Synapse structure
In synaptic expansion there are small vesicles, the 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 often on the presynaptic membranes, there are receptors for one or another mediator.
Classifications of synapses
Depending on the mechanism of nerve impulse transmission, there are
- electrical - 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.
For nervous system In mammals, electrical synapses are less common than chemical ones.
- 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 most common are chemical synapses.
Chemical synapses can be classified according to their location and belonging to the corresponding structures:
- peripheral
- 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;
- axo-dendritic- with dendrites, incl.
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.
Present at some synapses postsynaptic condensation- electron-dense zone consisting of proteins. Based on its presence or absence, synapses are distinguished asymmetrical And symmetrical. It is known that all glutamatergic synapses are asymmetric, and GABAergic synapses are symmetrical.
In cases where several synaptic extensions are in contact with the postsynaptic membrane, multiple synapses.
Special forms of synapses include spinous 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, the transmitter enters the synaptic cleft and attaches to 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 intracellular signal transduction reactions. The latter are associated with ion channels, which 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 with the help of carrier proteins across the postsynaptic membrane (direct uptake) and in the opposite direction through the presynaptic membrane (reverse uptake). In some cases, the transmitter is also taken up by neighboring neuroglial cells.
Two release mechanisms have been discovered: with 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 exit it into the synaptic cleft, while large molecules 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 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.
PNS: Schwann cells Neurolemma Node of Ranvier/Internodal segment Myelin notching
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Synonyms:See what “Synapse” is in other dictionaries:
- (from the Greek synapsis connection) the area of contact (connection) of nerve cells (neurons) with each other and with the cells of the executive organs. Interneuron synapses are usually formed by the branches of the axon of one nerve cell and the body, dendrites or axon... Big Encyclopedic Dictionary
In neural networks, communication between formal neurons. The output signal from the neuron enters the synapse, which transmits it to another neuron. Complex synapses can have memory. See also: Neural networks Financial Dictionary Finam... Financial Dictionary
synapse- A specialized zone of contact between neurons (interneuron synapse) or between neurons and other excitable formations (organ synapse), ensuring the transfer of excitation with the preservation, change or disappearance of its information... ... Technical Translator's Guide
Moscow Psychological Social Institute (MSSI)
Abstract on the Anatomy of the Central Nervous System on the topic:
SYNAPSES (structure, structure, functions).
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 the neuron and its structure.
3.Structure and functions of the synapse.
4.Chemical synapse.
5. Isolation of the mediator.
6. Chemical mediators and their types.
7.Epilogue.
8. List of references.
PROLOGUE:
Our body is one big clockwork mechanism. It consists of a huge amount 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 hard work nervous system - regulation of work internal organs, control of 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.
PHYSIOLOGY OF THE NEURON AND ITS STRUCTURE:
The simplest reaction of the nervous system to an external stimulus is it's a reflex. First of all, let us consider the structure and physiology of the structural elementary unit of 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 capable of not only perceiving and processing external stimulation, but also spontaneously producing 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 impulses per second (for motor neurons) to 600-800 impulses per second (for interneurons of the brain). The axon ends in many thin branches called terminals. From the terminals, the impulse passes to other cells, directly to their bodies or, more often, to their dendritic processes. The number of terminals in an axon can reach up to one thousand, which end in different cells. On the other hand, a typical vertebrate neuron has between 1,000 and 10,000 terminals from other cells.
Dendrites - shorter and more numerous processes neurons. They perceive excitation from neighboring neurons and conduct it to the cell body. There are pulpy and non-pulpate nerve cells and fibers.
Pulp fibers are part of the sensitive and motor nerves of skeletal muscles and sensory organs They are covered with a lipid myelin sheath. Pulp fibers are more “fast-acting”: in such fibers with a diameter of 1-3.5 micromillimeters, excitation spreads at a speed of 3-18 m/s. This is explained by the fact that the conduction of impulses along the myelinated nerve occurs spasmodically. In this case, the action potential “jumps” through the area of the nerve covered with myelin and at the node of Ranvier (the exposed area of the nerve), it passes to the sheath of the axial cylinder of the nerve fiber. The myelin sheath is a good insulator and prevents the transmission of excitation to the junction of parallel nerve fibers.
Non-muscle fibers make up the bulk of the sympathetic nerves. They do not have a myelin sheath and are separated from each other by neuroglial cells.
In pulpless fibers, cells act as insulators. neuroglia(nervous supporting tissue). Schwann cells - one of the types of glial cells. In addition to internal neurons that perceive and transform impulses coming from other neurons, there are neurons that perceive influences directly from environment- This receptors, as well as neurons that directly affect the executive organs - effectors, for example, on muscles or glands. If a neuron acts on a muscle, it is called a motor neuron or motor neuron. Among neuroreceptors, there are 5 types of cells, 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 influences. They are located in the organs of hearing and balance. Touch cells are also mechanoreceptors. Some mechanoreceptors are located in muscles and measure the degree of their stretch.
