Synapse structure: electrical and chemical synapses. Types of synapses, features of their structure. The mechanism of transmission of excitation through the synapse. Physiological properties of synapses Synapse functions in brief

The muscle and glandular cells are transmitted through a special structural formation - the synapse.

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

Synapse structure

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

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

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

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

Postsynaptic membrane - the thickened part of the cell membrane with which the presynaptic membrane contacts. It has ion channels and is capable of generating an action potential. In addition, special protein structures are located on it - 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. Synapse structure and processes carried out during synaptic signal transmission

Synapse types

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

By location secrete neuromuscular synapses, neuro-glandular and neuro-neuronal; the latter, in turn, are divided into axo-axonal, axo-dendritic, axo-somatic, dendro-somatic, dendro-dendrotic.

By the nature of the action on the perceiving structure, synapses can be excitatory and inhibitory.

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

Table 1. Classification and types of synapses

Synapse classification and excitation transmission mechanism

Synapses are classified as follows:

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

The excitement is transmitted by mediators(intermediaries).

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

Mediator properties

Synthesized in a neuron
Accumulate at the end of the cell
Are released when the 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 primarily as an excitatory neurotransmitter and is found in various parts of the central nervous system. The mediator is located in the vesicles of the presynaptic thickening (synaptic plaque). The mediator is synthesized in the cells of the neuron and can be resynthesized from the metabolites of its cleavage in the synaptic cleft.

When the axon terminals are excited, the membrane of the synaptic plaque is depolarized, causing the influx of calcium ions from the extracellular medium 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 penetrating into the gap, the mediator diffuses to the postsynaptic membrane, which contains receptors on its surface. The interaction of a mediator with receptors causes the opening of sodium channels, which contributes to depolarization of the postsynaptic membrane and the emergence of an excitatory postsynaptic potential. In the neuromuscular synapse, this potential is called end plate potential. Local currents arise between the depolarized postsynaptic membrane and the adjacent polarized regions of the same membrane, which depolarize the membrane to a critical level with the subsequent generation of an action potential. The action potential spreads across all membranes, for example, muscle fiber and causes its contraction.

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

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

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

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 mediator is released only from the synaptic plaque and interacts with receptors on the postsynaptic membrane;
the spread of excitation through synapses is slower than along the nerve fiber (synaptic delay);
the transmission of excitement is carried out using specific mediators;
the rhythm of excitation changes in synapses;
synapses are capable of fatigue;
synapses are highly sensitive to various chemicals and hypoxia.

One-way signal conduction. 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. It is caused by a synaptic delay in signal transmission from one cell to another. The delay is caused by the time spent on the processes of mediator release, its diffusion to the postsynaptic membrane, binding to the postsynaptic membrane receptors, depolarization and conversion of 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. Such summation appears if a subsequent signal arrives at the synapse a short time later (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 arousal. 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 synapses that transmit excitation from the nerve fiber to the skeletal muscle.

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

High sensitivity of synapses to the action of biologically active substances, drugs and poisons. For example, the poison strychnine blocks the function of inhibitory synapses of the central nervous system by binding to receptors that are 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 action of biologically active substances and poisons on signal transmission in neuromuscular synapses are discussed above.

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

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

There are other mechanisms for the development of relief. This phenomenon is also called in classical manuals 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. Facilitating signal transmission underlies the development of synaptic plasticity and improved function with frequent activation.

Depression (suppression) 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 the development of the phenomenon of depression, the depletion of the stores of the mediator in the presynaptic terminal, the decrease in the sensitivity of the receptors of the postsynaptic membrane to the mediator, the development of persistent depolarization of the postsynaptic membrane, which impede the generation of AP on the membrane of the postsynaptic cell, are of importance.

Electrical synapses

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

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

They are characterized by a higher rate of excitation conduction in comparison with chemical synapses and low sensitivity to the effects of chemicals.

Electrical synapses are with one- and two-way transmission of excitation.

There are also electrical inhibitory synapses in the body. The inhibitory effect develops due to the action of the current, which causes hyperpolarization of the postsynaptic membrane.

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

Moscow PsychologicalSocial Institute (MPSI)

Abstract on the Anatomy of the central nervous system on the topic:

Synapses(structure, structure, function).

1st year student of the Faculty of Psychology,

group 21 / 1-01 Logachev A.Yu.

Teacher:

Kholodova Marina Vladimirovna.

year 2001.

