What is a synapse and synaptic cleft. Chemical and electrical synapses

Depending on which structures of the neuron are involved in the formation of a synapse, axosomatic, axodendritic, axoaxonal and dendrodendritic synapses are distinguished. The synapse formed by the axon of the motor neuron and the muscle cell is called the end plate (neuromuscular junction, myoneural synapse). The indispensable structural attributes of the synapse are the presynaptic membrane, the postsynaptic membrane, and the synaptic gap between them. Let's take a closer look at each of them.

The presynaptic membrane is formed by the end of the terminal branches of the axon (or dendrite in the dendrodendritic synapse). The axon leaving the body of the nerve cell is covered with a myelin sheath, which accompanies it throughout, up to the branching into terminal terminals. The number of terminal branches of the axon can reach several hundred, and their length, now devoid of the myelin sheath, can be up to several tens of microns. The terminal branches of the axon have a small diameter - 0.5-2.5 microns, sometimes more. The endings of the terminals at the point of contact have a variety of shapes - in the form of a club, a reticulate plate, a ringlet, or may be multiple - in the form of a cup, a brush. The terminal terminal may have several extensions that contact in the course of movement with different parts of the same cell or with different cells, thus forming a plurality of synapses. Some researchers call such synapses tangent.

At the site of contact, the terminal terminal thickens somewhat and the part of its membrane adjacent to the membrane of the contacted cell forms a presynaptic membrane. In the zone of the terminal terminal, adjacent to the presynaptic membrane, electron microscopy revealed an accumulation of ultrastructural elements - mitochondria, the number of which fluctuates, sometimes reaching several tens, microtubules and synaptic vesicles (vesicles). The latter are of two types - agranular (light) and granular (dark). The former are 40-50 nm in size, the diameter of granular vesicles is usually more than 70 nm. Their membrane is cell-like and consists of a phospholipid bilayer and proteins. Most of the vesicles are fixed on the cytoskeleton with the help of a specific protein - synapsin, forming a transmitter reservoir. A minority of vesicles are attached to the inner side of the presynaptic membrane by means of the vesicle membrane protein, synaptobrevin, and the presynaptic membrane protein, syntaxin. There are two hypotheses regarding the origin of vesicles. According to one of them (Hubbard, 1973), they are formed in the region of the presynaptic ending from the so-called bordered vesicles. The latter are formed in the recesses of the cell membrane of the presynaptic ending and merge into cisterns, from which vesicles bud, filled with a mediator. According to another view, vesicles, as membrane formations, are formed in the soma of the neuron, transported empty along the axon to the area of ​​the presynaptic ending, and there they are filled with a mediator. After release of the neurotransmitter, the emptied vesicles are returned by retrograde axon transport to the soma, where they are degraded by lysosomes.

Synaptic vesicles are most densely located near the inner surface of the presynaptic membrane and their number is not constant. The vesicles are filled with a mediator; in addition, the so-called cotransmitters are concentrated here - substances of a protein nature that play an essential role in ensuring the activity of the main mediator. Small vesicles contain low molecular weight mediators, while large vesicles contain proteins and peptides. It has been shown that the mediator can also be located outside the vesicles. Calculations show that in the human neuromuscular junction the density of vesicles reaches 250-300 per 1 µm 2 , and their total number is about 2-3 million in one synapse. In one vesicle, from 400 to 4-6 thousand molecules of the mediator are concentrated, which is the so-called "quantum of the mediator", which is released into the synaptic cleft spontaneously or when an impulse arrives along the presynaptic fiber. The surface of the presynaptic membrane is heterogeneous - it has thickenings, active zones where mitochondria accumulate and the density of vesicles is the highest. In addition, voltage-gated calcium channels were found in the active zone, through which calcium passes through the presynaptic membrane into the presynaptic zone of the terminal terminal. In many synapses, so-called autoreceptors are built into the presynaptic membrane. When they interact with mediators released into the synaptic cleft, the release of the latter either increases or stops, depending on the type of synapse.

Synaptic cleft - the space between the presynaptic and postsynaptic membranes, limited by the contact area, the size of which for most neurons varies within a few microns 2. The area of ​​contact can vary in different synapses, which depends on the diameter of the presynaptic terminal, the form of contact, and the nature of the surface of the contacting membranes. Thus, for the most studied neuromuscular synapses, it has been shown that the contact area of ​​one presynaptic terminal with a myofibril can be tens of microns 2 . The size of the synaptic cleft ranges from 20 to 50-60 nm. Outside the contact, the cavity of the synaptic cleft communicates with the intercellular space, thus, a two-way exchange of various chemical agents is possible between them.

The postsynaptic membrane is a section of the membrane of a neuron, muscle or glandular cell in contact with the presynaptic membrane. As a rule, the area of ​​the postsynaptic membrane is somewhat thickened compared to neighboring areas of the contacted cell. In 1959, E. Gray proposed to divide the synapses in the cerebral cortex into two types. Type 1 synapses have a wider gap, their postsynaptic membrane is thicker and denser than type 2 synapses, the densified area is more extensive and occupies most of both synaptic membranes.

Protein-glycolipid complexes are embedded in the postsynaptic membrane, which act as receptors that can bind to mediators and form ion channels. Thus, the acetylcholine receptor in the myoneural synapse consists of five subunits that form a complex with a molecular weight of 5000-30000, penetrating the membrane. It has been shown by calculation that the density of such receptors can be up to 9 thousand per µm 2 of the surface of the postsynaptic membrane. The head of the complex protruding into the synaptic cleft has a so-called "recognizing center". When two molecules of acetylcholine are bound to it, the ion channel opens, its inner diameter becomes passable for sodium and potassium ions, while the channel remains impassable for anions due to the charges present on its inner walls. The most important role in the processes of synaptic transmission is played by a membrane protein called G-protein, which, in combination with guanine triphosphate (GTP), activates enzymes that include second messengers - intracellular regulators.

