissue are synthesized according to a general mechanism from tyrosine. The key enzyme in the synthesis is tyrosine hydroxylase, which is inhibited by the end products.

Rice. 23. Adrenergic synapse

Norepinephrine– a mediator in postganglionic fibers of the sympathetic system and in various parts of the central nervous system.

Dopamine– a mediator of pathways, the bodies of neurons of which are located in the brain. Dopamine is responsible for controlling voluntary movements. Therefore, when dopaminergic transmission is disrupted, the disease parkinsonism occurs.

Catecholamines, like acetylcholine, accumulate in synaptic vesicles and are also released into the synaptic cleft upon receipt of a nerve impulse. But regulation in the adrenergic receptor occurs differently. The presynaptic membrane has a special regulatory protein– achromogranin, which, in response to an increase in the concentration of the transmitter in the synaptic cleft, binds the already released transmitter and stops its further exocytosis. There is no enzyme that destroys the transmitter in adrenergic synapses. After the impulse is transmitted, the transmitter molecules are pumped by a special transport system through active transport with the participation of ATP back into the presynaptic membrane and are reincorporated into the vesicles. In the presynaptic nerve ending, excess transmitter can be inactivated by monoamine oxidase (MAO), as well as catecholamine-O-methyltransferase (COMT) by methylation at the hydroxy group.

Signal transmission at adrenergic synapses occurs with the participation of the adenylate cyclase system. Binding of the transmitter to the postsynaptic receptor almost instantly causes an increase in the concentration of cAMP, which leads to rapid phosphorylation of proteins of the postsynaptic membrane. As a result, the generation of nerve impulses in the postsynaptic membrane is inhibited. In some cases, the immediate cause of this is an increase in the permeability of the postsynaptic membrane for potassium, or a decrease in conductivity for sodium (this condition leads to hyperpolarization).

Taurine formed from the amino acid cysteine. First, sulfur in the HS group is oxidized (the process occurs in several stages), then decarboxylation occurs. Taurine is an unusual acid in which there is no carboxyl group, but a sulfuric acid residue. Taurine takes part in the conduction of nerve impulses in the process of visual perception.

GABA – inhibitory transmitter (about 40% of neurons). GABA increases the permeability of postsynaptic membranes for potassium ions. This leads to a change in membrane potential. GABA inhibits the inhibition of “unnecessary” information: attention, motor control.

Glycine– auxiliary inhibitory transmitter (less than 1% of neurons). In terms of the effects it causes, it is similar to GABA. Its function is inhibition of motor neurons.

Glutamic acid- the main excitatory transmitter (about 40% of neurons). Main function: conducting the main flows of information in the central nervous system (sensory signals, motor commands, memory).

Normal activity of the central nervous system is ensured by a delicate balance of glutamic acid and GABA. Violation of this balance (usually in the direction of decreasing inhibition) negatively affects many nervous processes. When the balance is disturbed, attention deficit hyperactivity disorder in children (ADHD) develops, nervousness and anxiety in adults, sleep disturbance, insomnia, and epilepsy increase.

Neuropeptides contain from three to several dozen amino acid residues. They function only in the higher parts of the nervous system. These peptides function not only as neurotransmitters, but also as hormones. They transmit information from cell to cell through the circulation system. These include:

Neurohypophyseal hormones (vasopressin, liberins, statins) - they are both hormones and mediators;

Gastrointestinal peptides (gastrin, cholecystokinin). Gastrin causes a feeling of hunger, cholecystokinin causes a feeling of fullness, and also stimulates contraction of the gallbladder and pancreatic function;

Opiate-like peptides (or analgesic peptides). They are formed through reactions of limited proteolysis of the proopiocortin precursor protein. Interacts with the same receptors as opiates (for example, morphine), thereby imitating their action. Common name- endorphins. They are easily destroyed by proteinases, so their pharmacological effect is negligible;

Sleep peptides. Their molecular nature has not been established. They induce sleep;

Memory peptides (scotophobin). Accumulates during training to avoid darkness;

Peptides are components of the renin-angiotensin system. Stimulate the thirst center and the secretion of antidiuretic hormone.

The formation of peptides occurs as a result of limited proteolysis reactions; they are destroyed under the action of proteinases.

Control questions

1. Describe chemical composition brain.

2. What are the features of metabolism in nervous tissue?

3. List the functions of glutamate in nervous tissue.

4. What is the role of mediators in the transmission of nerve impulses? List the main inhibitory and excitatory mediators.