Work plan:

1. Prologue.

2. Physiology of a neuron and its structure.

3. The structure and function of the synapse.

4. Chemical synapse.

5. Isolation of a pick.

6. Chemical mediators and their types.

7 Epilogue.

8. List of references.

PROLOGUE:

Our body is one big clockwork. It consists of a huge number of tiny particles that are located in strict order and each of them performs certain functions, and has its own unique properties. This mechanism - the body, consists of cells, connecting them tissues and systems: all this as a whole is a single chain, a super-system of the body. The greatest set of cellular elements could not work as a whole, if the body did not have a sophisticated regulation mechanism. The nervous system plays a special role in regulation. All the complex work of the nervous system - the regulation of the work of internal organs, the control of movements, whether they are simple and unconscious movements (for example, breathing) or complex, human hand movements - all this, in essence, is based on the interaction of cells with each other. All this, in essence, is based on the transmission of a signal from one cell to another. Moreover, each cell does its job, and sometimes has several functions. The variety of functions is provided by two factors: how cells are connected to each other, and how these connections are arranged.

PHYSIOLOGY OF THE NEURON AND ITS STRUCTURE:

The simplest reaction of the nervous system to an external stimulus is it is a reflex. First of all, let us consider the structure and physiology of the structural elementary unit of the nervous tissue of animals and humans - neuron. The functional and basic properties of a neuron are determined by its ability to excite and self-excite. The transmission of excitation is carried out along the processes of the neuron - axons and dendrites.

Axons are longer and wider processes. They have a number of specific properties: isolated conduction excitation and bilateral conduction.

Nerve cells are able not only to perceive and process external excitement, but also to spontaneously emit impulses that are not caused by external stimulation (self-excitation). In response to stimulation, the neuron responds impulse of activity- action potential, the generation frequency of which ranges from 50-60 pulses per second (for motoneurons), up to 600-800 pulses per second (for interneurons of the brain). The axon ends with many thin branches, which are called terminals. From the terminals, the impulse passes to other cells, directly to their bodies or, more often, to their dendrites. The number of terminals at the axon can reach up to one thousand, which end in different cells. On the other hand, a typical vertebrate neuron has 1,000 to 10,000 terminals from other cells.

Dendrites - shorter and more numerous processesneurons. They perceive excitation from neighboring neurons and conduct it to the cell body. Distinguish between fleshy and non-fleshy nerve cells and fibers.

Pulp fibers - are part of sensitive andmotor nerves of skeletal muscles and sensory organsThey are covered with a lipid myelin sheath. Fleshy fibers are more "fast-acting": in such fibers with a diameter of 1-3.5 micromillimeters, the excitement spreads at a speed of 3-18 m / s. This is due to the fact that the conduction of impulses along the myelinated nerve occurs abruptly. In this case, the action potential "jumps" over the area of the nerve covered with myelin and, at the site of interception of Ranvier (the bare area of the nerve), passes to the sheath of the axial cylinder of the nerve fiber. The myelin sheath is a good insulator and excludes the transmission of excitation to the junction of parallel nerve fibers.

Non-fleshy fibers - make up the bulk of the sympathetic nerves. They do not have a myelin sheath and are separated from each other by neuroglia cells.

Cells play the role of insulators in non-fleshy fibers. neuroglia(nerve supporting tissue). Schwann cells - one of the varieties of glial cells. In addition to internal neurons that perceive and transform impulses from other neurons, there are neurons that perceive influences directly from the environment - these are receptors, as well as neurons that directly affect the executive organs - effectors, for example, muscles or glands. If a neuron affects a muscle, it is called a motor neuron or motor neuron. Among neuroreceptors, 5 types of cells are distinguished, depending on the type of pathogen:

- photoreceptors, which are excited under the influence of light and provide the functioning of the organs of vision,

- mechanoreceptors, those receptors that respond to mechanical stress. They are located in the organs of hearing, balance. Tactile cells are also mechanoreceptors. Some mechanoreceptors are located in the muscles and measure the extent to which they stretch.

- chemoreceptors - they selectively react to the presence or change in the concentration of various chemicals, the work of the organs of smell and taste is based on them,

- thermoreceptors, react to a change in temperature or to its level - cold and heat receptors,

- electroreceptors react to current impulses, and are found in some fish, amphibians and mammals, for example, the platypus.

Based on the foregoing, I would like to note that for a long time among biologists who have studied the nervous system, there was an opinion that nerve cells form long complex networks that continuously merge into one another.