Receptors of postsynaptic membranes are located in the so-called "active zones" of synapses and among them two types are distinguished - ionotropic and metabotropic. In ionotropic (fast) receptors, their interaction with the mediator molecule is sufficient to open ion channels; the mediator directly opens the ion channel. Metabotropic (slow) receptors got their name in connection with the peculiarities of their functioning. The opening of ion channels in this case is associated with a cascade of metabolic processes involving various compounds (proteins, including G-protein, calcium ions, cyclic nucleotides - cAMP and cGMP, diacetylglycerols), which play the role of second messengers. Metobotropic receptors themselves are not ion channels; they only modify the operation of nearby ion channels, ion pumps, and other proteins through indirect mechanisms. Ionotropic receptors include GABA, glycine, glutamate, H-cholinergic receptors. To metabotropic - dopamine, serotonin, norepinephrine receptors, M-cholinergic receptors, some GABA, glutamate receptors.

Usually, receptors are located strictly within the postsynaptic membrane, so the influence of mediators is possible only in the synapse region. It was found, however, that a small number of acetylcholine-sensitive receptors exist outside the neuromuscular junction in the muscle cell membrane. Under certain conditions (during denervation, poisoning with certain poisons), zones sensitive to acetylcholine can form outside the synaptic contacts on the myofibril, which is accompanied by the development of muscle hypersensitivity to acetylcholine.

Receptors sensitive to acetylcholine are also widely distributed in CNS synapses and in peripheral ganglia. Excitatory receptors are divided into two classes, differing in pharmacological characteristics.

One of them is a class of receptors on which nicotine has similar effects to acetylcholine, hence their name - nicotine-sensitive (N-cholinergic receptors), the other class - sensitive to muscarine (fly agaric venom) are called M-cholinergic receptors. In this regard, synapses, where the main mediator is acetylcholine, are divided into groups of nicotinic and muscarinic types. Within these groups, many varieties are distinguished depending on the location and features of functioning. So, synapses with H-cholinergic receptors are described in all skeletal muscles, in the endings of preganglionic parasympathetic and sympathetic fibers, in the adrenal medulla, and muscarinic synapses in the central nervous system, smooth muscles (in synapses formed by the endings of parasympathetic fibers), in the heart.

In most synapses of the nervous system, chemicals are used to transmit signals from the presynaptic neuron to the postsynaptic neuron - mediators or neurotransmitters. Chemical signaling is carried out through chemical synapses(Fig. 14), including the membranes of pre- and postsynaptic cells and separating them synaptic cleft- area 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- vesicles 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 pre- and postsynaptic membranes together.

The following sequence of events has been established during transmission through a chemical synapse. When the action potential reaches the presynaptic ending, the membrane depolarizes in the synapse zone, the calcium channels of the plasma membrane are activated, and Ca 2+ ions enter the ending. An increase in intracellular calcium levels initiates exocytosis of mediator-filled vesicles. The contents of the vesicles are released into the extracellular space, and some of the mediator molecules, by 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 mediator-dependent channels (otherwise called ligand-activated channels or ionotropic receptors). They open and let the corresponding ions into the cell. The movement of ions along their electrochemical gradients generates sodium depolarizing(exciting) or potassium (chlorine) hyperpolarizing (braking) 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 rate of impulse conduction in the synapse is less than along the fiber, i.e. there is a synaptic delay, for example, in the neuromuscular synapse of a frog - 0.5 ms. The sequence of events described above is typical for the so-called. direct synaptic transmission.

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

G-proteins, so named for their ability to bind to guanine nucleotides, are trimers consisting of three subunits: α, β and g. There are a large number of varieties of each of the subunits (20 α, 6 β , 12γ). which creates the basis for a huge number of their combinations. G-proteins are divided into four main groups according to the structure and targets of their α-subunits: G s stimulates adenylate cyclase; G i inhibits adenylate cyclase; G q binds to phospholipase C; C 12 targets 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 of the G proteins 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 bound to 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 the 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 to form GDP. As a result, α -- and βγ subunits bind again, which leads to the termination of their activity.

To date, >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 an 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 neurotransmitter can be released not from nerve endings, but from varicose thickenings (nodules) located along the axon. In this case, there are no morphologically pronounced synapses, the nodules are not adjacent to any specialized receptive areas of the postsynaptic cell. Therefore, the mediator diffuses in a significant amount of the nervous tissue, acting (like a hormone) immediately on the receptor field in many nerve cells located in various parts of the nervous system and even beyond it. This is the so-called. indirect synaptic transmission.

In the course of functioning, synapses undergo functional and morphological rearrangements. This process is named 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 frequency of firing of intercalary neurons in the CNS 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 (seconds and minutes last) and long-term (hours, months, years) forms of synaptic plasticity. The latter are particularly interesting in that they are related to the processes of learning and memory. For example, long-term potentiation is a steady increase in synaptic transmission in response to high-frequency stimulation. This kind of plasticity can go on for days or months. Long-term potentiation is observed in all parts of the CNS, but is 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 with long-term potentiation, but develops at a low intracellular concentration of Ca2+ ions, while long-term potentiation develops at a high one.

