Regional University synapse as a functional contact of nervous tissue. Synapse structure: electrical and chemical synapses

Most synapses in the nervous system use chemicals to transmit signals from the presynaptic neuron to the postsynaptic neuron - mediators or neurotransmitters. Chemical signaling occurs through chemical synapses(Fig. 14), including membranes of pre- and postsynaptic cells and separating them synaptic cleft- a region of extracellular space about 20 nm wide.

Fig. 14. Chemical synapse

In the area of ​​the synapse, the axon usually expands, forming the so-called. presynaptic plaque or end plate. The presynaptic terminal contains synaptic vesicles- bubbles surrounded by a membrane with a diameter of about 50 nm, each of which contains 10 4 - 5x10 4 mediator molecules. The synaptic cleft is filled with mucopolysaccharide, which glues the pre- and postsynaptic membranes together.

The following sequence of events during transmission through a chemical synapse has been established. When the action potential reaches the presynaptic terminal, the membrane in the synapse zone depolarizes, the calcium channels of the plasma membrane are activated, and Ca 2+ ions enter the terminal. An increase in intracellular calcium levels initiates exocytosis of vesicles filled with mediator. The contents of the vesicles are released into the extracellular space, and some of the transmitter molecules, diffusing, bind to the receptor molecules of the postsynaptic membrane. Among them are receptors that can directly control ion channels. The binding of mediator molecules to such receptors is a signal for the activation of ion channels. Thus, along with the voltage-dependent ion channels discussed in the previous section, there are transmitter-dependent channels (otherwise called ligand-activated channels or ionotropic receptors). They open and allow the corresponding ions into the cell. The movement of ions along their electrochemical gradients generates sodium depolarizing(excitatory) or potassium (chloride) hyperpolarizing (inhibitory) current. Under the influence of a depolarizing current, a postsynaptic excitatory potential develops or end plate potential(PKP). If this potential exceeds the threshold level, voltage-gated sodium channels open and AP occurs. The speed of impulse conduction in the synapse is less than in the fiber, i.e. a synaptic delay is observed, for example, in the neuromuscular synapse of the frog - 0.5 ms. The sequence of events described above is typical for the so-called. direct synaptic transmission.

In addition to receptors that directly control ion channels, chemical transmission involves G protein-coupled receptors or metabotropic receptors.

G proteins, named for their ability to bind guanine nucleotides, are trimers consisting of three subunits: α, β and γ. There are a large number of varieties of each of the subunits (20 α, 6 β , 12 γ). which creates the basis for a huge number of their combinations. G proteins are divided into four main groups based on the structure and targets of their α-subunits: G s stimulates adenylate cyclase; G i inhibits adenylate cyclase; G q binds to phospholipase C; the targets of C 12 are not yet known. The G i family includes G t (transducin), which activates cGMP phosphodiesterase, as well as two G 0 isoforms that bind to ion channels. At the same time, each G protein can interact with several effectors, and different G proteins can modulate the activity of the same ion channels. In the inactivated state, guanosine diphosphate (GDP) is associated with the α subunit, and all three subunits are combined into a trimer. Interaction with the activated receptor allows guanosine triphosphate (GTP) to replace GDP on the α subunit, resulting in dissociation of α -- and βγ subunits (under physiological conditions β - and γ-subunits remain bound). Free α- and βγ-subunits bind to target proteins and modulate their activity. The free α-subunit has GTPase activity, causing hydrolysis of GTP with the formation of GDP. As a result α -- and βγ subunits rebind, resulting in the cessation of their activity.

Currently, >1000 metabotropic receptors have been identified. While channel-bound receptors cause electrical changes in the postsynaptic membrane in just a few milliseconds or faster, non-channel-bound receptors take several hundred milliseconds or more to achieve their effect. This is due to the fact that a series of enzymatic reactions must take place between the initial signal and the response. Moreover, the signal itself is often “blurred” not only in time, but also in space, since it has been established that the transmitter can be released not from nerve endings, but from varicose thickenings (nodules) located along the axon. In this case, there are no morphologically expressed synapses, the nodules are not adjacent to any specialized receptive areas of the postsynaptic cell. Therefore, the mediator diffuses throughout a significant volume of nervous tissue, acting (like a hormone) immediately on the receptor field of many nerve cells located in different parts of the nervous system and even beyond it. This is the so-called indirect synaptic transmission.