However, in 1875, an Italian scientist, professor of histology at the University of Pavia, came up with a new way of staining cells - silvering. When one of the thousands of adjacent cells is silvering, only it is stained - the only one, but completely, with all its processes. Golgi method greatly helped the study of the structure of nerve cells. Its use has shown that, despite the fact that the cells in the brain are located extremely close to each other, and their processes are entangled, yet each cell is clearly separated. That is, the brain, like other tissues, consists of separate cells that are not united into a common network. This conclusion was made by a Spanish histologist S. Ramon-i-Cajal, which thereby extended the cellular theory to the nervous system. The rejection of the idea of a united network meant that in the nervous system pulse passes from cell to cell not through direct electrical contact, but through break.

When the electron microscope, which was invented in 1931, began to be used in biology M. Knoll and E. Ruska, these ideas about the presence of a gap have been directly confirmed.

SYNAPSE STRUCTURE AND FUNCTIONS:

Every multicellular organism, every tissue, consisting of cells, needs mechanisms that ensure intercellular interactions. Consider how interneuronalinteractions. Information spreads along the nerve cell in the form action potentials. The transmission of excitation from axonal terminals to an innervated organ or other nerve cell occurs through intercellular structural formations - synapses(from the Greek. Synapsis-connection, communication). The concept of synapse was introduced by an English physiologist C. Sherrington in 1897 to indicate functional contact between neurons. It should be noted that back in the 60s of the last century THEM. Sechenov emphasized that outside the intercellular communication it is impossible to explain the ways of origin of even the most elementary nervous process. The more complex the nervous system is, and the greater the number of constituent nervous brain elements, the more important the importance of synaptic contacts becomes.

The various synaptic contacts are different from each other. However, with all the variety of synapses, there are certain general properties of their structure and function. Therefore, we first describe the general principles of their functioning.

Synapse is a complex structural a formation consisting of a presynaptic membrane (most often this is the terminal branching of an axon), a postsynaptic membrane (most often this is a section of the body membrane or dendrite of another neuron), as well as a synaptic cleft.

The mechanism of transmission through the synapse remained unclear for a long time, although it was obvious that the transmission of signals in the synaptic region is sharply different from the process of conducting an action potential along an axon. However, at the beginning of the 20th century, a hypothesis was formulated that synaptic transmission is carried out or electric or chemically. The electrical theory of synaptic transmission in the central nervous system was recognized until the early 1950s, but it significantly lost ground after the chemical synapse was demonstrated in a number of peripheral synapses. For example, A.V. Kibyakov, conducting an experiment on the nerve ganglion, as well as the use of microelectrode technology for intracellular registration of synaptic potentials

neurons of the central nervous system made it possible to draw a conclusion about the chemical nature of transmission in the interneuronal synapses of the spinal cord.

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

Synapse(Greek σύναψις, from συνάπτειν - to hug, clasp, shake hands) is a place of contact between two neurons or between an effector cell that receives a signal. Serves for transmission between two cells, and during synaptic transmission, the amplitude and frequency of the signal can be regulated.

The term was introduced in 1897 by the English physiologist Charles Sherrington.

Synapse structure

A typical synapse is axo-dendritic chemical. Such a synapse consists of two parts: presynaptic formed by the clavate extension of the end of the axon 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 that separates the membranes of contacting cells to which nerve endings come. 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.

There is a synaptic cleft between both parts - a 10-50 nm gap between the postsynaptic and presynaptic membranes, the edges of which are reinforced with intercellular contacts.

The part of the axolemma of the clavate expansion, adjacent to the synaptic cleft, is called presynaptic membrane... The area of the cytolemma of the receiving cell that limits the synaptic cleft on the opposite side is called postsynaptic membrane, in chemical synapses it is embossed and contains numerous.

There are small vesicles in the synaptic expansion, the so-called synaptic vesicles containing either a mediator (substance-mediator in transmission), or an enzyme that destroys this mediator. On the postsynaptic, and often on the presynaptic membranes, there are receptors for a particular mediator.

Synapse classification

Depending on the mechanism of transmission of the nerve impulse, they are distinguished

chemical;
electrical - cells are connected by highly permeable contacts using special connexons (each connexon consists of six protein subunits). The distance between the cell membranes in the electrical synapse is 3.5 nm (the usual intercellular distance is 20 nm)

Since the resistance of the extracellular fluid is small (in this case), the impulses pass without lingering through the synapse. Electrical synapses are usually excitatory.

