Nerve synapses. Synapse structure: electrical and chemical synapses The substance that promotes the organization of the synapse is called

Federal Agency for Education

State educational institution

higher professional education

"Ryazan State University named after S.A. Yesenin"

Institute of Psychology, Pedagogy and Social Work

Test work in the discipline “Neurophysiology and fundamentals of VND”

on the topic: “The concept of a synapse, the structure of a synapse.

Transmission of excitation in the synapse"

Completed by a student of group 13L

1st year OZO (3) A.I. Sharova

Checked:

professor of medical sciences

O.A. Belova

Ryazan 2010

1. Introduction……………………………………………………………..3

2. Structure and functions of the synapse……………………………………...6

3. Transmission of excitation at the synapse………………………………….8

4. Chemical synapse……………………………………………………………9

5. Isolation of the mediator……………………………………………...10

6. Chemical mediators and their types……………………………..12

7. Conclusion……………………………………………………………15

8. List of references………………………………………………………………....17

Introduction.

Our body is one big clockwork mechanism. It consists of a huge number of tiny particles that are located in in strict order and each of them performs certain functions and has its own unique properties. This mechanism - the body, consists of cells, connecting their tissues and systems: all this as a whole represents a single chain, a supersystem of the body. The greatest variety of cellular elements could not work as a single whole if a sophisticated regulatory mechanism did not exist in the body. The nervous system plays a special role in regulation. All the complex work of the nervous system - regulating the work of internal organs, controlling movements, whether simple and unconscious movements (for example, breathing) or complex movements of a person's hands - all this, in essence, is based on the interaction of cells with each other. All this is essentially based on the transmission of a signal from one cell to another. Moreover, each cell does its own job, and sometimes has several functions. The variety of functions is provided by two factors: the way cells are connected to each other, and the way these connections are arranged. The transition (transfer) of excitation from a nerve fiber to the cell it innervates (nerve, muscle, secretory) occurs through a specialized formation called a synapse.

Structure and functions of the synapse.

Every multicellular organism, every tissue consisting of cells needs mechanisms that ensure intercellular interactions. Let's look at how they are carried out interneuronalinteractions. Information travels along a nerve cell in the form action potentials. The transfer of excitation from axon terminals to an innervated organ or other nerve cell occurs through intercellular structural formations - synapses (from the Greek “Synapsis” - connection, connection). The concept of synapse was introduced by the English physiologist C. Sherrington in 1897, to denote the functional contact between neurons. It should be noted that back in the 60s of the last century THEM. Sechenov emphasized that without intercellular communication it is impossible to explain the methods of origin of even the most elementary nervous process. The more complex the nervous system is, and the larger number components of the nerve brain elements, the more important the importance of synaptic contacts becomes.

Different synaptic contacts differ from each other. However, with all the diversity of synapses, there are certain common properties of their structure and function. Therefore, we first describe the general principles of their functioning.

Synapse - is a complex structural formation consisting of

presynaptic membrane - electrogenic membrane at the axon terminal, forms a synapse on the muscle cell (most often this is the terminal branch of the axon)

postsynaptic membrane - the electrogenic membrane of the innervated cell on which a synapse is formed (most often this is a section of the body membrane or dendrite of another neuron)

synaptic cleft - the space between the presynaptic and postsynaptic membrane, filled with fluid, which in composition resembles blood plasma

Synapses can be between two neurons (interneuronal), between neuron and muscle fiber (neuromuscular), between receptor formations and processes of sensory neurons (receptor-neuronal), between neuron processes and other cells ( glandular).

There are several classifications of synapses.

1. By localization:

1) central synapses;

2) peripheral synapses.

Central synapses lie within the central nervous system and are also found in the ganglia of the autonomic nervous system.

Central synapses– these are contacts between two nerve cells, and these contacts are heterogeneous and, depending on the structure on which the first neuron forms a synapse with the second neuron, they are distinguished:

a) axosomatic, formed by the axon of one neuron and the body of another neuron;

b) axodendritic, formed by the axon of one neuron and the dendrite of another;

c) axoaxonal (the axon of the first neuron forms a synapse on the axon of the second neuron);

d) dendrodentrite (the dendrite of the first neuron forms a synapse on the dendrite of the second neuron).

There are several types peripheral synapses:

a) myoneural (neuromuscular), formed by the axon of a motor neuron and a muscle cell;

b) neuroepithelial, formed by the axon of a neuron and a secretory cell.

2. Functional classification of synapses:

1) excitatory synapses;

2) inhibitory synapses.

excitatory synapse- synapse in which the postsynaptic membrane is excited; an excitatory postsynaptic potential arises in it and the excitation that comes to the synapse spreads further.

Inhibitory synapse- A. Synapse, on the postsynaptic membrane of which an inhibitory postsynaptic potential arises, and the excitation that comes to the synapse does not spread further; B. excitatory axo-axonal synapse, causing presynaptic inhibition.

3. According to the mechanisms of excitation transmission in synapses:

1) chemical;

2) electric;

3) mixed

Peculiarity chemical synapses lies in the fact that the transfer of excitation is carried out using a special group of chemicals - mediators. It is more specialized than an electrical synapse.