During their functioning, synapses undergo functional and morphological rearrangements. This process is called synaptic plasticity. Such changes are most pronounced during high-frequency activity, which is a natural condition for the functioning of synapses in vivo. For example, the firing frequency of interneurons in the central nervous system reaches 1000 Hz. Plasticity can manifest itself as either an increase (potentiation) or a decrease (depression) in the efficiency of synaptic transmission. There are short-term (lasting seconds and minutes) and long-term (lasting hours, months, years) forms of synaptic plasticity. The latter are especially interesting because they relate to the processes of learning and memory. For example, long-term potentiation is a sustained increase in synaptic transmission in response to high-frequency stimulation. This type of plasticity can last for days or months. Long-term potentiation is observed in all parts of the central nervous system, but has been most fully studied at glutamatergic synapses in the hippocampus. Long-term depression also occurs in response to high-frequency stimulation and manifests itself as a long-term weakening of synaptic transmission. This type of plasticity has a similar mechanism to long-term potentiation, but develops at a low intracellular concentration of Ca2+ ions, while long-term potentiation occurs at a high one.

The release of mediators from the presynaptic terminal and the chemical transmission of the nerve impulse at the synapse can be influenced by mediators released from the third neuron. Such neurons and transmitters can inhibit synaptic transmission or, on the contrary, facilitate it. In these cases we talk about heterosynaptic modulation - heterosynaptic inhibition or facilitation depending on the final result.

Thus, chemical transmission is more flexible than electrical transmission, since both excitatory and inhibitory effects can be carried out without difficulty. In addition, when postsynaptic channels are activated by chemical agents, a sufficiently strong current can arise that can depolarize large cells.

Mediators - points of application and nature of action

One of the most difficult tasks facing neuroscientists is the precise chemical identification of transmitters acting at various synapses. To date, quite a lot of compounds are known that can act as chemical intermediaries in the intercellular transmission of nerve impulses. However, only a limited number of such mediators have been accurately identified; some of them will be discussed below. In order for the mediator function of a substance in any tissue to be irrefutably proven, certain criteria must be met:

1. when directly applied to the postsynaptic membrane, the substance should cause exactly the same physiological effects in the postsynaptic cell as when irritating the presynaptic fiber;

2. it must be proven that this substance is released upon activation of the presynaptic neuron;

3. the action of the substance must be blocked by the same agents that suppress the natural conduction of the signal.

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

There is the following classification of synapses:

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

Chemical;

Electrical (ephaps);

Mixed.

2) according to the released neurotransmitter:

Adrenergic - neurotransmitter norepinephrine;

Cholinergic - the neurotransmitter acetylcholine;

Dopaminergic - the neurotransmitter dopamine;

Serotonergic - the neurotransmitter serotonin;

GABAergic - neurotransmitter gamma-aminobutyric acid (GABA)

3) by influence:

Exciting;

Brake.

4) by location:

Neuromuscular;

Neuro-neural:

a) axo-somatic;

b) axo-axonal;

c) axo-dendritic;

d) dendrosomatic.

Consider three types of synapses: chemical, electrical and mixed(combining the properties of chemical and electrical synapses).

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

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

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

Rice. 2. Diagram of an electrical synapse

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Rice. 4. Post-synaptic membrane potentials

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

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

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

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

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

Rice. 5. Acetylcholine molecule

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

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

Rice. 6. Serotonin (5-hydroxytryptamine) molecule

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

Rice. 7. Catecholamines

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

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

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

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

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

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

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 soma of the neuron; 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 convergence, 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 physico-chemical 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, just 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. Neuron physiology: 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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What is a synapse? A synapse is a special structure that transmits a signal from the fibers of a nerve cell to another cell or fiber from a contact cell. Why do you need 2 nerve cells? In this case, the synapse is presented in 3 functional areas (presynaptic fragment, synaptic cleft and postsynaptic fragment) of nerve cells and is located in the area where the cell comes into contact with the muscles and glands of the human body.