5. What are the differences in the functioning of adrenergic and cholinergic synapses?

6. Give examples of compounds that affect the synaptic transmission of nerve impulses.

7. What biochemical changes can be observed in nervous tissue during mental illness?

8. What are the features of the action of neuropeptides?

Biochemistry of muscle tissue

Muscles make up 40-50% of a person's body weight.

Distinguish three types of muscles:

Striated skeletal muscles (contract voluntarily);

Striated cardiac muscle (contracts involuntarily);

Smooth muscles (vessels, intestines, uterus) (contract involuntarily).

Striated muscle consists of numerous elongated fibers.

Muscle fiber- multinucleate cell covered with an elastic membrane - sarcolemma. Muscle fiber contains motor nerves, transmitting to it a nerve impulse that causes contraction. Along the length of the fiber in semi-liquid sarcoplasm thread-like formations are located - myofibrils. Sarcomere- repeating element of myofibril, limited by Z-line(Fig. 24). In the middle of the sarcomere there is an A-disc, dark in a phase-contrast microscope, in the center of which there is an M-line, visible under electron microscopy. The H-zone occupies the middle part
A-disc. I-disks are bright under a phase-contrast microscope, and each of them is divided into equal halves by a Z-line. A-discs contain thick myosin and thin actin filaments. Thin filaments begin at the Z-line, pass through the I-disc and are interrupted in the H-zone. Electron microscopy showed that thick filaments are arranged in a hexagonal shape and extend across the entire A-disc. Between the thick threads are thin ones. When the muscle contracts, the I-discs practically disappear, and the area of ​​overlap between the thin and thick filaments increases.

Sarcoplasmic reticulum- an intracellular membrane system of interconnected flattened vesicles and tubules that surrounds the sarcomeres of myofibrils. Its inner membrane contains proteins that can bind calcium ions.

The conduction of a nerve impulse along the fiber occurs due to the propagation of a depolarization wave along the sheath of the process. Majority peripheral nerves through their motor and sensory fibers they ensure impulse conduction at speeds of up to 50-60 m/sec. The depolarization process itself is quite passive, while the restoration of the resting membrane potential and conductivity is carried out through the functioning of NA/K and Ca pumps. For their work, ATP is required, a prerequisite for the formation of which is the presence of segmental blood flow. Cutting off the blood supply to the nerve immediately blocks the conduction of the nerve impulse.

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

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

There are 5 laws of excitation:

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

Damage occurs:

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

Also in the literature there is a division of injuries into open (cut, punctured, torn, chopped, bruised, crushed wounds) and closed (concussion, contusion, compression, sprain, rupture and dislocation) injuries of the peripheral nervous system.

319. Conduction of nerve impulses

A nerve impulse is an excitation wave propagating along a nerve fiber that occurs when a neuron is stimulated and carries a signal about a change in the environment (centripetal impulse) or a command signal in response to a change that has occurred (centrifugal impulse).

Resting potential. The occurrence and conduction of an impulse is associated with a change in the state of some structural elements of the neuron. These structures include a sodium pump, including Na^1^-ATPase, and two types of ion-conducting channels - sodium and potassium. Their interaction gives, in a state of rest, a potential difference across different sides plasma membrane axons (resting potential). The existence of a potential difference is due to 1) the high concentration of potassium ions in the cell (20-50 times higher than in the environment); 2) the fact that intracellular anions (proteins and nucleic acids) cannot leave the cell; 3) with the fact that the permeability of the membrane for sodium ions is 20 times lower than for potassium ions. The potential exists ultimately because potassium ions tend to leave the cell to equalize external and internal concentrations. But potassium ions cannot leave the cell, and this leads to the appearance of a negative charge, which inhibits further equalization of the concentrations of potassium ions.

Chlorine ions must remain outside to compensate for the charge of poorly penetrating sodium, but tend to leave the cell along the concentration gradient. To maintain the membrane potential (about 75 mV), it is necessary to maintain a difference in the concentrations of sodium and potassium ions so that sodium ions entering the cell are removed back from it in exchange for potassium ions. "This is achieved due to the action of membrane Na +, g^-ATPase, which, due to the energy of ATP, transfers sodium ions from the cell in exchange for two potassium ions taken into the cell. When abnormal high concentration sodium ions in the pump increases the Na + /K + ratio.