There are several types chemical synapses, depending on the nature of the mediator:

a) cholinergic.

b) adrenergic.

c) dopaminergic. They transmit excitement using dopamine;

d) histaminergic. They transmit excitation with the help of histamine;

e) GABAergic. In them, excitation is transmitted with the help of gamma-aminobutyric acid, i.e., the process of inhibition develops.

Adrenergic synapse - synapse, the mediator of which is norepinephrine. It transmits excitation with the help of three catecholamines; There are a1-, b1-, and b2 - adrenergic synapses. They form neuroorgan synapses of the sympathetic nervous system and synapses of the central nervous system. Excitation of a-adrenoreactive synapses causes vasoconstriction and uterine contraction; b1- adrenoreactive synapses - increased heart function; b2 - adrenoreactive - dilation of the bronchi.

Cholinergic synapse - the mediator in it is acetylcholine. They are divided into n-cholinergic and m-cholinergic synapses.

In m-cholinergic At the synapse, the postsynaptic membrane is sensitive to muscarine. These synapses form neuroorgan synapses of the parasympathetic system and synapses of the central nervous system.

In n-cholinergic At the synapse, the postsynaptic membrane is sensitive to nicotine. This type of synapse is formed by neuromuscular synapses of the somatic nervous system, ganglion synapses, synapses of the sympathetic and parasympathetic nervous system, and synapses of the central nervous system.

Electrical synapse- in it, excitation from the pre- to the postsynaptic membrane is transmitted electrically, i.e. ephaptic transmission of excitation occurs - the action potential reaches the presynaptic terminal and then spreads through intercellular channels, causing depolarization of the postsynaptic membrane. In an electrical synapse, the transmitter is not produced, the synaptic cleft is small (2 - 4 nm) and there are protein bridges-channels, 1 - 2 nm wide, along which ions and small molecules move. This contributes to low postsynaptic membrane resistance. This type of synapse is much less common than chemical synapses and differs from them in a higher speed of excitation transmission, high reliability, and the possibility of two-way conduction of excitation.

Synapses have a number of physiological properties :

1) valve property of synapses, i.e., the ability to transmit excitation in only one direction from the presynaptic membrane to the postsynaptic;

2) synaptic delay property, due to the fact that the rate of excitation transmission decreases;

3) potentiation property(each subsequent impulse will be conducted with a shorter postsynaptic delay). This is due to the fact that the transmitter from the previous impulse remains on the presynaptic and postsynaptic membrane;

4) low synapse lability(100–150 pulses per second).

Transmission of excitation at the synapse.

The mechanism of transmission across synapses remained unclear for a long time, although it was obvious that signal transmission in the synaptic region differs sharply from the process of conducting an action potential along the axon. However, at the beginning of the 20th century, a hypothesis was formulated that synaptic transmission occurs either electric or chemically. The electrical theory of synaptic transmission in the central nervous system was recognized until the early 50s, but it lost ground significantly after the chemical synapse was demonstrated in a number of cases. peripheral synapses. For example, A.V. Kibyakov, Having conducted an experiment on the nerve ganglion, as well as the use of microelectrode technology for intracellular recording of the synaptic potential of CNS neurons, it was possible to draw a conclusion about the chemical nature of transmission in interneuronal synapses of the spinal cord.

Microelectrode studies recent years showed that at certain interneuron synapses there is an electrical transmission mechanism. It has now become obvious that there are synapses with both a chemical transmission mechanism and an electrical one. Moreover, in some synaptic structures both electrical and chemical transmission mechanisms function together - these are the so-called mixed synapses.

If electrical synapses are characteristic of the nervous system of more primitive animals (nervous diffusion system of coelenterates, some synapses of crayfish and annelids, synapses of the nervous system of fish), although they are found in the brain of mammals. In all the above cases, impulses are transmitted via depolarizing the action of an electric current that is generated in the presynaptic element. I would also like to note that in the case of electrical synapses, impulse transmission is possible in both one and two directions. Also in lower animals contact between presynaptic And postsynaptic element is carried out through just one synapse - monosynaptic form of communication, however, in the process of phylogenesis there is a transition to polysynaptic form of communication, that is, when the above contact is made through a larger number of synapses.

However, in this work, I would like to dwell in more detail on synapses with a chemical transmission mechanism, which make up the majority of the synaptic apparatus of the central nervous system of higher animals and humans. Thus, chemical synapses, in my opinion, are especially interesting, since they provide very complex cell interactions and are also associated with a number of pathological processes and change their properties under the influence of certain medications.

Synapse(Greek synapsis contact, connection) - a specialized zone of contact between the processes of nerve cells and other excitable and non-excitable cells, ensuring the transmission of an information signal. Morphologically, a synapse is formed by the contacting membranes of two cells. The membrane belonging to the processes of nerve cells is called presynaptic, the membrane of the cell to which the signal is transmitted is called postsynaptic. In accordance with the affiliation of the postsynaptic membrane of the synapse, they are divided into neurosecretory, neuromuscular and interneuronal. The term “synapse” was introduced in 1897 by the English physiologist Charles Sherrington.

A synapse is a special structure that ensures the transmission of a nerve impulse from a nerve fiber to some other nerve cell or nerve fiber, also from a receptor cell to a nerve fiber (the area of ​​contact of nerve cells with each other and with another nerve cell). To form a synapse, 2 cells are required.

