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, faster conduction speed, found in invertebrates and lower vertebrates).

Structure.

1. Pedsynaptic department

2. Synaptic cleft

3. Postsynaptic section

4. Visicles - bubbles with a mediator

5. Mediaor - a chemical substance that 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 cerebral cortex, 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 properties 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 the remaining 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 synaptic 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.

Structure and types of synapses

The terminal formations of neuron processes (nerve endings) are divided into receptor, effector and interneuronal. Receptor endings are the terminal formations of dendrites in organs. Effector endings are the terminal formations of axons in working organs. Interneuronal endings are the terminal formations of axons on the surface of the body of a neuron or processes of another nerve cell.

Efferent and interneuronal endings ensure the transition of excitation from a nerve fiber to a muscle, glandular or nerve cell. The structural formations that ensure this transition are called synapses.

Synapse- this is the connection through which each individual functional unit of the nervous system activates or inhibits the next functional unit, directing signals entering the central nervous system along one path or another, for example, in the direction from sensory units to motor ones.

Synapses are peripheral and central. An example of a peripheral synapse is the neuromuscular synapse, where a neuron makes contact with a muscle fiber. Synapses in the nervous system are called central synapses when two neurons come into contact.

There are five types of synapses, depending on what parts the neurons are in contact with: 1) axo-dendritic (the axon of one cell contacts the dendrite of another); 2) axo-somatic (the axon of one cell contacts the soma of another cell); 3) axo-axonal (the axon of one cell contacts the axon of another cell); 4) dendro-dendritic (the dendrite of one cell is in contact with the dendrite of another cell); 5) somo-somatic (the somas of two cells are in contact). The bulk of contacts are axo-dendritic and axo-somatic.

A synapse consists of three parts: presynaptic terminal, synaptic cleft and postsynaptic membrane. The presynaptic terminal (synaptic plaque) is an extended part of the axon terminal. The synaptic cleft is the space between two contacting neurons. The diameter of the synaptic cleft is 10-20 nm. The membrane of the presynaptic terminal facing the synaptic cleft is called the presynaptic membrane. The third part of the synapse is the postsynaptic membrane, which is located opposite the presynaptic membrane.

The type of information transmission through the synapse depends on the size of the synaptic gap. If the distance between neuron membranes does not exceed 2-4 nm or they are in contact with each other, then such a synapse is electric, since such a connection provides a low-resistance electrical connection between these cells, allowing electrical potential to be transferred directly or electrotonically from cell to cell. The proportion of electrical synapses in the central nervous system is very small. Chemical synapses - it is the most complex type of connections in the central nervous system. Morphologically, it differs from other forms of connections in the presence of a well-defined synaptic cleft and in the fact that with it the membranes are strictly oriented or polarized in the direction from neuron to neuron. In such synapses, interaction between neurons is carried out using mediator- a biologically active substance released from the presynaptic ending. At the presynaptic end of a chemical synapse there are vesicles - vesicles, which have a variety of sizes (from 20 to 150 and more) and are filled with various chemicals that facilitate the transfer of activity from one cell to another.

1. Based on the type of transmitter released, chemical synapses are classified into two types:

a) adrenergic (the mediator is adrenaline).

b) cholinergic (the mediator is acetylcholine).

2. Electrical synapses. They transmit excitation without the participation of a mediator at high speed and have two-way conduction of excitation. The structural basis of the electrical synapse is the nexus. These synapses are found in endocrine glands, epithelial tissue, the central nervous system, and the heart. In some organs, excitation can be transmitted through both chemical and electrical synapses.

3. According to the effect of action:

a) stimulating

b) brake

4. By location:

a) axoaxonal

b) axosomatic

c) axodendritic

d) dendrodendritic

e) dendrosomatic.

The mechanism of excitation transmission in the neuromuscular synapse.

AP reaching the nerve ending (presynaptic membrane) causes its depolarization. As a result, calcium ions enter the ending. An increase in calcium concentration in the nerve ending promotes the release of acetylcholine, which enters the synaptic cleft. The transmitter reaches the postsynaptic membrane and binds to receptors there. As a result, sodium ions enter the postsynaptic membrane and this membrane is depolarized.

If the initial MPP level was 85 mV, then it can decrease to 10 mV, i.e. partial depolarization occurs, i.e. the excitation does not yet spread further, but is located in the synapse. As a result of these mechanisms, a synaptic delay develops, which ranges from 0.2 to 1 mV. partial depolarization of the postsynaptic membrane is called excitatory postsynaptic potential (EPSP).

Under the influence of EPSP, a propagating PD arises in the adjacent sensitive area of ​​the muscle fiber membrane, which causes muscle contraction.

Acetylcholine is constantly released from the presynaptic ending, but its concentration is low, which is necessary to maintain muscle tone at rest.

To block the transmission of excitation through the synapse, the poison curare is used, which binds to the receptors of the postsynaptic membrane and prevents their interaction with acetylcholine. The poison butulin and other substances can block the conduction of excitation through the synapse.