Two release mechanisms have been discovered: with the complete fusion of the vesicle with the plasmalemma and the so-called "kissed and ran away" (eng. kiss-and-run), when the vesicle connects to the membrane, and small molecules leave it into the synaptic cleft, while the large ones remain in the vesicle. The second mechanism, presumably, is faster than the first, with the help of it synaptic transmission occurs with a high content of calcium ions in the synaptic plaque.

The consequence of this structure of the synapse is the unilateral 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 about - 0.5 ms.

The so-called "Dale principle" (one - one mediator) was recognized as erroneous. Or, as it is sometimes believed, it is specified: from one end of a cell, not one, but several mediators can be released, and their set is constant for a given cell.

Discovery history

In 1897, Sherrington formulated the concept of synapses.
For his studies of the nervous system, including synaptic transmission, Golgi and Ramon y Cajal received the Nobel Prize in 1906.
In 1921 the Austrian scientist O. Loewi established the chemical nature of the transmission of excitation through synapses and the role of acetylcholine in it. Received the Nobel Prize in 1936 together with H. Dale (N. Dale).
In 1933 the Soviet scientist A.V. Kibyakov established the role of adrenaline in synaptic transmission.
1970 - B. Katz (V. Katz, Great Britain), W. von Euler (U. v. Euler, Sweden) and J. Axelrod (USA) received the Nobel Prize for the discovery of rolinoradrenaline in synaptic transmission.

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

At the end of the 19th century, two theories of the organization of the nervous system (NS) existed in parallel. Reticular theory believed that NS is a functional syncytium: neurons are connected by processes like capillaries of the circulatory system. According to cell theory of Waldeyer(1981) NS consists of separate, separated by membranes, neurons. To solve the problem of the interaction of individual neurons, Sherrington in 1987 suggested the presence of a special membrane formation - synapse... Using an electron microscope, the presence of synapses was unconditionally 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 in the NS of crustaceans (electrical synapse).

Synapse- This is a membrane formation of two (or more) cells, in which the transfer of excitation (information) from one cell to another takes place.

There is the following classification of synapses:

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

Chemical;

Electric (efaps);

Mixed.

2) according to the released neurotransmitter:

Adrenergic - the neurotransmitter norepinephrine;

Cholinergic - the neurotransmitter acetylcholine;

Dopaminergic - the neurotransmitter dopamine;

Serotonergic - the neurotransmitter serotonin;

GABAergic - the neurotransmitter gamma-aminobutyric acid (GABA)

3) by influence:

Exciting;

Brake.

4) by location:

Neuromuscular;

Neuro-neuronal:

a) axo-somatic;

b) axo-axonal;

c) axo-dendric;

d) dendrosomatic.

Consider three types of synapses: chemical, electric 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, Sat); the final membrane of the SB is different from other sections of the neuron membrane and is called presynaptic membrane(PresM); the specialized membrane of the second cell is designated the postsynaptic membrane (PostSM); between the membranes of the synapse is synaptic cleft(SS, Fig. 1, 2).

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

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

Rice. 2. Diagram of electrical synapse

ES are more specialized compared to chemical synapses and provide a high excitation transfer rate... However, he, apparently, is 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 work of Claude Bernard, who in 1850 published an article "Research on the curare". Here 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 the sense that it can be introduced with impunity into the digestive tract of humans or animals, while injecting it under the skin or in 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, and the animals do not scream and show no signs of pain. " Although C. Bernard did not come to the idea of the chemical transmission of nerve impulses, his classical 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 a receptive substance. For the first time, the assumption of the transfer of excitation from a nerve to an effector organ with the help of a chemical was put forward by T. Eliot (1904).

However, only the works of G. Dale and O. Loewy have finally confirmed the hypothesis of the chemical synapse. Dale in 1914 established that the stimulation of the parasympathetic nerve is imitated by acetylcholine. Loewy in 1921 proved that acetylcholine is secreted from the nerve endings of the vagus nerve, and in 1926 he discovered acetylcholinesterase, an enzyme that destroys acetylcholine.

Excitation at a chemical synapse is transmitted by mediator... This process includes several stages. Let us consider these features using the example of the acetylcholine synapse, which is widespread in the central nervous system, autonomic and peripheral nervous system (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 a mediator in one vesicle is several thousand molecules ( mediator quantum). Some of the vesicles are located on the PresM and are ready to release the mediator (Fig. 3, 2).