Synapse structure

A typical synapse is axo-dendritic chemical. Such a synapse consists of two parts: presynaptic, formed by the club-shaped extension of the axon terminal of the transmitting cell, and postsynaptic, represented by the contacting area of ​​the cytolemma of the receiving cell (in this case, the area of ​​the dendrite). A synapse is a space separating the membranes of contacting cells to which nerve endings approach.

The transmission of impulses is carried out chemically with the help of mediators or electrically through the passage of ions from one cell to another. Between both parts there is a synaptic cleft, the edges of which are strengthened by intercellular contacts. The part of the axolemma of the clavate extension adjacent to the synaptic cleft is called presynaptic membrane. A section of the cytolemma of the receptive cell, limiting the synaptic cleft with opposite side, called postsynaptic membrane, in chemical synapses it is prominent and contains numerous receptors. In synaptic expansion there are small vesicles, so-called synaptic vesicles, containing either a mediator (a substance that mediates the transmission of excitation) or an enzyme that destroys this mediator. On the postsynaptic and presynaptic membranes there are receptors for one or another mediator.

Classifications of synapses

Depending on the mechanism of nerve impulse transmission, there are

• chemical;
• electric- cells are connected by highly permeable contacts using special connexons (each connexon consists of six protein subunits). The distance between cell membranes in the electrical synapse is 3.5 nm (usual intercellular distance is 20 nm); Since the resistance of the extracellular fluid is low (in this case), impulses pass through the synapse without delay. Electrical synapses are usually excitatory.
• mixed synapses: The presynaptic action potential produces a current that depolarizes the postsynaptic membrane of a typical chemical synapse where the pre- and postsynaptic membranes are not tightly adjacent to each other. Thus, at these synapses, chemical transmission serves as a necessary reinforcing mechanism. The first type is the most common.

Chemical synapses can be classified according to their location and belonging to the corresponding structures:

• peripheral
  • neuromuscular
  • neurosecretory (axo-vasal)
  • receptor-neuronal
• central
  • axo-dendritic - with dendrites, incl.
  • axo-spinous - with dendritic spines, outgrowths on dendrites;
  • axo-somatic - with the bodies of neurons;
  • axo-axonal - between axons;
  • dendro-dendritic - between dendrites;

Depending on the mediator, synapses are divided into

• aminergic, containing biogenic amines (for example, serotonin, dopamine;) o including adrenergic, containing adrenaline or norepinephrine;
• cholinergic, containing acetylcholine;
• purinergic, containing purines;
• peptidergic, containing peptides. At the same time, only one transmitter is not always produced at the synapse. Usually the main pick is released along with another one that plays the role of a modulator.

By action sign:

• stimulating
• brake

If the former contribute to the occurrence of excitation in the postsynaptic cell (in them, as a result of the arrival of an impulse, depolarization of the membrane occurs, which can cause an action potential under certain conditions), then the latter, on the contrary, stop or prevent its occurrence and prevent further propagation of the impulse. Typically inhibitory are glycinergic (mediator - glycine) and GABAergic synapses (mediator - gamma-aminobutyric acid).

Thus, inhibitory synapses are of two types:

1. a synapse in the presynaptic endings of which a transmitter is released, hyperpolarizing the postsynaptic membrane and causing the appearance of an inhibitory postsynaptic potential;
2. axo-axonal synapse, providing presynaptic inhibition.

Cholinergic synapse (s. cholinergica) - a synapse in which acetylcholine is the mediator. Some synapses have a postsynaptic seal, an electron-dense area made of proteins. Based on its presence or absence, synapses are distinguished as asymmetric and symmetric. It is known that all glutamatergic synapses are asymmetrical, while GABAergic synapses are symmetrical. In cases where several synaptic extensions come into contact with the postsynaptic membrane, multiple synapses are formed. Special forms of synapses include spiny apparatus, in which short single or multiple protrusions of the postsynaptic membrane of the dendrite contact the synaptic extension. Spine apparatuses significantly increase the number of synaptic contacts on a neuron and, consequently, the amount of information processed. Non-spine synapses are called sessile synapses. For example, all GABAergic synapses are sessile.

The mechanism of functioning of the chemical synapse When the presynaptic terminal is depolarized, voltage-sensitive calcium channels open, calcium ions enter the presynaptic terminal and trigger the fusion of synaptic vesicles with the membrane, as a result of which the transmitter enters the synaptic cleft and connects with receptor proteins of the postsynaptic membrane, which are divided into metabotropic and ionotropic. The former are associated with the G-protein and trigger a cascade of reactions of intracellular signal transmission, the latter are associated with ion channels that open when a neurotransmitter binds to them, which leads to a change in membrane potential.