The outer surface of the postsynaptic membrane contains the enzyme acetylcholinesterase, which breaks down acetylcholine and inactivates it.

Principles and features of excitation transfer

at interneural synapses.

The basic principle of excitation transmission in interneural synapses is the same as in the neuromuscular synapse. However, there are some peculiarities:

1. Many synapses are inhibitory.

2. EPSP during depolarization of one synapse is not enough to cause a propagating action potential, i.e. it is necessary to receive impulses to the nerve cell from many synapses.

Neuromuscular junction

Classification of synapses

1. By location and affiliation with the relevant structures:

peripheral (neuromuscular, neurosecretory, receptor-neuronal);

central (axo-somatic, axo-dendritic, axo-axonal, somato-dendritic. somato-somatic);

2. According to the effect of action:

stimulating

brake

3. According to the method of signal transmission:

Electrical,

chemical,

mixed.

4. By mediator:

cholinergic,

adrenergic,

serotonergic,

glycinergic. etc.

Brake mediators:

– gamma-aminobutyric acid (GABA)

– taurine

– glycine

Exciting mediators:

– aspartate

– glutamate

Both effects:

– norepinephrine

– dopamine

– serotonin

Mechanism of excitation transmission in synapses

(using the example of the neuromuscular synapse)

Release of transmitter into the synaptic cleft

Diffusion of ACh

The occurrence of excitation in the muscle fiber.

Removal of ACh from the synaptic cleft

MINISTRY OF EDUCATION AND SCIENCE OF RUSSIA

Federal State Budgetary Educational Institution of Higher Professional Education

"RUSSIAN STATE HUMANITIES UNIVERSITY"

INSTITUTE OF ECONOMICS, MANAGEMENT AND LAW

MANAGEMENT DEPARTMENT

Structure and function of the synapse. Classifications of synapses. Chemical synapse, transmitter

Final test in Developmental Psychology

2nd year student of distance (correspondence) form of education

Kundirenko Ekaterina Viktorovna

Supervisor

Usenko Anna Borisovna

Candidate of Psychological Sciences, Associate Professor

Moscow 2014

Maintaining. Physiology of the neuron and its structure. Structure and functions of the synapse. Chemical synapse. Isolation of the mediator. Chemical mediators and their types

Conclusion

synapse transmitter neuron

Introduction

The nervous system is responsible for the coordinated activity of various organs and systems, as well as for the regulation of body functions. It also connects the body with the external environment, thanks to which we feel various changes in the environment and respond to them. The main functions of the nervous system are receiving, storing and processing information from the external and internal environment, regulating and coordinating the activities of all organs and organ systems.

In humans, like in all mammals, the nervous system includes three main components: 1) nerve cells (neurons); 2) glial cells associated with them, in particular neuroglial cells, as well as cells forming neurilemma; 3) connective tissue. Neurons provide the conduction of nerve impulses; neuroglia performs supporting, protective and trophic functions both in the brain and in the spinal cord, and the neurilemma, consisting mainly of specialized, so-called. Schwann cells, participates in the formation of peripheral nerve fiber sheaths; Connective tissue supports and binds together the various parts of the nervous system.

The transmission of nerve impulses from one neuron to another is carried out using a synapse. 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 travels from the first cell, called presynaptic, to the second, called postsynaptic. Typically, a synapse refers to a chemical synapse in which signals are transmitted using neurotransmitters.

I. Physiology of the neuron and its structure

The structural and functional unit of the nervous system is the nerve cell - neuron.

Neurons are specialized cells capable of receiving, processing, encoding, transmitting and storing information, organizing reactions to stimuli, and establishing contacts with other neurons and organ cells. The unique features of the neuron are the ability to generate electrical discharges and transmit information using specialized endings - synapses.

The functions of a neuron are facilitated by the synthesis in its axoplasm of transmitter substances - neurotransmitters (neurotransmitters): acetylcholine, catecholamines, etc. The sizes of neurons range from 6 to 120 microns.

The number of neurons in the human brain is approaching 1011. One neuron can have up to 10,000 synapses. If only these elements are considered as information storage cells, then we can come to the conclusion that the nervous system can store 1019 units. information, i.e., it is capable of containing almost all the knowledge accumulated by humanity. Therefore, the idea that the human brain throughout life remembers everything that happens in the body and during its communication with the environment is quite reasonable. However, the brain cannot retrieve from memory all the information that is stored in it.

Different brain structures are characterized by certain types of neural organization. Neurons organizing a single function form so-called groups, populations, ensembles, columns, nuclei. In the cerebral cortex and cerebellum, neurons form layers of cells. Each layer has its own specific function.

Clumps of cells form the gray matter of the brain. Myelinated or unmyelinated fibers pass between nuclei, groups of cells and between individual cells: axons and dendrites.

One nerve fiber from the underlying brain structures in the cortex branches into neurons occupying a volume of 0.1 mm3, i.e. one nerve fiber can excite up to 5000 neurons. In postnatal development, certain changes occur in the density of neurons, their volume, and dendritic branching.