The release of mediators from the presynaptic ending and the chemical transmission of the nerve impulse in the synapse can be influenced by mediators released from the third neuron. Such neurons and mediators can inhibit synaptic transmission or, conversely, facilitate it. In these cases, one speaks of heterosynaptic modulation - heterosynaptic inhibition or facilitation depending on the end result.

Thus, chemical transmission is more flexible than electrical transmission, since both excitatory and inhibitory actions 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 - application points and nature of action

One of the most difficult tasks facing neurophysiologists is the precise chemical identification of neurotransmitters acting at different synapses. To date, quite a lot of compounds are known that can act as chemical mediators in the intercellular transmission of a nerve impulse. However, only a limited number of such mediators have been accurately identified; some of which 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 applied directly to the postsynaptic membrane, the substance should cause exactly the same physiological effects in the postsynaptic cell as when the presynaptic fiber is stimulated;

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.

The concept of synapse. Types of synapses

The term synapse (from the Greek sy "napsys - connection, connection) was introduced by I. Sherrington in 1897. Currently synapses are specialized functional contacts between excitable cells (nerve, muscle, secretory), which serve to transmit and transform nerve impulses. According to the nature of the contact surfaces, there are: axo-axonal, axo-dendritic, axo-somatic, neuromuscular, neuro-capillary synapses. Electron microscopic studies revealed that synapses have three main elements: the presynaptic membrane, the postsynaptic membrane and the synaptic cleft (Fig. 37).

Rice. 37. The main elements of the synapse.

The transmission of information across the synapse can be carried out chemically or electrically. Mixed synapses combine chemical and electrical transmission mechanisms. In the literature, based on the method of transmitting information, it is customary to distinguish three groups of synapses - chemical, electrical and mixed.

The structure of chemical synapses

The transmission of information in chemical synapses is carried out through the synaptic cleft - a region of the extracellular space 10-50 nm wide, separating the membranes of pre- and postsynaptic cells. The presynaptic ending contains synaptic vesicles (Fig. 38) - membrane vesicles with a diameter of about 50 nm., Each of which contains 1x104 - 5x104 mediator molecules. The total number of such vesicles in presynaptic endings is several thousand. The cytoplasm of the synaptic plaque contains mitochondria, smooth endoplasmic reticulum, microfilaments (Fig. 39).

Rice. 38. Structure of a chemical synapse

Rice. 39. Scheme of the neuromuscular synapse

The synaptic cleft is filled with mucopolysaccharide, which "sticks together" the pre- and postsynaptic membranes.

The postsynaptic membrane contains large protein molecules that act as mediator-sensitive receptors, as well as numerous channels and pores through which ions can enter the postsynaptic neuron.

Transfer of information in chemical synapses

When an action potential arrives at the presynaptic ending, the presynaptic membrane depolarizes and its permeability for Ca 2+ ions increases (Fig. 40). An increase in the concentration of Ca 2+ ions in the cytoplasm of the synaptic plaque initiates exocytosis of mediator-filled vesicles (Fig. 41).

The contents of the vesicles are released into the synaptic cleft, and some of the mediator molecules diffuse, binding to the receptor molecules of the postsynaptic membrane. On average, each vesicle contains about 3000 transmitter molecules, and diffusion of the transmitter to the postsynaptic membrane takes about 0.5 ms.

Rice. 40. The sequence of events occurring in a chemical synapse from the moment of excitation of the presynaptic ending to the occurrence of AP in the postsynaptic membrane.

Rice. 41. Exocytosis of synaptic vesicles with mediator. The vesicles fuse with the plasma membrane and eject their contents into the synaptic cleft. The mediator diffuses to the postsynaptic membrane and binds to receptors located on it. (Eccles, 1965).

When the mediator molecules bind to the receptor, its configuration changes, which leads to the opening of ion channels (Fig. 42) and the entry of ions through the postsynaptic membrane into the cell, causing the development of the end plate potential (EPP). PKP is the result of a local change in the permeability of the postsynaptic membrane for Na + and K + ions. However, PEP does not activate other chemo-excitable channels of the postsynaptic membrane and its value depends on the concentration of the mediator acting on the membrane: the greater the concentration of the mediator, the higher (up to a certain limit) the PEP. Thus, the EPP, in contrast to the action potential, is gradual. In this respect, it is similar to the local response, although the mechanism of its occurrence is different. When the PCR reaches a certain threshold value, local currents arise between the area of ​​the depolarized postsynaptic membrane and adjacent sections of the electrically excitable membrane, which causes the generation of an action potential.

Rice. 42. Structure and operation of a chemoexcitable ion channel. The channel is formed by a protein macromolecule immersed in the lipid bilayer of the membrane. Before the mediator molecule interacts with the receptor, the gate is closed (A). They open when the mediator binds to the receptor (B). (According to Khodorov B.I.).

Thus, the process of excitation transmission through a chemical synapse can be schematically represented as the following chain of events: action potential on the presynaptic membrane entry of Ca 2+ ions into the nerve ending release of the mediator diffusion of the mediator through the synaptic cleft to the postsynaptic membrane interaction of the mediator with the receptor activation of chemo-excitable channels of the postsynaptic membranes the emergence of the potential of the end plate critical depolarization of the postsynaptic electrically excitable membrane generation of the action potential.

Chemical synapses have two properties in common:

1. Excitation through a chemical synapse is transmitted in only one direction - from the presynaptic membrane to the postsynaptic membrane (unilateral conduction).