The mediator acts for a very short time, after which it is destroyed by a specific enzyme. For example, in cholinergic synapses, the enzyme that destroys the transmitter in the synaptic cleft is acetylcholinesterase. At the same time, part of the transmitter can move through the postsynaptic membrane (direct uptake) and in the opposite direction through the presynaptic membrane (reverse uptake). In some cases, the mediator is also absorbed by neighboring neuroglial cells. Two release mechanisms have been discovered: with complete fusion of the vesicle with the plasmalemma and the so-called “kiss-and-run”, when the vesicle connects to the membrane, and small molecules exit from it into the synaptic cleft, while large ones remain in the vesicle . The second mechanism is presumably faster than the first, with the help of it synaptic transmission occurs when the content of calcium ions in the synaptic plaque is high. The consequence of this structure of the synapse is the one-sided conduction of the nerve impulse.

There is a so-called synaptic delay - the time required for the transmission of a nerve impulse. Its duration is 0.5 ms. The so-called “Dale principle” (one neuron - one transmitter) has been recognized as erroneous. Or, as is sometimes believed, it is more precise: not one, but several mediators can be released from one end of a cell, and their set is constant for a given cell.

Muscle and glandular cells are transmitted through a special structural formation - a synapse.

Synapse- a structure that ensures the conduction of a signal from one to another. The term was introduced by the English physiologist C. Sherrington in 1897.

Synapse structure

Synapses consist of three main elements: the presynaptic membrane, the postsynaptic membrane and the synaptic cleft (Fig. 1).

Rice. 1. Structure of the synapse: 1 - microtubules; 2 - mitochondria; 3 - synaptic vesicles with a transmitter; 4 - presynaptic membrane; 5 - postsynaptic membrane; 6 - receptors; 7 - synaptic cleft

Some elements of synapses may have other names. For example, a synaptic plaque is a synapse between, an end plate is a postsynaptic membrane, a motor plaque is the presynaptic ending of an axon on a muscle fiber.

Presynaptic membrane covers the expanded nerve ending, which is a neurosecretory apparatus. The presynaptic part contains vesicles and mitochondria that provide mediator synthesis. Mediators are deposited in granules (bubbles).

Postsynaptic membrane - the thickened part of the cell membrane with which the presynaptic membrane is in contact. It has ion channels and is capable of generating action potentials. In addition, it contains special protein structures - receptors that perceive the action of mediators.

Synaptic cleft is a space between the presynaptic and postsynaptic membranes, filled with a liquid similar in composition to.

Rice. The structure of the synapse and the processes carried out during synaptic signal transmission

Types of synapses

Synapses are classified by location, nature of action, and method of signal transmission.

By location They distinguish neuromuscular synapses, neuroglandular and neuroneuronal; the latter, in turn, are divided into axo-axonal, axo-dendritic, axo-somatic, dendro-somatic, dendro-dendrotic.

By the nature of the action Synapses on a perceptive structure can be excitatory or inhibitory.

By signal transmission method Synapses are divided into electrical, chemical, and mixed.

Table 1. Classification and types of synapses

Classification of synapses and mechanism of excitation transmission

Synapses are classified as follows:

• by location - peripheral and central;
• by the nature of their action - exciting and inhibitory;
• by signal transmission method - chemical, electrical, mixed;
• according to the mediator through which transmission is carried out - cholinergic, adrenergic, serotonergic, etc.

Excitement is transmitted through mediators(intermediaries).

Mediators- molecules of chemical substances that ensure the transmission of excitation in synapses. In other words, chemical substances involved in the transfer of excitation or inhibition from one excitable cell to another.

Properties of mediators

• Synthesized in a neuron
• Accumulate at the end of the cell
• Released when Ca2+ ion appears in the presynaptic terminal
• Have a specific effect on the postsynaptic membrane

By chemical structure Mediators can be divided into amines (norepinephrine, dopamine, serotonin), amino acids (glycine, gamma-aminobutyric acid) and polypeptides (endorphins, enkephalins). Acetylcholine is known mainly as an excitatory neurotransmitter and is found in various parts of the central nervous system. The transmitter is located in the vesicles of the presynaptic thickening (synaptic plaque). The mediator is synthesized in neuron cells and can be resynthesized from metabolites of its cleavage in the synaptic cleft.

When axon terminals are excited, the membrane of the synaptic plaque depolarizes, causing calcium ions to flow from the extracellular environment into the nerve ending through calcium channels. Calcium ions stimulate the movement of synaptic vesicles to the presynaptic membrane, their fusion with it and the subsequent release of the transmitter into the synaptic cleft. After penetration into the gap, the transmitter diffuses to the postsynaptic membrane containing receptors on its surface. The interaction of the transmitter with the receptors causes the opening of sodium channels, which contributes to the depolarization of the postsynaptic membrane and the appearance of an excitatory postsynaptic potential. At the neuromuscular synapse this potential is called end plate potential. Local currents arise between the depolarized postsynaptic membrane and the adjacent polarized sections of the same membrane, which depolarize the membrane to a critical level, followed by the generation of an action potential. The action potential spreads across all membranes of, for example, a muscle fiber and causes its contraction.

The transmitter released into the synaptic cleft binds to the receptors of the postsynaptic membrane and is cleaved by the corresponding enzyme. Thus, cholinesterase destroys the neurotransmitter acetylcholine. After this, a certain amount of mediator breakdown products enters the synaptic plaque, where acetylcholine is resynthesized from them again.