The structure of a neuron.

Functionally, the following parts are distinguished in a neuron: perceptive - dendrites, membrane of the neuron soma; integrative - soma with axon hillock; transmitting - axon hillock with axon.

The body of the neuron (soma), in addition to the informational one, performs a trophic function in relation to its processes and their synapses. Transection of an axon or dendrite leads to the death of processes lying distal to the transection, and, consequently, the synapses of these processes. The soma also ensures the growth of dendrites and axons.

The neuron soma is enclosed in a multilayer membrane, which ensures the formation and propagation of electrotonic potential to the axon hillock.

Neurons are able to perform their information function mainly due to the fact that their membrane has special properties. The neuron membrane is 6 nm thick and consists of two layers of lipid molecules, which with their hydrophilic ends face the aqueous phase: one layer of molecules faces inward, the other faces outward of the cell. The hydrophobic ends are turned towards each other - inside the membrane. Membrane proteins are embedded in the lipid bilayer and perform several functions: “pump” proteins ensure the movement of ions and molecules against the concentration gradient in the cell; proteins embedded in the channels provide selective membrane permeability; receptor proteins recognize the desired molecules and fix them on the membrane; enzymes, located on the membrane, facilitate the occurrence of chemical reactions on the surface of the neuron. In some cases, the same protein can be a receptor, an enzyme, and a “pump.”

Ribosomes are located, as a rule, near the nucleus and carry out protein synthesis on tRNA templates. Neuronal ribosomes come into contact with the endoplasmic reticulum of the lamellar complex and form basophilic substance.

Basophilic substance (Nissl substance, tigroid substance, tigroid) is a tubular structure covered with small grains, contains RNA and is involved in the synthesis of protein components of the cell. Prolonged excitation of a neuron leads to the disappearance of the basophilic substance in the cell, and therefore to the cessation of the synthesis of a specific protein. In newborns, neurons of the frontal lobe of the cerebral cortex do not have basophilic substance. At the same time, in the structures that provide vital reflexes - the spinal cord, brain stem, neurons contain a large amount of basophilic substance. It moves from the cell soma to the axon by axoplasmic current.

The lamellar complex (Golgi apparatus) is an organelle of a neuron that surrounds the nucleus in the form of a network. The lamellar complex is involved in the synthesis of neurosecretory and other biologically active cell compounds.

Lysosomes and their enzymes provide hydrolysis of a number of substances in the neuron.

Neuronal pigments - melanin and lipofuscin are found in the neurons of the substantia nigra of the midbrain, in the nuclei of the vagus nerve, and in the cells of the sympathetic system.

Mitochondria are organelles that provide the energy needs of a neuron. They play an important role in cellular respiration. They are most numerous in the most active parts of the neuron: the axon hillock, in the area of ​​synapses. When a neuron is active, the number of mitochondria increases.

Neurotubules penetrate the soma of the neuron and take part in the storage and transmission of information.

The neuron nucleus is surrounded by a porous two-layer membrane. Through the pores, exchange occurs between the nucleoplasm and the cytoplasm. When a neuron is activated, the nucleus, due to protrusions, increases its surface, which enhances the nuclear-plasmic relationship, stimulating the functions of the nerve cell. The nucleus of a neuron contains genetic material. The genetic apparatus ensures differentiation, the final shape of the cell, as well as connections typical for a given cell. Another essential function of the nucleus is the regulation of neuron protein synthesis throughout its life.

The nucleolus contains a large amount of RNA and is covered with a thin layer of DNA.

There is a certain relationship between the development of the nucleolus and basophilic substance in ontogenesis and the formation of primary behavioral reactions in humans. This is due to the fact that the activity of neurons and the establishment of contacts with other neurons depend on the accumulation of basophilic substances in them.

Dendrites are the main receptive field of a neuron. The membrane of the dendrite and the synaptic part of the cell body is capable of responding to mediators released by axon endings by changing the electrical potential.

Typically a neuron has several branching dendrites. The need for such branching is due to the fact that a neuron as an information structure must have a large number of inputs. Information comes to it from other neurons through specialized contacts, the so-called spines.

“Spikes” have a complex structure and ensure the perception of signals by the neuron. The more complex the function of the nervous system, the more different analyzers send information to a given structure, the more “spines” there are on the dendrites of neurons. The maximum number of them is contained on pyramidal neurons of the motor zone of the cerebral cortex and reaches several thousand. They occupy up to 43% of the surface of the soma membrane and dendrites. Due to the “spines,” the receptive surface of the neuron increases significantly and can reach, for example, 250,000 μm in Purkinje cells.

Let us recall that motor pyramidal neurons receive information from almost all sensory systems, a number of subcortical formations, and from associative systems of the brain. If a given “spike” or group of “spikes” stops receiving information for a long time, then these “spikes” disappear.