- chemoreceptors - selectively react to the presence or change in concentration of various chemicals, the work of the organs of smell and taste is based on them,
- thermoreceptors, react to changes in temperature or its level - cold and heat receptors,
- electroreceptors react to current impulses, and are present in some fish, amphibians and mammals, for example, the platypus.
Based on the above, I would like to note that for a long time Among biologists who studied the nervous system, there was an opinion that nerve cells form long complex networks that continuously flow into one another.
However, in 1875, an Italian scientist, professor of histology at the University of Pavia, came up with new way cell staining - silvering. When one of the thousands of nearby cells turns silver, 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 showed that, despite the fact that the cells in the brain are located extremely close to each other, and their processes are confused, each cell is still clearly separated. That is, the brain, like other tissues, consists of individual cells that are not united into a common network. This conclusion was made by a Spanish histologist S. Ramon y Cahalem, who thereby extended the cell theory to the nervous system. The rejection of the concept of a connected network meant that in the nervous system pulse passes from cell to cell not through direct electrical contact, but through gap
When did the electron microscope, which was invented in 1931, begin to be used in biology? M. Knollem And E. Ruska, these ideas about the presence of a gap received direct confirmation.
STRUCTURE AND FUNCTION OF SYNAPSE:
Every multicellular organism, every tissue consisting of cells needs mechanisms that ensure intercellular interactions. Let's look at how they are carried out interneuronal interactions. 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 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 greater the number of constituent neural 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 general properties their structures and functions. Therefore, we first describe general principles their functioning.
Synapse - is a complex structural a formation consisting of a presynaptic membrane (most often this is the terminal branch 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 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 synaptic potentials
neurons of the central nervous system allowed us to conclude that chemical nature transmission at 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.
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
Based on their 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 departments CNS. 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 through 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 the action of biologically active substances, medicines 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 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 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 two membranes. Due to the presence of transverse channels between the membranes and low resistance, an electrical impulse easily passes through the 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.
Synapse (synapse, from the Greek synapsys - connection): specialized intercellular contacts through which cells of the nervous system (neurons) transmit a signal (nerve impulse) to each other or to non-neuronal cells. Information in the form of action potentials comes from the first cell, called presynaptic, to the second, called postsynaptic (Fig. 129, Fig. 130). Typically, a synapse is understood as a chemical synapse in which signals are transmitted using neurotransmitters.
At synapses, electrical signals are converted into chemical signals and back - chemical into electrical signals. Thus, a synapse is a place of functional contact between neurons, in which information is transferred from one cell to another. There are axodendritic synapses and axosomatic synapses.
Typical synapses are formations formed by the axon terminals of one neuron and the dendrites of another (axodendritic synapses). But there are other types: axosomatic, axoaxonal and dendrodendritic. The synapse between a motor neuron axon and a skeletal muscle fiber is called the motor end plate, or neuromuscular junction.
There are two types of synapses in the nervous system: excitatory and inhibitory synapses. At excitatory synapses, one cell causes another to fire. In this case, the excitatory transmitter causes depolarization - a flow of Na+ ions rushes into the cell. In inhibitory synapses, one cell inhibits the activation of another. This is due to the fact that the inhibitory transmitter causes a flow of negatively charged ions into the cells, so depolarization does not occur.
The nerve impulse enters the synapse through the presynaptic terminal, which is limited by the presynaptic membrane (presynaptic part) and is perceived by the postsynaptic membrane (postsynaptic part). Between the membranes there is a synaptic cleft. The presynaptic terminal contains many mitochondria and presynaptic vesicles containing transmitter. A nerve impulse entering the presynaptic terminal causes the release of a transmitter into the synaptic cleft. Transmitter molecules react with specific receptor proteins of the cell membrane, changing its permeability to certain -ions, which leads to the occurrence of an action potential (see Fig. 130). Along with chemical synapses, there are electrotonic synapses, in which impulses are transmitted directly by bioelectrical pathways between contacting cells.
Depending on the nature of the signals passing through the synapses, synapses are divided into
A synapse is the place of contact of one neuron with another, which is affected by the innervated organ.
Types of synapses:
· At the place of contacts (neuronal, axodendritic, dendrodendritic, axomal, axosamal, dendrosomal, neuromuscular, neurosecretory)
· Excitatory and inhibitory
· Chemical (conduct an impulse in one direction) and electrical (conduct a nerve impulse in any direction, narrower synaptic cleft, fast speed conduction, are found in invertebrates and lower vertebrates).