The body contains not only excitatory, but also inhibitory synapses. According to the mechanism of excitation transmission, they are similar to excitatory synapses. At inhibitory synapses, a transmitter (for example, gamma-aminobutyric acid) binds to receptors on the postsynaptic membrane and promotes opening in it. In this case, the penetration of these ions into the cell is activated and hyperpolarization of the postsynaptic membrane develops, causing the appearance of an inhibitory postsynaptic potential.

It has now been found that one mediator can bind to several different receptors and induce different reactions.

Chemical synapses

Physiological properties of chemical synapses

Synapses with chemical transmission of excitation have certain properties:

• excitation is carried out in one direction, since the transmitter is released only from the synaptic plaque and interacts with receptors on the postsynaptic membrane;
• the spread of excitation through synapses occurs more slowly than along the nerve fiber (synaptic delay);
• transmission of excitation is carried out using specific mediators;
• the rhythm of excitation changes in synapses;
• synapses can become tired;
• synapses are highly sensitive to various chemicals and hypoxia.

One-way signal transmission. The signal is transmitted only from the presynaptic membrane to the postsynaptic membrane. This follows from the structural features and properties of synaptic structures.

Slow signal transmission. Caused by a synaptic delay in signal transmission from one cell to another. The delay is caused by the time required for the processes of release of the transmitter, its diffusion to the postsynaptic membrane, binding to the receptors of the postsynaptic membrane, depolarization and conversion of the postsynaptic potential into AP (action potential). The duration of the synaptic delay ranges from 0.5 to 2 ms.

The ability to summarize the effect of signals arriving at the synapse. This summation appears if the subsequent signal arrives at the synapse a short time (1-10 ms) after the previous one. In such cases, the EPSP amplitude increases and a higher AP frequency can be generated on the postsynaptic neuron.

Transformation of the rhythm of excitement. The frequency of nerve impulses arriving at the presynaptic membrane usually does not correspond to the frequency of APs generated by the postsynaptic neuron. The exception is the synapses that transmit excitation from the nerve fiber to the skeletal muscle.

Low lability and high fatigue of synapses. Synapses can conduct 50-100 nerve impulses per second. This is 5-10 times less than the maximum AP frequency that nerve fibers can reproduce when electrically stimulated. If nerve fibers are considered practically tireless, then at synapses fatigue develops very quickly. This occurs due to depletion of transmitter reserves, energy resources, development of persistent depolarization of the postsynaptic membrane, etc.

High sensitivity of synapses to biological action active substances, drugs and poisons. For example, the poison strychnine blocks the function of inhibitory synapses in the central nervous system by binding to receptors sensitive to the mediator glycine. Tetanus toxin blocks inhibitory synapses, disrupting transmitter release from the presynaptic terminal. In both cases, life-threatening phenomena develop. Examples of the effect of biologically active substances and poisons on signal transmission at neuromuscular synapses are discussed above.

Facilitation and depression properties of synoptic transmission. Facilitation of synaptic transmission occurs when nerve impulses arrive at the synapse after a short time (10-50 ms) one after another, i.e. often enough. Moreover, over a certain period of time, each subsequent PD arriving at the presynaptic membrane causes an increase in the content of the transmitter in the synaptic cleft, an increase in the amplitude of EPSPs and an increase in the efficiency of synaptic transmission.

One of the mechanisms of facilitation is the accumulation of Ca 2 ions in the presynaptic terminal. It takes several tens of milliseconds for the calcium pump to remove the portion of calcium that entered the synaptic terminal upon arrival of the AP. If at this time a new action potential arrives, then new portion calcium enters the terminal and its effect on neurotransmitter release is combined with the residual amount of calcium that the calcium pump did not have time to remove from the neuroplasm of the terminal.

There are other mechanisms for the development of relief. This phenomenon is also called in classical textbooks on physiology post-tetanic potentiation. Facilitation of synaptic transmission is important in the functioning of memory mechanisms, for the formation of conditioned reflexes and learning. Facilitation of signal transmission underlies the development of synaptic plasticity and improvement of their functions with frequent activation.

Depression (inhibition) of signal transmission in synapses develops when very frequent (for a neuromuscular synapse more than 100 Hz) nerve impulses arrive at the presynaptic membrane. In the mechanisms of development of the phenomenon of depression, depletion of transmitter reserves in the presynaptic terminal, a decrease in the sensitivity of receptors of the postsynaptic membrane to the transmitter, and the development of persistent depolarization of the postsynaptic membrane, which complicate the generation of APs on the membrane of the postsynaptic cell, are important.

Electrical synapses

In addition to synapses with chemical transmission of excitation, the body has synapses with electrical transmission. These synapses have a very narrow synaptic cleft and reduced electrical resistance between the two membranes. Due to the presence of transverse channels between the membranes and low resistance, electrical impulse easily passes through membranes. Electrical synapses are usually characteristic of cells of the same type.

As a result of exposure to a stimulus, the presynaptic action potential excites the postsynaptic membrane, where a propagating action potential occurs.

They are characterized by a higher speed of excitation compared to chemical synapses and low sensitivity to the effects of chemicals.

Electrical synapses have one- and two-way transmission of excitation.

Electrical inhibitory synapses are also found in the body. The inhibitory effect develops due to the action of a current that causes hyperpolarization of the postsynaptic membrane.

In mixed synapses, excitation can be transmitted using both electrical impulses and mediators.