An axon is an outgrowth of the cytoplasm, adapted to carry information collected by dendrites, processed in a neuron and transmitted to the axon through the axon hillock - the place where the axon exits the neuron. The axon of a given cell has a constant diameter, in most cases it is covered in a myelin sheath formed from glia. The axon has branched endings. The endings contain mitochondria and secretory formations.

Types of neurons.

The structure of neurons largely corresponds to their functional purpose. Based on their structure, neurons are divided into three types: unipolar, bipolar and multipolar.

True unipolar neurons are found only in the mesencephalic nucleus of the trigeminal nerve. These neurons provide proprioceptive sensitivity to the masticatory muscles.

Other unipolar neurons are called pseudounipolar; in fact, they have two processes (one comes from the periphery from the receptors, the other into the structures of the central nervous system). Both processes merge near the cell body into a single process. All these cells are located in sensory nodes: spinal, trigeminal, etc. They provide the perception of pain, temperature, tactile, proprioceptive, baroceptive, vibration signaling.

Bipolar neurons have one axon and one dendrite. Neurons of this type are found mainly in the peripheral parts of the visual, auditory and olfactory systems. Bipolar neurons are connected by a dendrite to the receptor, and by an axon - to a neuron at the next level of organization of the corresponding sensory system.

Multipolar neurons have several dendrites and one axon. Currently, there are up to 60 different variants of the structure of multipolar neurons, but they all represent varieties of fusiform, stellate, basket and pyramidal cells.

Metabolism in a neuron.

Necessary nutrients and salts are delivered to the nerve cell in the form of aqueous solutions. Metabolic products are also removed from the neuron in the form of aqueous solutions.

Neuron proteins serve plastic and informational purposes. The nucleus of a neuron contains DNA, while RNA predominates in the cytoplasm. RNA is concentrated predominantly in the basophilic substance. The intensity of protein metabolism in the nucleus is higher than in the cytoplasm. The rate of protein renewal in phylogenetically newer structures of the nervous system is higher than in older ones. The highest rate of protein turnover is in the gray matter of the cerebral cortex. Less - in the cerebellum, the smallest - in the spinal cord.

Neuronal lipids serve as energy and plastic material. The presence of lipids in the myelin sheath determines their high electrical resistance, reaching 1000 Ohm/cm2 of surface in some neurons. Lipid metabolism in a nerve cell occurs slowly; excitation of the neuron leads to a decrease in the amount of lipids. Usually, after prolonged mental work and fatigue, the amount of phospholipids in the cell decreases.

Carbohydrates of neurons are the main source of energy for them. Glucose, entering a nerve cell, is converted into glycogen, which, if necessary, under the influence of the enzymes of the cell itself, is converted back into glucose. Due to the fact that glycogen reserves during neuron operation do not fully support its energy expenditure, blood glucose serves as the source of energy for the nerve cell.

Glucose is broken down in the neuron aerobically and anaerobically. The breakdown occurs predominantly aerobically, which explains the high sensitivity of nerve cells to a lack of oxygen. An increase in adrenaline in the blood and active body activity lead to an increase in carbohydrate consumption. During anesthesia, carbohydrate intake decreases.

Nerve tissue contains salts of potassium, sodium, calcium, magnesium, etc. Among the cations, K+, Na+, Mg2+, Ca2+ predominate; from anions - Cl-, HCO3-. In addition, the neuron contains various trace elements (for example, copper and manganese). Due to their high biological activity, they activate enzymes. The amount of microelements in a neuron depends on its functional state. Thus, with reflex or caffeine excitation, the content of copper and manganese in the neuron decreases sharply.

The energy exchange in a neuron in a state of rest and excitation is different. This is evidenced by the value of the respiratory coefficient in the cell. At rest it is 0.8, and when excited it is 1.0. When excited, oxygen consumption increases by 100%. After excitation, the amount of nucleic acids in the cytoplasm of neurons sometimes decreases by 5 times.

The intrinsic energy processes of a neuron (its soma) are closely related to the trophic influences of neurons, which affects primarily axons and dendrites. At the same time, the nerve endings of the axons have trophic effects on the muscle or cells of other organs. Thus, disruption of muscle innervation leads to its atrophy, increased protein breakdown, and death of muscle fibers.

Classification of neurons.

There is a classification of neurons that takes into account the chemical structure of substances released at their axon terminals: cholinergic, peptidergic, noradrenergic, dopaminergic, serotonergic, etc.

Based on their sensitivity to the action of stimuli, neurons are divided into mono-, bi-, and polysensory.

Monosensory neurons. They are most often located in the primary projection zones of the cortex and respond only to signals from their sensory system. For example, a significant part of the neurons in the primary visual area of ​​the cerebral cortex reacts only to light stimulation of the retina.

Monosensory neurons are divided functionally according to their sensitivity to different qualities of a single stimulus. Thus, individual neurons of the auditory zone of the cerebral cortex can respond to presentations of a tone of 1000 Hz and not respond to tones of a different frequency. They are called monomodal. Neurons that respond to two different tones are called bimodal; neurons that respond to three or more are called polymodal.