The system of neuronal synapses is carried out according to their localization, type of activity and method of transit of available signal data. Regarding the localization of synapses, they are distinguished: neuroneuronal, neuromuscular. Neuroneuronal into axosomatic, dendrosomatic, axodendritic, axoaxonal.

According to the type of activity on perception, synapses are usually divided into: excitatory and no less important inhibitory. Regarding the method of transit of the information signal, they are classified into:

1. Electric type.
2. Chemical type.
3. Mixed type.

Etiology of neuron contact comes down to the type of docking, which can be distant, contact, and also borderline. The connection of a distant property is carried out through 2 neurons located in many parts of the body.

Thus, in the tissues of the human brain, neurohormones and neuropeptide substances are generated that affect the neurons present in the body in another location. The contact connection comes down to special junctions of membrane films of typical neurons that make up chemical synapses, as well as electrical components.

Adjacent (border) work of neurons is carried out during the time during which the membrane films of neurons are blocked only by the synaptic cleft. As a rule, such a merger is observed if between 2 special membrane films no glial tissue. This contiguity is characteristic of parallel fibers of the cerebellum, axons of a special olfactory nerve, and so on.

There is an opinion that adjacent contact provokes the work of nearby neurons in the production of a common function. This is observed due to the fact that metabolites, the fruits of the action of a human neuron, penetrating into the cavity located between the cells influence nearby active neurons. Moreover, an edge connection can often transmit electrical data from 1 working neuron to the 2nd participant in the process.

Electrical and chemical synapses

The action of film-membrane fusion is considered to be electrical synapses. In conditions where the required synaptic cleft is discontinuous with interstices of monolithic junctions. These partitions form an alternating structure of synapse compartments, while the compartments are separated by fragments of approximate membranes, the gap between which in synapses of the usual type is 0.15 - 0.20 nm in representatives of mammals. At the junction of membrane films there are pathways through which part of the fruit is exchanged.

In addition to individual types of synapses, there are the necessary electrical typical synapses in the form of a single synaptic cleft, the total perimeter of which extends to 1000 μm. Thus, a similar synaptic phenomenon is represented in ciliary ganglion neurons.

Electrical synapses are capable of conducting high-quality excitation unilaterally. This fact is noted when fixing the electrical reserve of the synaptic component. For example, at the moment when the afferent tubules are touched, the synaptic film-membrane is depolarized, when when the efferent particles of the fibers are touched, it becomes hyperpolarized. It is believed that synapses of active neurons with common responsibilities can carry out the required excitation (between 2 transmitting areas) in both directions.

On the contrary, the synapses of the present neurons with a different list of actions (motor and sensory) carry out the act of arousal unilaterally. The main work of synaptic components is determined by the production of immediate reactions of the body. The electrical synapse is subject to an insignificant amount of fatigue and has a significant percentage of resistance to internal-external factors.

Chemical synapses have the appearance of a presynaptic segment, a functional synaptic cleft with a fragment of the postsynaptic component. The presynaptic fragment is formed by an increase in the size of the axon within its own tubule or towards its termination. This fragment contains granular and agranular special sacs containing a mediator.

The presynaptic increase observes the localization of active mitochondria, generating particles of the substance glycogen, as well as required mediator production and other. Under conditions of frequent contact with the presynaptic field, the transmitter reserve in the existing sacs is lost.

There is an opinion that small granular vesicles contain a substance such as norepinephrine, and large ones contain catecholamines. Moreover, acetylchonine is located in the agranular cavities (vesicles). In addition, mediators of increased excitation are considered to be substances formed according to the type of aspartic acid produced or the equally important glutamine acid.

The active synapse contacts are often located between:

• Dendrite and axon.
• Soma and axon.
• Dendrites.
• Axons.
• Cell soma and dendrites.