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The connection between two adjacent neurons (nerve cells) is called a synapse. Synapses are connections that connect one neuron (presynaptic) to another (postsynaptic). Essentially, synapses are small constrictions. There is no physical connection between cells. Small densities, called synaptic knobs, at the end of each presynaptic axon approach the dendrites, axons, or postsynaptic cell bodies. It is through synaptic cones that neurotransmitters come out.

Neurotransmitters

Neurotransmitters are molecules that act as chemical signals, transmitting electrical impulses from one cell to another. They are located at the synapses between the synaptic pathways of one neuron and the dendrites of another. Chemical substances, which allow the smooth transmission of impulses through neurons, are called excitatory neurotransmitters. Inhibitory neurotransmitters block electrical impulses.

Connection between two neurons

Anatomy of a synapse

At the end of the axon there is a synaptic cone. It does not touch the neighboring neuron, but leaves a small gap, or synapse, between the pre- and postsynaptic membranes. Mitochondria in the axon produce the energy needed to release neurotransmitters. They reside in small vesicles (cavities) before exiting through the presynaptic lattice, crossing the cleft, and moving to the postsynaptic membrane.

How do synapses work?

1 The nerve impulse enters the synaptic cone of the neuron.

2 Neurotransmitters are released at the synapse.

3 Neurotransmitters quickly pass through the gap, and the molecules land on receptors on the membrane of the postsynaptic neuron.

4 This causes changes in the permeability of the postsynaptic membrane to sodium ions, and its positive ions pass into the postsynaptic neuron, causing depolarization. As a result nerve impulse passed on to the next neuron.

I.A. Borisova

Lecture 2. Physiology of synapses: structure, classification and mechanisms of activity. Mediators, neurochemical basis of behavior.

At the end of the 19th century, there were two parallel theories of the organization of the nervous system (NS). Reticular theory believed that the NS is a functional syncytium: neurons are connected by processes, similar to the capillaries of the circulatory system. According to Waldeyer's cell theory(1981) The NS consists of individual neurons separated by membranes. To resolve the issue of interaction between individual neurons, Sherrington in 1987 he suggested the presence of a special membrane formation - synapse. Using an electron microscope, the presence of synapses was unequivocally confirmed. However, the cellular theory of the structure of the NS became generally accepted; ironically, in 1959 Fershpan and Potter discovered a synapse with gap junctions (electrical synapse) in the NS of crustaceans.

Synapse is a membrane formation of two (or more) cells in which excitation (information) is transferred from one cell to another.

There is the following classification of synapses:

1) by the mechanism of excitation transmission (and by structure):

Chemical;

Electrical (ephaps);

Mixed.

2) according to the released neurotransmitter:

Adrenergic – neurotransmitter norepinephrine;

Cholinergic – neurotransmitter acetylcholine;

Dopaminergic – the neurotransmitter dopamine;

Serotonergic – neurotransmitter serotonin;

GABAergic – neurotransmitter gamma-aminobutyric acid (GABA)

3) by influence:

Exciting;

Brake.

4) by location:

Neuromuscular;

Neuro-neural:

a) axo-somatic;

b) axo-axonal;

c) axo-dendritic;

d) dendrosomatic.

Let's consider three types of synapses: chemical, electrical and mixed(combining the properties of chemical and electrical synapses).

Regardless of the type, synapses have common structural features: the nerve process at the end forms an extension ( synaptic plaque, SB); the terminal membrane of the SB is different from other parts of the neuron membrane and is called presynaptic membrane(PreSM); the specialized membrane of the second cell is designated the postsynaptic membrane (PostSM); located between the membranes of the synapse synaptic cleft(SCH, Fig. 1, 2).

Rice. 1. Scheme of the structure of a chemical synapse

Electrical synapses(ephapses, ES) are today found in the NS of not only crustaceans, but also mollusks, arthropods, and mammals. ES have a number of unique properties. They have a narrow synaptic cleft (about 2-4 nm), due to which excitation can be transmitted electrochemically (as through a nerve fiber due to EMF) at high speed and in both directions: both from PreSM membrane to PostSM, and from PostSM to PreSM. Between cells there are gap junctions (connexes or connexons), formed by two connexin proteins. The six subunits of each connexin form the PreSM and PostSM channels, through which cells can exchange low-molecular-weight substances molecular weight 1000-2000 Dalton. The work of connexons can be regulated by Ca 2+ ions (Fig. 2).

Rice. 2. Diagram of an electrical synapse

ES have greater specialization compared to chemical synapses and provide high excitation transmission speed. However, it appears to be deprived of the possibility of a more subtle analysis (regulation) of the transmitted information.

Chemical synapses dominate the NS. The history of their study begins with the works of Claude Bernard, who in 1850 published the article “Research on Curare.” This is what he wrote: “Curare is a strong poison prepared by some peoples (mostly cannibals) living in the forests... of the Amazon.” And further, “Curare is similar to snake venom in that it can be introduced with impunity into the digestive tract of humans or animals, while injection under the skin or into any part of the body quickly leads to death. ...after a few moments the animals lie down as if they were tired. Then breathing stops and their sensitivity and life disappear, without the animals uttering a cry or showing any signs of pain.” Although C. Bernard did not come to the idea of ​​chemical transmission of nerve impulses, his classic experiments with curare allowed this idea to arise. More than half a century passed when J. Langley established (1906) that the paralyzing effect of curare is associated with a special part of the muscle, which he called the receptive substance. The first suggestion about the transfer of excitation from a nerve to an effector organ using a chemical substance was made by T. Eliot (1904).