Bisensory neurons. They are more often located in the secondary zones of the cortex of some analyzer and can respond to signals from both their own and other sensory systems. For example, neurons in the secondary visual area of ​​the cerebral cortex respond to visual and auditory stimuli.

Polysensory neurons. These are most often neurons of the associative areas of the brain; they are able to respond to irritation of the auditory, visual, skin and other receptive systems.

Nerve cells of different parts of the nervous system can be active outside of influence - background, or background active (Fig. 2.16). Other neurons exhibit impulse activity only in response to some kind of stimulation.

Background active neurons are divided into inhibitory ones - reducing the frequency of discharges and excitatory ones - increasing the frequency of discharges in response to any irritation. Background active neurons can generate impulses continuously with some slowing down or increasing the frequency of discharges - this is the first type of activity - continuously arrhythmic. Such neurons provide the tone of the nerve centers. Background active neurons are of great importance in maintaining the level of excitation of the cortex and other brain structures. The number of background active neurons increases during wakefulness.

Neurons of the second type produce a group of impulses with a short interpulse interval, after which a period of silence begins and a group, or burst, of impulses appears again. This type of activity is called bursting. The significance of the burst type of activity is to create conditions for the conduction of signals while reducing the functionality of the conducting or perceptive structures of the brain. Interpulse intervals in a burst are approximately 1-3 ms; between bursts this interval is 15-120 ms.

The third form of background activity is group activity. The group type of activity is characterized by the aperiodic appearance in the background of a group of pulses (interpulse intervals range from 3 to 30 ms), followed by a period of silence.

Functionally, neurons can also be divided into three types: afferent, interneurons (interneurons), efferent. The first perform the function of receiving and transmitting information to the overlying structures of the central nervous system, the second - ensure interaction between neurons of the central nervous system, the third - transmit information to the underlying structures of the central nervous system, to nerve nodes lying outside the central nervous system, and to the organs of the body.

The functions of afferent neurons are closely related to the functions of receptors.

Structure and function of the synapse

Synapses are the contacts that establish neurons as independent entities. The synapse is a complex structure and consists of a presynaptic part (the end of the axon that transmits the signal), a synaptic cleft and a postsynaptic part (the structure of the receiving cell).

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

Based on location, neuromuscular synapses and neuro-neuronal synapses are distinguished, the latter in turn are divided into axo-somatic, axo-axonal, axodendritic, dendro-somatic.

According to the nature of the effect on the perceptive structure, synapses can be excitatory or inhibitory.

According to the method of signal transmission, synapses are divided into electrical, chemical, and mixed.

The nature of the interaction of neurons. The method of interaction is determined: distant, adjacent, contact.

Distant interaction can be ensured by two neurons located in different structures of the body. For example, in the cells of a number of brain structures, neurohormones and neuropeptides are formed, which are able to have a humoral effect on neurons of other parts.

Adjacent interaction between neurons occurs when the membranes of neurons are separated only by intercellular space. Typically, such interaction occurs where there are no glial cells between the membranes of neurons. Such contiguity is characteristic of axons of the olfactory nerve, parallel fibers of the cerebellum, etc. It is believed that contiguous interaction ensures the participation of neighboring neurons in the performance of a single function. This occurs, in particular, because metabolites, products of neuron activity, entering the intercellular space, affect neighboring neurons. Adjacent interaction can, in some cases, ensure the transfer of electrical information from neuron to neuron.

Contact interaction is caused by specific contacts of neuron membranes, which form so-called electrical and chemical synapses.

Electrical synapses. Morphologically they represent a fusion, or rapprochement, of membrane sections. In the latter case, the synaptic cleft is not continuous, but is interrupted by full contact bridges. These bridges form a repeating cellular structure of the synapse, with the cells limited by areas of adjacent membranes, the distance between which in mammalian synapses is 0.15-0.20 nm. At membrane fusion sites there are channels through which cells can exchange certain products. In addition to the described cellular synapses, among the electrical synapses there are others - in the form of a continuous gap; the area of ​​each of them reaches 1000 µm, as, for example, between the neurons of the ciliary ganglion.

Electrical synapses have one-way conduction of excitation. This is easy to prove by recording the electrical potential at the synapse: when the afferent pathways are stimulated, the synapse membrane is depolarized, and when the efferent fibers are stimulated, it hyperpolarizes. It turned out that synapses of neurons with the same function have bilateral conduction of excitation (for example, synapses between two sensitive cells), and synapses between differently functional neurons (sensory and motor) have unilateral conduction. The functions of electrical synapses are primarily to ensure urgent reactions of the body. This apparently explains their location in animals in structures that provide the reaction of flight, salvation from danger, etc.

The electrical synapse is relatively less fatigued and is resistant to changes in the external and internal environment. Apparently, these qualities, along with speed, ensure high reliability of its operation.