2. Excitation is conducted through the synapse much more slowly than the synaptic delay along the nerve fiber.

The one-sidedness of conduction is due to the release of the mediator from the presynaptic membrane and the localization of receptors on the postsynaptic membrane. Slowing down of conduction through the synapse (synaptic delay) occurs due to the fact that conduction is a multi-stage process (transmitter secretion, transmitter diffusion to the postsynaptic membrane, activation of chemoreceptors, PKD growth to a threshold value) and each of these stages requires time. In addition, the presence of a relatively wide synaptic cleft prevents impulse conduction using local currents.

Chemical mediators

Mediators (from Latin - mediator - conductor) - biologically active substances through which intercellular interactions are carried out in synapses.

In general, chemical mediators are low molecular weight substances. However, some high molecular weight compounds, such as polypeptides, can also act as chemical messengers. Currently, a number of substances are known that play the role of mediators in the CNS of mammals. These include acetylcholine, biogenic amines: adrenaline, norepinephrine, dopamine, serotonin, acidic amino acids: glycines, gamma-aminobutyric acid (GABA), polypeptides: substance P, enkephalin, somatostatin, etc. (Fig. 43).

Rice. 43. Structural formulas of some mediators.

The function of mediators can also be performed by such compounds as ATP, histamine, prostaglandins. In 1935, G. Dale formulated a rule (the Dale principle), according to which each nerve cell releases only one specific mediator. Therefore, it is customary to designate neurons according to the type of mediator that is released in their endings. So, neurons that release acetylcholine are called cholinergic, norepinephrine - adrenergic, serotonin - serotonergic, amines - aminergic, etc.

Quantum extraction of mediators

Studying the mechanisms of neuromuscular transmission, Paul Fett and Bernard Katz in 1952 registered miniature postsynaptic potentials (MPSPs). MPSP can be registered in the area of ​​the postsynaptic membrane. As the intracellular recording electrode moves away from the postsynaptic membrane, the MPSP gradually decreases. The amplitude of the MCSP is less than 1 mV. (Fig. 44).

Rice. 44. Miniature postsynaptic potentials recorded in the region of the end plate of a skeletal muscle fiber. It can be seen that the amplitude of the MCSP is small and constant. (According to R. Eckert).

Katz and his collaborators investigated the relationship between SMSPs and common PEPs that occur when motor nerves are stimulated. It has been proposed that the MCCS is the result of the separation of the "quantum" of the mediator, and the CPP is formed as a result of the summation of many MCCS. It is now known that the "quantum" of the mediator is a "package" of mediator molecules in the synaptic vesicle of the presynaptic membrane. According to calculations, each MSP corresponds to the release of a transmitter quantum consisting of 10,000 - 40,000 mediator molecules, which leads to the activation of about 2000 postsynaptic ion channels. For the emergence of an end plate potential (EPP) or an excitatory postsynaptic potential (EPSP), it is necessary to release 200-300 transmitter quanta.

Action potential generation

Miniature postsynaptic potential, end plate potential and excitatory postsynaptic potential are local processes. They cannot propagate and therefore cannot provide information transfer between cells. The site for generating action potentials in the motor neuron is the initial segment of the axon, located immediately behind the axon hillock (Fig. 45). This area is most sensitive to depolarization and has a lower critical level of depolarization than the body and dendrites of the neuron. Therefore, it is in the region of the axon hillock that action potentials arise. In order to cause excitation, PKP (or EPSP) must reach a certain threshold level in the region of the axon hillock (Fig. 46). Rice. 46. ​​Spatial attenuation of EPSPs and action potential generation. Excitatory synaptic potentials that arise in the dendrite decay as they spread through the neuron. The AP generation threshold (critical level of depolarization) depends on the density of sodium channels (black dots). Although the synaptic potential (shown at the top of the figure) decays as it propagates from the dendrite to the axon, AP still occurs in the region of the axon hillock. It is here that the density of sodium channels is the highest, and the threshold level of depolarization is the lowest. (R. Eckert).

The summation of excitatory synaptic influences is important for the emergence of an action potential in a nerve cell, since the depolarization created by one synapse is often not enough to reach the threshold level and generate an action potential. So, if there is an increase in EPSP due to the addition of potentials arising due to the work of different synapses, then spatial summation takes place (Fig. 48). The critical level of depolarization can also be achieved due to temporary summation (Fig. 47). Rice. 47. Scheme of somoto-dentritic synapses, providing summation of excitation. So, if after one postsynaptic potential another arises, then the second potential is "superimposed" on the first, as a result of which a total potential with a larger amplitude is formed (Fig. 49.).

The shorter the interval between two successive synaptic potentials, the higher the amplitude of the total potential. Under natural conditions, both spatial and temporal summations usually occur simultaneously. Thus, during the period between the release of the mediator into the synaptic cleft and the occurrence of an action potential on the postsynaptic structure (neuron, muscle, gland), a number of bioelectric phenomena occur, the sequence and specific features of which are presented in (Table 1) and (Fig. 51.).

Rice. 48. Spatial summation in a motor neuron Fig 49. Time summation. With a high repetition rate of stimuli, it is possible to “impose” one postsynaptic potential on another, resulting in the formation of a total potential with a larger amplitude.

1. Excitatory postsynaptic potentials arising in two different synapses (A and B).

2. Potentials arising on the membrane in the zone of impulse generation when fiber A or B is stimulated, or both of these fibers simultaneously (A + B).

3. In order for the potential in the region of the axon hillock to exceed the threshold level, the spatial summation of SNPSs that occur in several synapses is necessary. (R. Eckert).