However, only the works of G. Dale and O. Löwy finally approved the hypothesis of chemical synapse. Dale in 1914 established that irritation of the parasympathetic nerve is imitated by acetylcholine. Löwy proved in 1921 that acetylcholine is released from the nerve ending of the vagus nerve, and in 1926 he discovered acetylcholinesterase, an enzyme that destroys acetylcholine.

Excitation in a chemical synapse is transmitted using mediator. This process includes several stages. Let us consider these features using the example of acetylcholine synapse, which is widespread in the central nervous system, autonomic and peripheral nervous systems (Fig. 3).

Rice. 3. Scheme of the functioning of a chemical synapse

1. The mediator acetylcholine (ACh) is synthesized in the synaptic plaque from acetyl-CoA (acetyl-coenzyme A is formed in mitochondria) and choline (synthesized by the liver) using acetylcholine transferase (Fig. 3, 1).

2. The pick is packed in synaptic vesicles ( Castillo, Katz; 1955). The amount of mediator in one vesicle is several thousand molecules ( mediator quantum). Some of the vesicles are located on the PreSM and are ready for mediator release (Fig. 3, 2).

3. The mediator is released by exocytosis upon excitation of the PreSM. The incoming current plays an important role in membrane rupture and quantum release of the transmitter. Sa 2+ (Fig. 3, 3).

4. Released pick binds to a specific receptor protein PostSM (Fig. 3, 4).

5. As a result of the interaction between the mediator and the receptor ionic conductivity changes PostSM: when Na + channels open, depolarization; the opening of K + or Cl - channels leads to hyperpolarization(Fig. 3, 5).

6 . Following depolarization, biochemical processes are launched in the postsynaptic cytoplasm (Fig. 3, 6).

7. The receptor is freed from the mediator: ACh is destroyed by acetylcholinesterase (AChE, Fig. 3. 7).

Beginning of the form

Please note that the mediator normally interacts with a specific receptor with a certain strength and duration. Why is curare poison? The site of action of curare is precisely the ACh synapse. Curare binds more firmly to the acetylcholine receptor and deprives it of interaction with the neurotransmitter (ACh). Excitation from somatic nerves to skeletal muscles, including from the phrenic nerve to the main respiratory muscle (diaphragm) is transmitted with the help of ACh, so curare causes muscle relaxation and cessation of breathing (which, in fact, causes death).

Let's note the main Features of excitation transmission in a chemical synapse.

1. Excitation is transmitted using a chemical intermediary - a mediator.

2. Excitation is transmitted in one direction: from PreSm to PostSm.

3. At the chemical synapse occurs temporary delay in conducting excitation, therefore the synapse has low lability.

4. The chemical synapse is highly sensitive to the action of not only mediators, but also other biologically active substances, drugs and poisons.

5. In a chemical synapse, a transformation of excitations occurs: the electrochemical nature of excitation on the PreSM continues into the biochemical process of exocytosis of synaptic vesicles and binding of a mediator to a specific receptor. This is followed by a change in the ionic conductivity of the PostSM (also an electrochemical process), which continues with biochemical reactions in the postsynaptic cytoplasm.

In principle, such a multi-stage transfer of excitation should have significant biological significance. Please note that at each stage it is possible to regulate the process of excitation transfer. Despite the limited number of mediators (a little more than a dozen), in a chemical synapse there are conditions for a wide variety in deciding the fate of nerve excitation coming to the synapse. The combination of features of chemical synapses explains the individual biochemical diversity of nervous and mental processes.

Now let us dwell on two important processes occurring in the postsynaptic space. We noted that as a result of the interaction of ACh with the receptor on the PostSM, both depolarization and hyperpolarization can develop. What determines whether a mediator will be excitatory or inhibitory? The result of the interaction between a mediator and a receptor determined by the properties of the receptor protein(Another important property chemical synapse - PostSM is active in relation to excitation coming to it). In principle, a chemical synapse is a dynamic formation; by changing the receptor, the cell receiving the excitation can influence it future fate. If the properties of the receptor are such that its interaction with the transmitter opens Na + channels, then when by isolating one quantum of the mediator on the PostSM, local potential develops(for the neuromuscular junction it is called the miniature end plate potential - MEPP).

When does PD occur? PostSM excitation (excitatory postsynaptic potential - EPSP) arises as a result of the summation of local potentials. You can select two types of summation processes. At sequential release of several mediator quanta at the same synapse(water wears away stone) arises temporaryA I'm summation. If quanta mediators are released simultaneously at different synapses(there can be several thousand of them on the membrane of a neuron) occurs spatial summation. Repolarization of the PostSM membrane occurs slowly and after the release of individual quanta of the mediator, the PostSM is in a state of exaltation for some time (the so-called synaptic potentiation, Fig. 4). Perhaps, in this way, synapse training occurs (the release of transmitter quanta in certain synapses can “prepare” the membrane for a decisive interaction with the transmitter).