Chemical synapses. Structurally represented by the presynaptic part, the synaptic cleft and the postsynaptic part. The presynaptic part of a chemical synapse is formed by the expansion of the axon along its course or termination. The presynaptic part contains agranular and granular vesicles (Fig. 1). Bubbles (quanta) contain a mediator. In the presynaptic extension there are mitochondria that provide the synthesis of the transmitter, glycogen granules, etc. With repeated stimulation of the presynaptic ending, the reserves of the transmitter in the synaptic vesicles are depleted. It is believed that small granular vesicles contain norepinephrine, large ones contain other catecholamines. Agranular vesicles contain acetylcholine. Derivatives of glutamic and aspartic acids can also be excitation mediators.

Rice. 1. Scheme of the process of nerve signal transmission at a chemical synapse.

Chemical synapse

The essence of the mechanism for transmitting an electrical impulse from one nerve cell to another through a chemical synapse is as follows. An electrical signal traveling along the process of a neuron of one cell arrives at the presynaptic region and causes the release of a certain chemical compound - an intermediary or transmitter - into the synaptic cleft. The transmitter, diffusing along the synaptic cleft, reaches the postsynaptic region and chemically binds to a molecule located there, called a receptor. As a result of this binding, a series of physicochemical transformations are triggered in the postsynaptic zone, as a result of which an electric current pulse appears in its area, spreading further to the second cell.

The presynaptic region is characterized by several important morphological formations that play a major role in its operation. In this area there are specific granules - vesicles - containing one or another chemical compound, generally called a mediator. This term has a purely functional meaning, like, for example, the term hormone. The same substance can be classified as either mediators or hormones. For example, norepinephrine must be called a transmitter if it is released from presynaptic vesicles; If norepinephrine is released into the blood by the adrenal glands, then in this case it is called a hormone.

In addition, in the presynaptic zone there are mitochondria containing calcium ions and specific membrane structures - ion channels. The activation of the presynapse begins at the moment when an electrical impulse from the cell arrives in this area. This impulse causes large amounts of calcium to enter the presynapse through ion channels. In addition, in response to an electrical impulse, calcium ions leave the mitochondria. Both of these processes lead to an increase in calcium concentration in the presynapse. The appearance of excess calcium leads to the connection of the presynaptic membrane with the membrane of the vesicles, and the latter begin to be drawn towards the presynaptic membrane, eventually releasing their contents into the synaptic cleft.

The main structure of the postsynaptic region is the membrane of the region of the second cell in contact with the presynapse. This membrane contains a genetically determined macromolecule - a receptor, which selectively binds to a mediator. This molecule contains two sections. The first section is responsible for recognizing “one’s” mediator, the second section is responsible for physicochemical changes in the membrane, leading to the appearance of an electrical potential.

The activation of the postsynapse begins at the moment when a transmitter molecule arrives in this area. The recognition center “recognizes” its molecule and binds to it with a certain type of chemical bond, which can be visualized as the interaction of a lock with its key. This interaction involves the work of a second region of the molecule, and its work results in an electrical impulse.

The features of signal transmission through a chemical synapse are determined by the features of its structure. First, an electrical signal from one cell is transmitted to another using a chemical messenger - a transmitter. Secondly, the electrical signal is transmitted only in one direction, which is determined by the structural features of the synapse. Thirdly, there is a slight delay in signal transmission, the time of which is determined by the time of diffusion of the transmitter along the synaptic cleft. Fourth, conduction through a chemical synapse can be blocked in various ways.

The functioning of a chemical synapse is regulated both at the level of the presynapse and at the level of the postsynapse. In the standard mode of operation, after the arrival of an electrical signal there, a transmitter is released from the presynapse, which binds to the post-synapse receptor and causes the emergence of a new electrical signal. Before a new signal arrives at the presynapse, the amount of transmitter has time to recover. However, if signals from a nerve cell go too often or for a long time, the amount of transmitter there is depleted and the synapse stops working.

At the same time, the synapse can be “trained” to transmit very frequent signals over a long period of time. This mechanism is extremely important for understanding the mechanisms of memory. It has been shown that in vesicles, in addition to the substance that plays the role of a mediator, there are other substances of a protein nature, and on the membrane of the presynapse and postsynapse there are specific receptors that recognize them. These receptors for peptides are fundamentally different from receptors for mediators in that interaction with them does not cause the emergence of potentials, but triggers biochemical synthetic reactions.

Thus, after the impulse arrives at the presynapse, regulatory peptides are also released along with the transmitters. Some of them interact with peptide receptors on the presynaptic membrane, and this interaction includes the mechanism of transmitter synthesis. Consequently, the more often the mediator and regulatory peptides are released, the more intense the mediator synthesis will take place. Another part of the regulatory peptides, together with the mediator, reaches the postsynapse. The mediator binds to its receptor, and the regulatory peptides to theirs, and this last interaction triggers the processes of synthesis of receptor molecules for the mediator. As a result of such a process, the receptor field sensitive to the mediator increases so that all of the mediator molecules contact their receptor molecules. Overall, this process results in what is called facilitation of conduction across the chemical synapse.