Thus, at rest, potassium ions move outward along the gradient. At the same time, a certain amount of potassium is returned by diffusion. The difference between these processes is compensated by the action of the K" 1 ", N8"" pump. Sodium ions enter along a gradient at a rate limited by the permeability of the membrane for them. At the same time, sodium ions are pumped out by a pump against the concentration gradient due to the energy of ATP. Action potential - sequence of processes caused in a nerve by a stimulus. Irritation of the nerve entails local depolarization of the membrane, a decrease in membrane potential. This occurs due to the entry of a certain amount of sodium ions into the cell. When the potential difference drops to threshold level(about 50 mV), the permeability of the membrane to sodium increases approximately 100 times. Sodium rushes along a gradient into the cell, extinguishing the negative charge on

inner surface

membranes.

The potential value can change from -75 at rest to +50. Not only will the negative charge on the inner surface of the membrane be extinguished, but a positive charge will appear (polarity inversion). This charge prevents further sodium from entering the cell, and the conductivity for sodium drops. The pump restores the original state. The immediate cause of these transformations is discussed below.

Changes in the concentration of calcium ions inside the axons may be associated with the movement of the action potential and its conduction. All intracellular calcium, except for a small fraction, is bound to protein (the concentration of free calcium is about 0.3 mM), while around the cell its concentration reaches 2 mM. Therefore, there is a gradient that tends to push calcium ions into the cell. The nature of the calcium extrusion pump is unclear.

It is known, however, that each calcium ion is exchanged for 3 sodium ions, which penetrate the cell at the moment the action potential increases. Sodium channel structure

has not been sufficiently studied, although a number of facts are known: 1) an essential structural element of the channel is an integral membrane protein; 2) for every square micrometer of the Ranvier interception surface there are about 500 channels; 3) during the rising phase of the action potential, approximately 50,000 sodium ions pass through the channel; 4) rapid removal of ions is possible due to the fact that for each channel in the membrane there are from 5 to 10 Na+,\K^-ATPase molecules.

Each ATPase molecule must push 5-10 thousand sodium ions out of the cell so that the next excitation cycle can begin.

Comparison of the speed of passage of molecules of different sizes made it possible to establish the diameter of the channels - approximately 0.5 nm. The diameter can increase by 0.1 nm. The rate of passage of sodium ions through the channel under real conditions is 500 times higher than the rate of passage of potassium ions and remains 12 times higher even at the same concentrations of these ions.

Spontaneous release of potassium from the cell occurs through independent channels, the diameter of which is about

The threshold level of membrane potential at which its permeability to sodium increases depends on the calcium concentration outside the cell; its decrease during hypocalcemia causes convulsions. The occurrence of an action potential and the propagation of an impulse in an unmyelinated nerve occurs due to the opening of a sodium channel. The channel is formed by integral protein molecules, its conformation changes in response to an increase in positive charge environment

. The increase in charge is associated with the entry of sodium through the adjacent channel.

In the myelinated nerve, sodium channels are concentrated in the unmyelinated nodes of Ranvier (more than ten thousand per 1 micron). In this regard, in the interception zone, the sodium flow is 10-100 times greater than on the conducting surface of the unmyelinated nerve. Na^K^-ATPase molecules in large quantities are located on adjacent sections of the nerve.

Depolarization of one of the nodes causes a potential gradient between the nodes, so the current quickly flows through the axoplasm to the adjacent node, reducing the potential difference there to a threshold level. This ensures a high speed of impulse transmission along the nerve - no less than 2 times faster than through a non-myelinated nerve (up to 50 m/s in a non-myelinated nerve and up to 100 m/s in a myelinated one). , 320.Transmission of nerve impulses - those. its spread to another cell is carried out using special structures synapses

, connecting the nerve ending and the neighboring cell. The synaptic cleft separates the cells. If the gap width is below 2 nm, signal transmission occurs by current propagation, as along the axon. In most synapses, the gap width approaches 20 nm. In these synapses, the arrival of an action potential leads to the release of a transmitter substance from the presynaptic membrane, which diffuses through the synaptic gap and binds to a specific receptor on the postsynaptic membrane, transmitting a signal to it. Mediator substances (neurotransmitters) - compounds that are present in the presynaptic structure in sufficient concentration, are released during impulse transmission, and cause an electrical impulse after binding to the postsynaptic membrane. An essential feature of a neurotransmitter is the presence of a transport system for its removal from the synapse. Moreover, this transport system

must have a high affinity for the mediator.