The influence of the produced mediator relative to the presence of the postsynaptic membrane film occurs due to excessive penetration of its sodium particles. The generation of powerful outpourings of sodium particles from the working synaptic cleft through the postsynaptic membrane film forms its depolarization, forming the excitation of the postsynaptic reserve. The transit of the chemical direction of the synapse data is characterized by a synaptic suspension of excitation for a time of 0.5 ms with the development of a postsynaptic reserve, as a reaction to the presynaptic flow.

This possibility at the moment of excitation appears in the depolarization of the postsynaptic film-membrane, and at the moment of suspension in its hyperpolarization. What causes suspended postsynaptic reserve. As a rule, during strong excitation the level of permeability of the postsynaptic membrane film increases.

The required excitatory property is fixed inside neurons if norepinephrine, dopamine, acetyl choline, important serotonin, substance P and glutamine acid work in typical synapses.

The restraining potential is formed during the influence on synapses from gamma-aminobutyric acid and glycine.

Mental performance of children

A person’s performance directly determines his age, when all values ​​increase simultaneously with the development and physical growth of children.

The accuracy and speed of mental actions vary unevenly with age, depending on other factors that determine the development and physical growth of the body. Students of any age who have there are health deviations, characterized by a low level of performance relative to the surrounding strong children.

In healthy first-graders with a reduced readiness of the body for the constant learning process, according to some indicators, the ability to act is low, which complicates the fight against problems that arise during the learning process.

The rate of onset of weakness is determined by the initial state of the children's sensory nervous system, the work tempo and the volume of load. At the same time, children are prone to overwork during prolonged immobility and when the actions performed are uninteresting to the child. After a break, performance becomes the same or becomes higher than before, and it is better to make the rest not passive, but active, switching to a different activity.

The first part of the educational process for ordinary primary school children is accompanied by excellent performance, but by the end of the 3rd lesson they have there is a decrease in concentration:

• They look out the window.
• They do not listen attentively to the teacher's words.
• Change the position of their body.
• They start talking.
• They get up from their place.

The work capacity values ​​are particularly high for high school students studying in the 2nd shift. It is especially important to pay attention to the fact that the time for preparing for classes before the start of the educational activity in the classroom is quite short and does not guarantee complete relief from harmful changes in the central nervous system. Mental activity quickly depletes in the first hours of lessons, which is clearly reflected in negative behavior.

Therefore, qualitative changes in performance are observed in students of the junior block in lessons 1 - 3, and in the middle-senior blocks in lessons 4 - 5. In turn, lesson 6 takes place in conditions of particularly reduced ability to act. At the same time, the duration of classes for grades 2-11 is 45 minutes, which weakens the condition of the children. Therefore, it is recommended to periodically change the type of work, and take an active break in the middle of the lesson.

Moscow Psychological and Social Institute (MPSI)

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 number of tiny particles that are located in in strict order and each of them performs certain functions and has its own unique properties. This mechanism - the body, consists of cells, connecting their tissues and systems: all this as a whole represents a single chain, a supersystem of the body.

The greatest variety of cellular elements could not work as a single whole if a sophisticated regulatory mechanism did not exist in the body. The nervous system plays a special role in regulation. All the complex work of the nervous system - regulating the work of internal organs, controlling movements, whether simple and unconscious movements (for example, breathing) or complex movements of a person's hands - all this, in essence, is based on the interaction of cells with each other.

All this is essentially based on the transmission of a signal from one cell to another. Moreover, each cell does its own job, and sometimes has several functions. The variety of functions is provided by two factors: the way cells are connected to each other, and the way these connections are arranged.

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 of excitation and bilateral conductivity.

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 are shorter and more numerous processes of 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 sensory 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 the environment - these are 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 transform into one another.

However, in 1875, an Italian scientist, professor of histology at the University of Pavia, came up with a new way of staining cells - silvering. When one of the thousands of 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 WITH.

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 provide intercellular interactions.

Let's look at how they are carried out interneuronalinteractions. Information travels along a nerve cell in the form action potentials. The transfer of excitation from axon terminals to an innervated organ or other nerve cell occurs through intercellular structural formations - synapses(from 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 nerve brain elements, the more important the value of synaptic contacts becomes.

Different synaptic contacts differ from each other.

However, with all the diversity of synapses, there are certain common properties of their structure and function. Therefore, we first describe the general principles of their functioning.