Selecting a mediator

The factor that performs the transmitter function is produced in the body of the neuron, and from there it is transported to the axon terminal. The transmitter contained in the presynaptic endings must be released into the synoptic cleft in order to act on the receptors of the postsynaptic membrane, providing transsynaptic signal transmission. Substances such as acetylcholine, catecholamine group, serotonin, neuropyptids and many others can act as a mediator; their general properties will be described below.

Even before many of the essential features of the process of transmitter release were clarified, it was established that presynaptic endings can change the state of spontaneous secretory activity. Constantly released small portions of the transmitter cause so-called spontaneous, miniature postsynaptic potentials in the postsynaptic cell. This was established in 1950 by English scientists Fett and Katz, who, while studying the work of the frog neuromuscular synapse, discovered that without any action on the nerve in the muscle in the area of ​​the postsynaptic membrane, small potential fluctuations arise on their own at random intervals, with an amplitude of approximately at 0.5mV.

The discovery of the release of a transmitter, not associated with the arrival of a nerve impulse, helped to establish the quantum nature of its release, that is, it turned out that in a chemical synapse the transmitter is released at rest, but occasionally and in small portions. Discreteness is expressed in the fact that the mediator leaves the ending not diffusely, not in the form of individual molecules, but in the form of multimolecular portions (or quanta), each of which contains several.

This happens as follows: in the axoplasm of the neuron terminals in close proximity to the presynaptic membrane, when examined under an electron microscope, many vesicles or vesicles were discovered, each of which contains one quantum of the transmitter. Action currents caused by presynaptic impulses do not have a noticeable effect on the postsynaptic membrane, but lead to the destruction of the membrane of the vesicles with the transmitter. This process (exocytosis) consists in the fact that the vesicle, having approached the inner surface of the membrane of the presynaptic terminal in the presence of calcium (Ca2+), merges with the presynaptic membrane, as a result of which the vesicle is emptied into the synoptic cleft. After the destruction of the vesicle, the membrane surrounding it is included in the membrane of the presynaptic terminal, increasing its surface. Subsequently, as a result of the process of endomitosis, small sections of the presynaptic membrane are invaginated inward, again forming vesicles, which are subsequently again able to turn on the transmitter and enter into the cycle of its release.

V. Chemical mediators and their types

In the central nervous system, a large group of heterogeneous chemical substances performs a mediator function. The list of newly discovered chemical mediators is steadily growing. According to the latest data, there are about 30 of them. I would also like to note that according to Dale’s principle, each neuron secretes the same transmitter in all its synoptic endings. Based on this principle, it is customary to designate neurons by the type of transmitter that their endings release. Thus, for example, neurons that release acetylcholine are called cholinergic, serotonin - serotonergic. This principle can be used to designate various chemical synapses. Let's look at some of the most well-known chemical mediators:

Acetylcholine. One of the first neurotransmitters discovered (it was also known as “vagus nerve substance” due to its effects on the heart).

A feature of acetylcholine as a mediator is its rapid destruction after release from presynaptic terminals using the enzyme acetylcholinesterase. Acetylcholine functions as a mediator in synapses formed by recurrent collaterals of axons of motor neurons of the spinal cord on Renshaw intercalary cells, which in turn, with the help of another mediator, have an inhibitory effect on motor neurons.

Neurons of the spinal cord innervating chromaffin cells and preganglionic neurons innervating nerve cells of the intramural and extramural ganglia are also cholinergic. It is believed that cholinergic neurons are present in the reticular formation of the midbrain, cerebellum, basal ganglia and cortex.

Catecholamines. These are three chemically related substances. These include: dopamine, norepinephrine and adrenaline, which are tyrosine derivatives and perform a mediator function not only in peripheral, but also in central synapses. Dopaminergic neurons are found primarily within the midbrain in mammals. Dopamine plays a particularly important role in the striatum, where particularly large amounts of this neurotransmitter are found. In addition, dopaminergic neurons are present in the hypothalamus. Noradrenergic neurons are also contained in the midbrain, pons and medulla oblongata. The axons of noradrenergic neurons form ascending pathways that go to the hypothalamus, thalamus, limbic cortex and cerebellum. Descending fibers of noradrenergic neurons innervate the nerve cells of the spinal cord.

Catecholamines have both excitatory and inhibitory effects on CNS neurons.

Serotonin. Like catecholamines, it belongs to the group of monoamines, that is, it is synthesized from the amino acid tryptophan. In mammals, serotonergic neurons are located primarily in the brainstem. They are part of the dorsal and medial raphe, nuclei of the medulla oblongata, pons and midbrain. Serotonergic neurons extend their influence to the neocortex, hippocampus, globus pallidus, amygdala, subthalamic region, stem structures, cerebellar cortex, and spinal cord. Serotonin plays an important role in the descending control of spinal cord activity and in the hypothalamic control of body temperature. In turn, disturbances in serotonin metabolism that occur under the influence of a number of pharmacological drugs can cause hallucinations. Dysfunction of serotonergic synapses is observed in schizophrenia and other mental disorders. Serotonin can cause excitatory and inhibitory effects depending on the properties of the receptors of the postsynaptic membrane.