Structure.
1. Pedsynaptic section
2. Synaptic cleft
3. Postsynaptic section
4. Visicles - bubbles with a mediator
5. Mediaor – Chemical substance, which either conducts excitation or blocks it
The postsynaptic membrane contains receptors that are sensitive to this type of transmitter. In most synapses, the postsynaptic membrane is folded to increase the surface area.
Role in conducting.
Excitation through synapses is transmitted chemically with the help of a special substance - an intermediary, or transmitter, located in synaptic vesicles located in the presynaptic terminal. Different transmitters are produced at different synapses. Most often it is acetylcholine, adrenaline or norepinephrine.
There are also electrical synapses. They are distinguished by a narrow synaptic cleft and the presence of transverse channels crossing both membranes, i.e. there is a direct connection between the cytoplasms of both cells. The channels are formed by protein molecules of each membrane, connected in a complementary manner. The pattern of excitation transmission in such a synapse is similar to the pattern of action potential transmission in a homogeneous nerve conductor.
In chemical synapses, the mechanism of impulse transmission is as follows. The arrival of a nerve impulse at the presynaptic terminal is accompanied by the synchronous release of a transmitter into the synaptic cleft from synaptic vesicles located in close proximity to it. Typically, a series of impulses arrive at the presynaptic terminal; their frequency increases with increasing strength of the stimulus, leading to an increase in the release of the transmitter into the synaptic cleft. The dimensions of the synaptic cleft are very small, and the transmitter, quickly reaching the postsynaptic membrane, interacts with its substance. As a result of this interaction, the structure of the postsynaptic membrane temporarily changes, its permeability to sodium ions increases, which leads to the movement of ions and, as a consequence, the appearance of an excitatory postsynaptic potential. When this potential reaches a certain value, a spreading excitation occurs - an action potential. After a few milliseconds, the mediator is destroyed by special enzymes.
There are also special inhibitory synapses. It is believed that in specialized inhibitory neurons, in the nerve endings of axons, a special transmitter is produced that has an inhibitory effect on the subsequent neuron. In the cortex cerebral hemispheres In the brain, gamma-aminobutyric acid is considered such a mediator. The structure and mechanism of operation of inhibitory synapses are similar to those of excitatory synapses, only the result of their action is hyperpolarization. This leads to the emergence of an inhibitory postsynaptic potential, resulting in inhibition
Synapse mediators
Mediator (from Latin Media - transmitter, intermediary or middle). Such synaptic mediators are very important in the process of transmitting nerve impulses.
The morphological difference between inhibitory and excitatory synapses is that they do not have a mechanism for transmitter release. The transmitter in the inhibitory synapse, motor neuron and other inhibitory synapse is considered to be the amino acid glycine. But the inhibitory or excitatory nature of the synapse is determined not by their mediators, but by the property of the postsynaptic membrane. For example, acetylcholine has an stimulating effect at the neuromuscular synapse terminals (vagus nerves in the myocardium).
Acetylcholine serves as an excitatory transmitter in cholinergic synapses (the presynaptic membrane in it is played by the ending of the spinal cord of the motor neuron), in the synapse on Renshaw cells, in the presynaptic terminal of the sweat glands, the adrenal medulla, in the intestinal synapse and in the ganglia of the sympathetic nervous system. Acetylcholinesterase and acetylcholine were also found in fractions of different parts of the brain, sometimes in large quantities, but apart from the cholinergic synapse on Renshaw cells, they have not yet been able to identify other cholinergic synapses. According to scientists, the mediator excitatory function of acetylcholine in the central nervous system is very likely.
Catelchomines (dopamine, norepinephrine and epinephrine) are considered adrenergic mediators. Adrenaline and norepinephrine are synthesized at the end of the sympathetic nerve, in the brain cell of the adrenal gland, spinal cord and brain. Amino acids (tyrosine and L-phenylalanine) are considered the starting material, and adrenaline is the final product of the synthesis. The intermediate substance, which includes norepinephrine and dopamine, also functions as mediators in the synapse created at the endings of the sympathetic nerves. This function can be either inhibitory (secretory glands of the intestine, several sphincters and smooth muscle of the bronchi and intestines) or excitatory (smooth muscles of certain sphincters and blood vessels, in the myocardial synapse - norepinephrine, in the subcutaneous nuclei of the brain - dopamine).
When synapse mediators complete their function, catecholamine is absorbed by the presynaptic nerve ending, and transmembrane transport is activated. During the absorption of transmitters, synapses are protected from premature depletion of the supply during long and rhythmic work.