A synapse is a complex structural 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 the dendrite of another neuron), as well as a synaptic cleft.

The mechanism of transmission through the synapse remained unclear for a long time, although it was obvious that the transmission of signals in the synaptic region 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 or electric or chemically. The electrical theory of synaptic transmission in the CNS enjoyed recognition until the early 1950s, but it lost ground significantly after the chemical synapse was demonstrated in a number of 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

CNS neurons allowed us to draw a conclusion about the chemical nature of transmission in interneuronal synapses of the spinal cord.

Microelectrode studies in recent years have shown that an electrical transmission mechanism exists at certain interneuron synapses.

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.

Synapse: structure, functions

Synapse(Greek synapsis - union) ensures unidirectional transmission of nerve impulses. Synapses are sites of functional contact between neurons or between neurons and other effector cells (for example, muscle and glandular cells).

Function synapse consists of converting an electrical signal (impulse) transmitted by a presynaptic cell into a chemical signal that affects another cell, known as a postsynaptic cell.

Most synapses transmit information by releasing neurotransmitters as part of the signal propagation process.

Neurotransmitters- these are chemical compounds that, by binding to a receptor protein, open or close ion channels or trigger second messenger cascades. Neuromodulators are chemical messengers that do not directly act on synapses, but change (modify) the sensitivity of a neuron to synaptic stimulation or synaptic inhibition.

Some neuromodulators are neuropeptides or steroids and are produced in nervous tissue, others are steroids circulating in the blood. The synapse itself includes an axon terminal (presynaptic terminal), which brings the signal, a site on the surface of another cell in which a new signal is generated (postsynaptic terminal), and a narrow intercellular space - the synoptic fissure.

If the axon ends on the cell body, it is an axosomatic synapse, if it ends on a dendrite, then such a synapse is known as axodendritic, and if it forms a synapse on an axon, it is an axoaxonal synapse.

Most of synapses- chemical synapses, because they use chemical messengers, but individual synapses transmit ionic signals through gap junctions that penetrate the pre- and postsynaptic membranes, thereby allowing direct transmission of neuronal signals.

Such contacts are known as electrical synapses.
Presynaptic terminal always contains synaptic vesicles with neurotransmitters and numerous mitochondria.

Neurotransmitters usually synthesized in the cell body; then they are stored in vesicles in the presynaptic part of the synapse. During transmission of a nerve impulse, they are released into the synaptic cleft through a process known as exocytosis.

5. Mechanism of information transmission in synapses

Endocytosis promotes the return of excess membrane, which accumulates in the presynaptic part as a result of exocytosis of synaptic vesicles.

Returned membrane fuses with the agranular endoplasmic reticulum (aERP) of the presynaptic compartment and is reused to form new synaptic vesicles.

Some neurotransmitters synthesized in the presynaptic compartment using enzymes and precursors that are delivered by the axonal transport mechanism.

The first to be described neurotransmitters there were acetylcholine and norepinephrine. The axon terminal releasing norepinephrine is shown in the figure.

Most neurotransmitters are amines, amino acids, or small peptides (neuropeptides). Some inorganic substances, such as nitric oxide, can also act as neurotransmitters. Certain peptides that act as neurotransmitters are used in other parts of the body, for example as hormones in the digestive tract.

Neuropeptides are very important in regulating sensations and impulses such as pain, pleasure, hunger, thirst and sex drive.

Sequence of phenomena during signal transmission at a chemical synapse

Phenomena occurring during transmission signal in a chemical synapse, illustrated in the figure.

Nerve impulses traveling rapidly (within milliseconds) across the cell membrane cause explosive electrical activity (depolarization) that spreads across the cell membrane.

Such impulses briefly open calcium channels in the presynaptic region, allowing an influx of calcium that triggers exocytosis of synaptic vesicles.

In areas of exopitosis there are neurotransmitters, which react with receptors located at the postsynaptic site, causing transient electrical activity (depolarization) of the postsynaptic membrane.