Neutral amino acids. These are two main dicarboxylic acids, L-glutamate and L-aspartate, which are found in large quantities in the central nervous system and can act as mediators. L-glutamic acid is part of many proteins and peptides. It does not pass well through the blood-brain barrier and therefore does not enter the brain from the blood, being formed mainly from glucose in the nervous tissue itself. Glutamate is found in high concentrations in the mammalian central nervous system. It is believed that its function is mainly associated with the synoptic transmission of excitation.

Polypeptides. In recent years, it has been shown that some polypeptides can perform a mediator function in CNS synapses. Such polypeptides include substances-P, hypothalamic neurohormones, enkephalins, etc. Substance-P refers to a group of agents first extracted from the intestine. These polypeptides are found in many parts of the central nervous system. Their concentration is especially high in the area of ​​the substantia nigra. The presence of substance-P in the dorsal roots of the spinal cord suggests that it may serve as a mediator at synapses formed by the central endings of the axons of some primary afferent neurons. Substance-P has an excitatory effect on certain neurons in the spinal cord. The mediator role of other neuropeptides is even less clear.

Conclusion

The modern understanding of the structure and function of the central nervous system is based on the neural theory, which is a special case of the cellular theory. However, if the cellular theory was formulated back in the first half of the 19th century, then the neural theory, which considers the brain as the result of the functional unification of individual cellular elements - neurons, gained recognition only at the turn of this century. The studies of the Spanish neurohistologist R. Cajal and the English physiologist C. Sherrington played a major role in the recognition of the neural theory. Final evidence of the complete structural isolation of nerve cells was obtained using an electron microscope, the high resolution of which made it possible to establish that each nerve cell is surrounded throughout its entire length by a limiting membrane, and that there are free spaces between the membranes of different neurons. Our nervous system is built from two types of cells - nerve and glial. Moreover, the number of glial cells is 8-9 times higher than the number of nerve cells. The number of nervous elements, being very limited in primitive organisms, in the process of evolutionary development of the nervous system reaches many billions in primates and humans. At the same time, the number of synaptic contacts between neurons is approaching an astronomical figure. The complexity of the organization of the central nervous system is also manifested in the fact that the structure and functions of neurons in different parts of the brain vary significantly. However, a necessary condition for analyzing brain activity is to identify the fundamental principles underlying the functioning of neurons and synapses. After all, it is these connections of neurons that provide all the variety of processes associated with the transmission and processing of information.

One can only imagine what will happen if there is a failure in this complex exchange process... what will happen to us. This can be said about any structure of the body; it may not be the main one, but without it the activity of the entire organism will not be entirely correct and complete. It's the same as in a watch. If one, even the smallest part in the mechanism is missing, the watch will no longer work absolutely accurately. And soon the clock will break. In the same way, our body, if one of the systems is disrupted, gradually leads to the failure of the whole organism, and subsequently to the death of this very organism. So it is in our interests to monitor the condition of our body and avoid making mistakes that can lead to serious consequences for us.

List of sources and literature

1. Batuev A. S. Physiology of higher nervous activity and sensory systems: textbook / A. S. Batuev. - St. Petersburg. : Peter, 2009. - 317 p.

Danilova N. N. Psychophysiology: Textbook / N. N. Danilova. - M.: ASPECT PRESS, 2000. - 373 p.

Danilova N. N. Physiology of higher nervous activity: textbook / N. N. Danilova, A. L. Krylova. - M.: Educational literature, 1997. - 428 p.

Karaulova L.K. Physiology: textbook / L.K. Karaulova, N.A. Krasnoperova, M.M. Rasulov. - M.: Academy, 2009. - 384 p.

Katalymov, L. L. Physiology of the neuron: textbook / L. L. Katalymov, O. S. Sotnikov; Min. people. Education of the RSFSR, Ulyanovsk. state ped. int. - Ulyanovsk: B. i., 1991. - 95 p.

Semenov, E.V. Physiology and anatomy: textbook / E.V. Semenov. - M.: Dzhangar, 2005. - 480 p.

Smirnov, V. M. Physiology of the central nervous system: textbook / V. M. Smirnov, V. N. Yakovlev. - M.: Academy, 2002. - 352 p.

Smirnov V. M. Human physiology: textbook / V. M. Smirnova. - M.: Medicine, 2002. - 608 p.

Rossolimo T. E. Physiology of higher nervous activity: a textbook: textbook / T. E. Rossolimo, I. A. Moskvina - Tarkhanova, L. B. Rybalov. - M.; Voronezh: MPSI: MODEK, 2007. - 336 p.

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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.

Electrical synapses are less common in the mammalian nervous system 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;

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 asymmetrical, 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 notch

Connective tissue Epineurium · Perineurium · Endoneurium · Nerve bundles · Meninges: dura, arachnoid, soft

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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