3. The mediator is released by exocytosis when preSM is excited. Incoming current plays an important role in rupture of membranes and quantum release of the transmitter. Ca 2+ (Fig. 3, 3).

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

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

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

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

Form start

take 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 the curare is precisely the AX synapse. Curare binds more tightly to the acetylcholine receptor and deprives it of interaction with the mediator (ACh). Excitation from somatic nerves to skeletal muscles, including from the phrenic nerve to the main respiratory muscle (diaphragm) is transmitted using the AX, therefore, curare causes relaxation (relaxation) of the muscles and cessation of breathing (which, in fact, causes death).

Let's note the main features of the transfer of excitation in a chemical synapse.

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

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

3. In the chemical synapse occurs temporary delay in conduction of excitement, 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. The transformation of excitations occurs in the chemical synapse: the electrochemical nature of excitation on the PreSM continues into the biochemical process of exocytosis of synaptic vesicles and the 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 multistage transmission of excitation should have significant biological significance. Please note that at each of the stages, regulation of the excitation transfer process is possible. Despite the limited number of mediators (a little more than a dozen), in the chemical synapse there are conditions for a wide variety in deciding the fate of the nervous excitement coming to the synapse. The totality of the features of chemical synapses explains the individual biochemical diversity of nervous and mental processes.

Now let's dwell on two important processes taking place in the postsynaptic space. We noted that as a result of the interaction of ACh with the receptor on PostSM, both depolarization and hyperpolarization can develop. What determines whether the mediator will be exciting or inhibitory? The result of the interaction of the mediator and the receptor determined by the properties of the receptor protein(one more important property of the chemical synapse - PostSM is active in relation to the excitation coming to it). In principle, a chemical synapse is a dynamic formation, by changing the receptor, the cell that receives the excitation can influence its further fate. If the properties of the receptor are such that its interaction with the mediator opens Na + channels, then at the allocation of one quantum of the mediator on the PostSM develops a local potential(for the neuromuscular synapse, it is called the miniature potential of the end plate - IPPC).

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

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

Rice. 4. Potentials of the postsynaptic membrane

Naturally, in the case of TPSP development, further propagation of excitation can be stopped. Another option for terminating the arousal process is presynaptic inhibition. If an inhibitory synapse is formed on the membrane of the synaptic plaque, then as a result of the hyperpolarization of the PreSM, the exocytosis of the 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 phosphorylation. These biochemical reactions can "descend" deep into the cytoplasm down to the neuron nucleus, regulating the processes of protein synthesis. Thus, a nerve cell can respond to the excitement that has come not only by deciding its further fate (to respond to EPSP or TPSP, i.e. to carry out or not to carry it out further), but to change the number of receptors, or synthesize a receptor protein with new properties in relation to a certain mediator. Therefore, another important property of the 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 synapses is determined by the mediator, more precisely the name of the receptor for a specific mediator. Therefore, we will consider the classification of the main mediators and receptors of the nervous system (see also the material handed out at the lecture !!).

We have already noted that the effect of the interaction of a mediator and a receptor is determined by the properties of the receptor. Therefore, the known mediators, with the exception of g-aminobutyric acid, can function as both excitatory and inhibitory mediators. The following groups of mediators are distinguished according to their chemical structure.

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

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

Rice. 6. Serotonin (5-hydroxytryptamine) molecule

Three other neurotransmitters are synthesized from the essential amino acid phenylalanine, therefore they are united by a common name catecholamines- this is 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 only an inhibitory mediator), glycine, glutamic acid, aspartic acid.

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

Sleep delta peptide got its name for the ability to induce 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 excitement and mediators of reinforcement (joy and pleasure). In other words, our brains are 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 released when we enjoy ourselves. Do everything with pleasure, make your endogenous factory synthesize opiates! We are naturally given this opportunity from birth - the overwhelming majority of neurons are reactive to positive reinforcement.

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

In principle, the history of neurotransmitters is not yet complete; there are a number of substances that are involved in the regulation of nervous excitement. 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.

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 on the discipline "Neurophysiology and the basics of GNI"

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

Transmission of excitation in the synapse "

Completed by student 13L group

1 course OZO (3) A.I. Sharova

Checked:

professor of medical sciences

O.A. Belova

Ryazan 2010

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

2. The structure and functions of the synapse …………………………………… ... 6

3. Transfer of excitation in the synapse ………………………………… .8

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

5. Isolation of a mediator ………………………………………… ... 10

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

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

8. References ……………………………………………… .... 17

Introduction.