Such synapses are known as excitatory synapses because their activity promotes the generation of impulses in the postsynaptic cell membrane. In some synapses, the interaction between the neurotransmitter and the receptor produces the opposite effect - hyperpolarization occurs, and there is no transmission of the nerve impulse. These synapses are known as inhibitory synapses. Thus, synapses can either enhance or inhibit the transmission of impulses, thereby they are able to regulate neural activity.

After use neurotransmitters are quickly removed due to enzymatic destruction, diffusion or endocytosis mediated by specific receptors on the presynaptic membrane. This removal of neurotransmitters has important functional significance because it prevents unwanted prolonged stimulation of the postsynaptic neuron.

Training video - structure of a synapse

1. Nerve cell body - neuron: structure, histology
2. Dendrites of nerve cells: structure, histology
3. Nerve cell axons: structure, histology
4. Membrane potentials of nerve cells.

   Physiology

5. Synapse: structure, functions
6. Glial cells: oligodendrocytes, Schwann cells, astrocytes, ependymal cells
7. Microglia: structure, histology
8. Central nervous system (CNS): structure, histology
9. Histology of the meninges. Structure
10. Blood-brain barrier: structure, histology

Synapse structure Let us consider the structure of a synapse using an axosomatic one as an example. The synapse consists of three parts: the presynaptic terminal, the synaptic cleft and the postsynaptic membrane (Fig. 9). The presynaptic terminal (synaptic plaque) is an expanded part of the axon terminal. The synaptic cleft is the space between two neurons in contact. 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 presynaptic terminal is filled with vesicles and mitochondria. The vesicles contain biologically active substances - mediators. Mediators are synthesized in the soma and transported via microtubules to the presynaptic terminal. The most common mediators are adrenaline, norepinephrine, acetylcholine, serotonin, gamma-aminobutyric acid (GABA), glycine and others. Typically, a synapse contains one of the transmitters in greater quantities compared to other transmitters. It is customary to designate synapses based on the type of mediator: adrenergic, cholinergic, serotonergic, etc. The postsynaptic membrane contains special protein molecules - receptors that can attach molecules of mediators. The synaptic cleft is filled with intercellular fluid, which contains enzymes that promote the destruction of neurotransmitters. One postsynaptic neuron can have up to 20,000 synapses, some of which are excitatory, and some are inhibitory. In addition to chemical synapses, in which neurotransmitters are involved in the interaction of neurons, electrical synapses are found in the nervous system. In electrical synapses, the interaction of two neurons is carried out through biocurrents. Chemical synapse Nerve fiber PD (AP - action potential) what membrane receptors Rice. 9. Scheme of the structure of a synapse. The central nervous system is dominated by chemical synapses. In some interneuron synapses, electrical and chemical transmission occurs simultaneously - this is a mixed type of synapse. The influence of excitatory and inhibitory synapses on the excitability of the postsynaptic neuron is additive, and the effect depends on the location of the synapse. The closer the synapses are located to the axonal hillock, the more effective they are. On the contrary, the further the synapses are located from the axonal hillock (for example, at the end of dendrites), the less effective they are. Thus, synapses located on the soma and axonal hillock influence the excitability of the neuron quickly and efficiently, while the influence of distant synapses is slow and smooth. Amps iipinl system Neural networks Thanks to synaptic connections, neurons are united into functional units - neural networks. Neural networks can be formed by neurons located at a short distance. Such a neural network is called local. In addition, neurons remote from each other from different areas of the brain can be combined into a network. The highest level of organization of neuronal connections reflects the connection of several areas of the central nervous system. Such a nervous network is called a pathway, or system. There are descending and ascending paths. Along ascending pathways, information is transmitted from underlying areas of the brain to higher ones (for example, from the spinal cord to the cerebral cortex). Descending tracts connect the cerebral cortex with the spinal cord. The most complex networks are called distribution systems. They are formed by neurons in different parts of the brain that control behavior, in which the body participates as a whole. Some nerve networks provide convergence (convergence) of impulses on a limited number of neurons. Nervous networks can also be built according to the type of divergence (divergence). Such networks enable the transmission of information over considerable distances. In addition, neural networks provide integration (summarization or generalization) of various types of information (Fig. 10).