In addition to excitatory synapses, through which excitation is transmitted, there are inhibitory synapses, in which mediators (in particular, GABA) cause inhibition on the postsynaptic membrane (Fig. 50). In such synapses, excitation of the presynaptic membrane leads to the release of an inhibitory mediator, which, acting on the postsynaptic membrane, causes the development of IPSP (inhibitory postsynaptic potential). The mechanism of its occurrence is associated with an increase in the permeability of the postsynaptic membrane for K + and Cl -, resulting in its hyperpolarization. The braking mechanism will be described in more detail in the next lecture.

Rice. 50. Scheme of spatial summation in the presence of excitatory and inhibitory synapses.

TABLE #1. Types of Potentials Place of origin The nature of the process Type of electric potentials Amplitude Miniature postsynaptic potential (MPSP) Neuromuscular and interneuronal synapses Miniature local depolarization Gradual End plate potential (EPP) neuromuscular junction Local depolarization Gradual Excitatory postsynaptic potential (EPSP) Interneuronal synapses Local depolarization Gradual Action potential (AP) Nerve, muscle, secretory cells A propagating process Impulse (according to the law "all or nothing")

Rice. 51. The sequence of bioelectric phenomena in the chemical synapse occurring during the time between the release of the mediator and the occurrence of AP on the postsynaptic structure.

Metabolism of mediators

Acetylcholine, secreted by the endings of cholinergic neurons, is hydrolyzed to choline and acetate by the enzyme acetylcholinesterase. Hydrolysis products do not act on the postsynaptic membrane. The resulting choline is actively absorbed by the presynaptic membrane and, interacting with acetyl coenzyme A, forms a new acetylcholine molecule. (Fig. 52.).

Rice. 52. Metabolism of acetylcholine (Ach) in the cholinenergic synapse. ACh coming from the presynaptic ending is hydrolyzed in the synaptic cleft by the enzyme acetylcholinesterase (ACChE). Choline enters the presynaptic fiber and is used to synthesize acetylcholine molecules (Mountcastle and Baldessarini, 1968)

A similar process occurs with other mediators. Another well-studied neurotransmitter, norepinephrine, is secreted by postganglionic synaptic cells and chromaffin cells of the adrenal medulla. The biochemical transformations that norepinephrine undergoes in adrenergic synapses are schematically shown in Figure 53.

Rice. 53. Biochemical transformations of the mediator in the adrenergic synapse. Norepinephrine (NA) is synthesized from the amino acid phenylalanine to form an intermediate product, tyrosine. The resulting NA is stored in synaptic vesicles. After release from the synapse, part of the HA is reuptaken by the presynaptic fiber, while the other part is inactivated by methylation and removed from the bloodstream. NA that enters the cytoplasm of the presynaptic ending is either taken up by synaptic vesicles or degraded by monoamine oxidase (MAO). (Mountcastle and Baldessarini, 1968).

synaptic modulation

The biochemical processes taking place in the synapse are largely influenced by various factors, primarily chemical ones. Thus, acetylcholinesterase can be inactivated by certain nerve agents and insecticides. In this case, acetylcholine accumulates in synapses. This leads to a violation of the repolarization of the postsynaptic membrane and inactivation of cholinergic receptors (Fig. 54.). As a result, the activity of interneuronal and neuromuscular synapses is disrupted and the body quickly dies. However, a large number of substances are formed in the nervous system that play the role of synaptic modulators - substances that affect synaptic conduction.

Rice. 54. Effect of a cholinesterase inhibitor (neostigmine) on the duration of the postsynaptic potential of a single muscle fiber.a - before the use of neostigmine; b - after the application of neostigmine. (According to B.I. Khodorov).

By chemical nature, these substances are peptides, but they are often called neuropeptides, although not all of them are formed in the nervous system. So, a number of substances are synthesized in the endocrine cells of the intestine, and some neuropeptides were originally found in the internal organs. The best known substances of this kind are the hormones of the gastrointestinal tract - glucagon, gastrin, cholecystokinin, substance P, gastric inhibitory peptide (GIP).

Two groups of neuropeptides, endorphins and enkephalins, are of considerable interest to researchers. These substances have analgesic (pain-reducing), hallucinogenic, and some other properties (cause a feeling of satisfaction and euphoria, their activation speeds up the pulse and raises body temperature). The analgesic effect of these compounds may be due to the fact that these neuropeptides interfere with the release of neurotransmitters from certain nerve endings. This point of view is in good agreement with the fact that enkephalins and endorphins are present in the posterior horns of the spinal cord, i.e. in the area where sensory pathways enter the spinal cord. Pain sensations can be reduced as a result of the release of neuropeptides that disrupt synaptic conduction in efferent pathways, transmitting pain signals. The content of endorphins and enkephalins is not constant: for example, during meals, pain, listening to pleasant music, their release increases. Thus, the body protects itself from excessive pain and bestows for biologically beneficial actions. Due to these properties, as well as the fact that these neuropeptides bind in the nervous system to the same receptors as opiates (opium and its derivatives), they are called endogenous opioids . It is now known that on the surface of the membrane of some neurons there are opioid receptors with which, under natural conditions, the enkephalins and endorphins produced by the nervous system bind. But with the use of narcotic opiates - alkaloid substances secreted from plants, opiates bind to opioid receptors, causing them to be unnaturally powerful stimulation. This causes extremely pleasant subjective sensations. With repeated use of opioids, compensatory changes in the metabolism of nerve cells occur, and then, after their withdrawal, the state of the nervous system becomes such that the patient experiences extreme discomfort (withdrawal syndrome) without the administration of the next dose of the drug. This metabolic addiction is called addiction.