Synapse structure and function.

Every multicellular organism, every tissue, consisting of cells, needs mechanisms that ensure intercellular interactions. Consider how interneuronalinteractions. Information spreads along the nerve cell in the form action potentials. The transmission of excitation from axonal 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 an English physiologist C. Sherrington in 1897 to indicate functional contact between neurons. It should be noted that back in the 60s of the last century THEM. Sechenov emphasized that outside the intercellular communication it is impossible to explain the ways of origin of even the most elementary nervous process. The more complex the nervous system is, and the greater the number of constituent nervous brain elements, the more important the importance of synaptic contacts becomes.

Synapse - is a complex structural formation, consisting of

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

postsynaptic membrane - an electrogenic membrane of an 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 membranes, filled with a fluid that resembles blood plasma in composition

Synapses can be between two neurons (interneuronal), between neuron and muscle fiber (neuromuscular), between receptor formations and processes of sensory neurons (receptor-neural), between the processes of the neuron 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 located 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) dendrodentritic (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 the motor neuron and the muscle cell;

b) neuro-epithelial, formed by the axon of the neuron and the secretory cell.

2. Functional classification of synapses:

1) excitatory synapses;

2) inhibitory synapses.

Synapse exciting- a synapse in which the postsynaptic membrane is excited; an exciting postsynaptic potential arises in it and the excitation that has come to the synapse spreads further.

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

3. By the mechanisms of transmission of excitation in synapses:

1) chemical;

2) electrical;

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 the electrical synapse.

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

a) cholinergic.

b) adrenergic.

c) dopaminergic. In them, arousal is transmitted with the help of dopamine;

d) histaminergic. In them, the transfer of excitement occurs with the help of histamine;

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

Synapse adrenergic - a synapse in which norepinephrine is a mediator. In it, excitement is transmitted with the help of three catecholamines; distinguish between 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, contraction of the uterus; b1 - adrenergic synapses - increased heart function; b2 - adrenergic - bronchial dilatation.

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

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

In n-cholinergic the synapse, the postsynaptic membrane is sensitive to nicotine. This type of synapses form neuromuscular synapses of the somatic nervous system, ganglionic synapses, synapses of the sympathetic and parasympathetic nervous systems, synapses of the central nervous system.

Synapse electrical- in it, excitation from the pre- to postsynaptic membrane is transmitted electrically, i.e. an efaptic transfer of excitation occurs - the action potential reaches the presynaptic end and then spreads through the intercellular channels, causing depolarization of the postsynaptic membrane. In the electrical synapse, the mediator is not produced, the synaptic cleft is small (2 - 4 nm) and it contains protein bridges-channels, 1 - 2 nm wide, along which ions and small molecules move. This contributes to the low resistance of the postsynaptic membrane. This type of synapse occurs much less frequently than chemical synapses and differ from them in a higher rate of excitation transfer, high reliability, and the possibility of two-way excitation.

Synapses have a number of physiological properties :

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

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

3) potentiation property(each subsequent impulse will be carried out with a smaller postsynaptic delay). This is due to the fact that a mediator from the previous impulse remains on the presynaptic and postsynaptic membranes;

4) low lability of the synapse(100-150 pulses per second).

Transfer of excitement at the synapse.

The mechanism of transmission through the synapse remained unclear for a long time, although it was obvious that the transmission of signals in the synaptic region is sharply different from the process of conducting an action potential along an axon. However, at the beginning of the 20th century, a hypothesis was formulated that synaptic transmission is carried out or electric or chemically. The electrical theory of synaptic transmission in the central nervous system was recognized until the early 1950s, but it significantly lost ground after the chemical synapse was demonstrated in a number of peripheral synapses. For example, A.V. Kibyakov, After conducting an experiment on the nervous 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 the interneuronal synapses of the spinal cord.

If electrical synapses are characteristic of the nervous system of more primitive animals (nervous diffusion system of coelenterates, some synapses of cancer and annelids, synapses of the nervous system of fish), although they are found in the brain of mammals. In all of the above cases, the pulses are transmitted through 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, impulses can be transmitted 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 more synapses.

However, in this work, I would like to dwell in more detail on synapses with a chemical transmission mechanism, which make up a large part 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 interactions of cells, and are also associated with a number of pathological processes and change their properties under the influence of certain drugs.