When K + or Cl - channels open on the PostSM, an inhibitory postsynaptic potential (IPSP, Fig. 4) appears.

Rice. 4. Post-synaptic membrane potentials

Naturally, if IPSP develops, further propagation of excitation can be stopped. Another option for stopping the excitation process is presynaptic inhibition. If an inhibitory synapse is formed on the membrane of a synaptic plaque, then as a result of hyperpolarization of the PreSM, the exocytosis of synaptic vesicles can be blocked.

The second important process is the development of biochemical reactions in the postsynaptic cytoplasm. A change in the ionic conductivity of PostSM activates the so-called secondary messengers (intermediaries): cAMP, cGMP, Ca 2+ -dependent protein kinase, which in turn activate various protein kinases by phosphorylating them. These biochemical reactions can “descend” deep into the cytoplasm all the way to the nucleus of the neuron, regulating the processes of protein synthesis. Thus, a nerve cell can respond to incoming excitation not only by deciding its further fate (respond with an EPSP or IPSP, i.e., carry out or not carry on further), but change the number of receptors, or synthesize a receptor protein with new properties in relation to a certain to the mediator. Consequently, another important property of a chemical synapse: thanks to the biochemical processes of the postsynaptic cytoplasm, the cell prepares (learns) for future interactions.

A variety of synapses function in the nervous system, which differ in mediators and receptors. The name of the synapse is determined by the mediator, or more precisely, by the name of the receptor for a specific mediator. Therefore, let’s consider the classification of the main mediators and receptors of the nervous system (see also the material distributed at the lecture!!).

We have already noted that the effect of interaction between the mediator and the receptor is determined by the properties of the receptor. Therefore, known mediators, with the exception of g-aminobutyric acid, can perform the functions of both excitatory and inhibitory mediators. According chemical structure The following groups of mediators are distinguished.

Acetylcholine, widely distributed in the central nervous system, is a mediator in cholinergic synapses of the autonomic nervous system, as well as in somatic neuromuscular synapses (Fig. 5).

Rice. 5. Acetylcholine molecule

Known two types of cholinergic receptors: nicotine ( H-cholinergic receptors) and muscarinics ( M-cholinergic receptors). The name was given to the substances that cause an effect similar to acetylcholine in these synapses: N-cholinomimetic is nicotine, A M-cholinomimetic- fly agaric toxin Amanita muscaria ( muscarine). H-cholinergic receptor blocker (anticholinergic) is d-tubocurarine(the main component of curare poison), and M-anticholinergic is a belladonna toxin of Atropa belladonna – atropine. Interestingly, the properties of atropine have long been known and there was a time when women used atropine from belladonna to cause dilation of the visual pupils (to make the eyes dark and “beautiful”).

The following four main mediators have similarities in chemical structure, so they are classified as monoamines. This serotonin or 5-hydroxytryptamins (5-HT), plays an important role in the mechanisms of reinforcement (the hormone of joy). It is synthesized from the essential amino acid for humans - tryptophan (Fig. 6).

Rice. 6. Serotonin (5-hydroxytryptamine) molecule

Three other mediators are synthesized from the essential amino acid phenylalanine, and therefore are united under the common name catecholamines- This dopamine (dopamine), norepinephrine (norepinephrine) and adrenaline (epinephrine, Fig. 7).

Rice. 7. Catecholamines

Among amino acids mediators include gamma-aminobutyric acid(g-AMK or GABA - known as the only inhibitory neurotransmitter), glycine, glutamic acid, aspartic acid.

Mediators include a number of peptides. In 1931, Euler discovered a substance in extracts of the brain and intestines that causes contraction of intestinal smooth muscles and dilation of blood vessels. This transmitter was isolated in its pure form from the hypothalamus and was named substance P(from the English powder - powder, consists of 11 amino acids). It was later established that substance P plays an important role in the conduction of painful excitations (the name did not have to be changed, since pain in English is pain).

Delta sleep peptide received its name for its ability to cause slow, high-amplitude rhythms (delta rhythms) in the electroencephalogram.

A number of protein mediators of a narcotic (opiate) nature are synthesized in the brain. These are pentapeptides Met-enkephalin And Leu-enkephalin, and endorphins. These are the most important blockers of pain excitations and mediators of reinforcement (joy and pleasure). In other words, our brain is a great factory endogenous drugs. The main thing is to teach the brain to produce them. "How?" - you ask. It's simple - endogenous opiates are produced when we experience pleasure. Do everything with pleasure, force your endogenous factory to synthesize opiates! We are naturally given this opportunity from birth - the vast majority of neurons are reactive to positive reinforcement.

Research in recent decades has made it possible to discover another very interesting mediator - nitric oxide (NO). It turned out that NO not only plays an important role in regulating the tone of blood vessels (the nitroglycerin you know is a source of NO and dilates coronary vessels), but is also synthesized in neurons of the central nervous system.

In principle, the history of mediators is not over yet; there are a number of substances that are involved in the regulation of nervous excitation. It’s just that the fact of their synthesis in neurons has not yet been precisely established, they have not been found in synaptic vesicles, and receptors specific to them have not been found.