In the study of opioid receptors, the substance naloxone, a competitive blocker of these receptors, turned out to be very useful. Since naloxone interferes with the binding of opiates to target cells, it can be used to determine whether a particular reaction is caused by excitation of such receptors. Naloxone, for example, has been found to largely reverse the analgesic effects of placebos (a neutral substance given to patients with the assurance that it will relieve their pain). It is likely that belief in a drug (or other treatment) that is supposed to relieve pain leads to the release of opioid peptides; perhaps this is the pharmacological mechanism of placebo action. Naloxone also removes the analgesic effect of acupuncture. From this it was concluded that natural opioid peptides are released from the CNS during acupuncture.

Thus, the efficiency of synaptic transmission can be significantly changed under the influence of substances (modulators) that are not directly involved in the transmission of information.

Features of the structure and functioning of electrical synapses

Electrical synapses are widespread in the nervous system of invertebrates, and are extremely rare in mammals. At the same time, electrical synapses in higher animals are widespread in the heart muscle, smooth muscles of the internal organs of the liver, epithelial and glandular tissues.

The width of the synaptic gap in electrical synapses is only 2-4 nm, which is much less than in chemical synapses. An important feature of electrical synapses is the presence between the presynaptic and postsynaptic membranes of peculiar bridges formed by protein molecules. They are channels 1-2 nm wide (Fig. 55.).

Rice. 55. The structure of the electrical synapse. Characteristic features: a narrow (2-4 nm) synaptic cleft and the presence of channels formed by protein molecules.

Due to the presence of channels, the size of which allows inorganic ions and even small molecules to pass from cell to cell, the electrical resistance of such a synapse, called a gap or high-permeability junction, is very low. Such conditions allow the presynaptic current to spread to the postsynaptic cell with virtually no extinction. An electric current flows from an excited area to an unexcited one and flows out there, causing its depolarization (Fig. 56.).

Rice. 56. Scheme of excitation transfer in chemical (A) and electrical synapse (B). The arrows show the propagation of electric current through the membrane of the presynaptic ending and the postsynaptic membrane to the neuron. (According to B.I. Khodorov).

Electrical synapses have a number of specific functional properties: There is practically no synaptic delay; there is no interval between the arrival of an impulse at the presynaptic ending and the beginning of the postsynaptic potential. In electrical synapses, conduction is bidirectional, although the geometry of the synapse makes conduction in one direction more efficient. Electrical synapses, unlike chemical synapses, can ensure the transmission of only one process - excitation. Electrical synapses are less affected by various factors (pharmacological, thermal, etc.)

Along with chemical and electrical synapses, there are so-called mixed synapses between some neurons. Their main feature is that electrical and chemical transmission is carried out in parallel, since the gap between the pre- and postsynaptic membranes has sections with the structure of chemical and electrical synapses (Fig. 57.).

Rice. 57. Structure of a mixed synapse. A - chemical transmission site. B - electric transmission section. 1. Presynaptic membrane. 2. Postsynaptic membrane. 3. Synaptic cleft.

The main functions of synapses

The significance of the mechanisms of cell functioning becomes clear when the processes of their interaction necessary for the exchange of information are clarified. Information is exchanged through nervous system and in herself. The points of contact between nerve cells (synapses) play an important role in the transfer of information. Information in the form of a series of action potentials comes from the first ( presynaptic) neuron to the second ( postsynaptic). This is possible directly through the formation of a local current between neighboring cells or, more often, indirectly through chemical carriers.

There is no doubt about the importance of cell functions for the successful functioning of the whole organism. However, in order for an organism to function as a whole, an interconnection must be carried out between its cells - the transfer of various chemicals and information. In the transmission of information involved, for example, hormones delivered to the cells by the blood. But, first of all, the transmission of information is carried out in the nervous system in the form of nerve impulses. Thus, the sense organs receive information from the surrounding world, for example, in the form of sound, light, smell, and transmit it further along the corresponding nerves to the brain. central nervous system, for its part, must process this information and, as a result, again issue some information to the periphery, which can be figuratively represented in the form of certain orders to peripheral effector organs, such as muscles, glands, and sensory organs. This will be the answer to external irritations.

The transmission of information, for example, from the receptors of the organ of hearing to the brain also includes its processing in the central nervous system. To do this, millions of nerve cells must interact with each other. Only on the basis of this processing of the received information is it possible to form the final response, for example, directed actions or the termination of these actions, flight or attack. These two examples indicate that information processing in the CNS can lead to reactions involving either excitatory or inhibitory processes. The contact zones between nerve cells - synapses - also take part in the transmission of information and the formation of the response of the central nervous system. In addition to synaptic contacts between interneurons in the CNS, these processes are carried out by synaptic contacts lying on the transmission path efferent information, synapses between axon and the executive neuron and outside the CNS (on the periphery) between the executive neuron and the effector organ. The concept of "synapse" was introduced in 1897 by the English physiologist F. Sherrington. Synapse between an axon motor neuron and fiber skeletal muscle called myoneural synapse .

It has been shown that when excited, a neuron generates an action potential. A series of action potentials are information carriers. The task of the synapse is to transmit these signals from one neuron to another or to effector cells. As a rule, the result of recoding is the emergence of action potentials, which in this case can be suppressed under the influence of other synaptic contacts. Ultimately, synaptic conduction again leads to electrical phenomena. There are two possibilities here. Fast signal transmission is carried out electrical synapses, slower - chemical in which the carrier chemical takes on the role of signal transduction. However, in this case, there are two fundamental possibilities. In one case, a chemical carrier can cause direct electrical phenomena on the membrane of a neighboring cell, and the effect is relatively rapid. In other cases, this substance causes only a chain of further chemical processes, which, in turn, lead to electrical phenomena on the membrane of the subsequent neuron, which is associated with large time costs.

The following terminology is generally accepted. If the cell from which directed information is carried out is located in front of the synapse, then it presynaptic. The cell after the synapse is called postsynaptic .

A synapse is a point of contact between two cells. Information in the form of action potentials comes from the first cell, called the presynaptic, to the second, called the postsynaptic.

The signal through the synapse is transmitted electrically by the occurrence of local currents between two cells (electrical synapses), chemically, in which the electrical signal is transmitted indirectly using a transmitter (chemical synapses), and using both of these mechanisms simultaneously (mixed synapses).

Synapse electrical

Rice. 8.2. Scheme nicotinic cholinergic synapse. Presynaptic nerve ending contains components for the synthesis of a neurotransmitter (here acetylcholine). After synthesis(I) the neurotransmitter is packed into vesicles (vesicles) (II). These synaptic vesicles merge (perhaps temporarily) with the presynaptic membrane (1P), and the neurotransmitter is released in this way in synaptic cleft. It diffuses to the postsynaptic membrane and binds there with specific receptor(IV). IN education neurotransmitter- receptor complex postsynaptic membrane becomes permeable to cations (V), i.e., depolarizes. (If the depolarization is high enough, then action potential, i.e. chemical signal turns back to electric nerve impulse.) Finally, the mediator is inactivated, i.e., either cleaved by an enzyme(VI) or removed from synaptic cleft through special absorption mechanism. In the above diagram only one cleavage product mediator - choline - is absorbed nerve ending(VII) and reused. basement membrane- diffuse structure identified by electron microscopy V synaptic cleft(Fig. 8.3, a), not shown here.

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Electrical and chemical synapses     Electrical properties synapse

Transmission of signals from cell to cell. can be carried out either by direct passage of action potentials (electrical synapses), or with special molecules - neurotransmitters ( chemical synapses). Depending on their specific functions synapses have very different structures. IN chemical synapses distance between cells is - 20-40 nm synaptic cleft between cells- is a part intercellular space it contains liquid low electrical resistance, So electrical signal dissipates before it reaches the next cell. electrical transmission, on the contrary, is carried out only in specialized structures - gap junctions, where the cells are at a distance of 2 nm and are connected by conductive channels. In fact, there is something similar to the previously postulated syncytium, or multicellular cytoplasmic continuum. Ironically, the history of science     Passive systems transport, hereinafter referred to as channels, are not a single group of functional elements in the membrane. At rest, the channels are closed and become conductive only after they are opened. opening, or gate mechanism, starts electrically, i.e. when changing membrane potential, or chemically- when interacting with a specific molecule. Chemical nature gate mechanism in close connection with the biochemistry of the synapse is considered in Chap. 8 and 9. I would only like to note that gate mechanism also different from other transport systems in their pharmacology, ion selectivity and kinetics. Among the many examples pointing to the importance communication links, can be brought phenomenon of electric cell conjugation. Cell membranes usually have very high electrical resistance, however, in the membranes of adjoining cells there are areas with low resistance- presumably areas gap junctions. One of the most perfect forms communication is a synapse specialized contact between neurons. nerve impulse passing through the membrane of one neuron, stimulates excretion quantum chemical(mediator) who goes through synapse cleft and initiates occurrence of a nerve impulse in the second neuron.     nerve fiber is yourself a highly elongated tube of gelatinous substance filled with saline solution of one composition and washed saline solution other composition. These solutions contain electrically charged ions, in relation to which resembling membrane sheath nerve has selective permeability. Due to the difference in diffusion rates negative and positive charged ions between internal And outer surface nerve fiber there is some potential difference. If it is instantly lowered, that is, local depolarization is caused, this depolarization will spread to neighboring sections of the membrane, as a result of which its wave will run along the fiber. This is the so-called spike potential, or nerve impulse. The membrane cannot be partially discharged; it depolarizes completely all the way or does not depolarize at all. In addition, after impulse passage it takes some time to restore the original membrane potential, and, until then while the membrane potential will not recover nerve fiber will not be able to skip the next pulse. nature occurrence of a nerve impulse(according to the law all or nothing) and the following the passage of an impulse refractory period(or the period of return of the fiber to its original state) we will consider in more detail in the last chapter of the book. If the excitation was received somewhere in the middle of the fiber, the impulse would have to propagate in both directions. But this usually does not happen because nervous tissue constructed Thus so that the signal at any given moment goes in some certain direction. For this nerve fibers connected between yourself in the nerve by special formations, synapses, passing signals in only one direction. Channels passive ion transport passing through excitable membranes, contain two functional components gate mechanism And selective filter. gate mechanism, capable of opening or closing the channel, can be activated electrically by changes membrane potential or chemically, for example in a synapse, by binding to neurotransmitter molecule. selective filter has the same dimensions and such a structure, which allow you to skip whether Synapses are the places where nerve cells communicate. Chemical and electrical synapses differ in transfer mechanism information. In ch. 1 has already been said about the fact that almost all neuron functions to a greater or lesser extent due to membrane properties. In particular, phenomena such as propagation of nerve impulses, their electrical or chemical transfer from cell to cell active ion transport, cellular recognition and synapse development, interaction with neuromodulators, neuropharmacological agents and neurotoxins. This somewhat one-sided view is clarified in this chapter by consideration of the cytoplasm of neurons. Although basically it is similar to the cytoplasm of other cells - the same organelles were found in it (and also synaptic vesicles) and enzymes (and, in addition, involved in metabolism mediators), however neuronal the cytoplasm is adapted specifically to the functions of neurons. FROM microtubule formation or from the presence of mediator nli Ca2+ synaptic contact not due to the presence of a mediator, electrical activity or formation of functional receptors. None of the studies done so far provide a complete answer to the question of education mechanism, specificity and synapse stabilization and not solves problems staged education neural network responsible for higher nervous function systems. At first this chapter we have highlighted this issue as one of the most important in neuroscience, but we will look at it in more detail a little later. Physostigmine played important role V history of science. It inhibits the enzyme cholinesterase, which breaks down acetylcholine (see section 6.2). Due to this, the latter, as a neurotransmitter, is stored for a long time in nerve endings. This made it possible to isolate it from them, determine its function and generally develop theory of chemical electrical transmission momentum through synapses of the nervous systems. basis nervous system form nervous cells - neurons, which are connected between yourself synapses. Thanks to such a structure nervous system capable of transmitting nerve impulses. nerve impulse- This electrical signal, who moves By cage for now will not reach nerve ending, where under by the action of an electric signals, molecules called neurotransmitters are released. They and carry a signal(information) through the synapse, reaching another nerve cell.     Biochemical Research structures and mechanism of action electrical synapses have not yet been carried out. However gap contacts connected not only nerve cells, but also liver cells, epithelium, muscles and many others fabrics. Among them, it was possible to identify and characterize biochemical methods And electron microscopy membrane fragments. which are definitely kept the zones intercellular contacts.electron micrographs show ordered structures particles, which Goodenough called connexons and which form channels between cells separated by 2 nm from each other. From these membranes, two polypeptides with M 25,000 and 35,000 were isolated, called connexins. It is possible that two connexons of neighboring cells, through dimerization, can form a channel(Fig. 8.1). It is shown that this channel transmits not only alkali metal ions, but n molecules with M 1000-2000. Thus, connexons, except electrical interface, provide cells with the opportunity to exchange metabolites. The permeability of such channels can regulate ions calcium. neurons represent yourself cells with long processes capable of led electric signals. Signals are usually received by dendrites and cell body, and then transmitted along the axon in the form of action potentials. Communication with other neurons takes place at synapses, where signals are transmitted from using a chemical-neurotransmitter. Apart from neurons nervous tissue always contains various glial cells that perform a supporting function. Rps. 19-4. Diagram of a typical synapse. electrical signal, coming in the trenches cell axon, leads to the release of synaptic cleft chemical messenger (neurotransmitter) that causes electrical change in the dendrite membrane of cell B In neurochemical terms, the electromotor synapse of the electrical organ of fish, where ACh serves as a neurotransmitter, has been studied better than other synapses. In the early 70s, in the laboratory of W. Whittaker in Germany, for the first time, it was possible to isolate an isolated fraction of synaptic vesicles from electric organ stingray Torpedo marmorata. It is on this object biochemical, immunocytochemical methods and nuclear magnetic Neurons are characterized by an unusually high level of metabolism, a significant part of which is directed to ensuring the work sodium pump in membranes and maintenance states of excitation. Chemical basis of nerve impulse transmission on the axon have already been discussed in Chap. 5, sec. B, 3. Sequential opening of first sodium and then potassium channels it could be considered firmly established. Less clear is the question of whether change in ion permeability required for action potential propagation, with any special enzymatic processes. Nachmanzon points out that acetylcholinesterase is present in high concentration throughout neuron membranes and not just at synapses. He assumes that increase in permeability To sodium ions due to cooperative binding of several molecules acetylcholine with membrane receptors, which either make up sodium channels themselves or regulate the degree of their opening. Wherein acetylcholine is released from accumulation sites located on the membrane as a result of depolarization. Actually, sequence of events must be is such that electrical change fields in the membrane induces protein conformation change, and this already leads to the release of acetylcholine. Under the action of acetylcholinesterase disintegrates quickly, And membrane permeability For sodium ions returns to the original level. In general, the description given differs from that described earlier schemes synaptic transmission in only one respect in neurons does acetylcholine accumulate in the protein form, while in synapses - in special bubbles. There is an opinion that the work of potassium channels regulated by ions calcium. sensitive to change in electrical fields Ca-binding protein releases Ca +, which in turn activates channels for K ", the latter occurs with some delay relative to opening time sodium channels, which is due to the difference in the rate constants of these two processes. The closure of potassium channels is provided hydrolysis energy APR. There are also other assumptions O mechanisms of the nervous conductivity . Some of them proceed from the fact that nerve conduction is entirely provided by the work sodium pump.     Distance between presynaptic and post-synaptic membranes - synaptic cleft- can reach 15-20 nm. in the myoneural connection gap even more - up to 50-100 nm. At the same time, there are synapses with strongly contiguous and even merging presynaptic and postsynaptic membranes. Accordingly, two transmission type. For large gaps, the transmission is chemical, for close contact Maybe direct electric interaction. Here we will look at chemical transfer. Finding out electrical properties cells at rest, consider the processes associated with membrane excitation. State of arousal can be defined as a temporary deviation membrane potential from the resting potential caused by an external stimulus. This electrical or chemical stimulus excites the membrane, changing it ionic conductivity, i.e., the resistance in the circuit decreases (Fig. 5.4). Excitation spreads from the stimulated site to nearby areas of the membrane, in which there is a change conductivity, and hence the potential. Such propagation (generation) of excitation is called an impulse. There are two types action potential impulses when the signal propagates unchanged from the site of excitation to nerve ending, And local potential,. rapidly decreasing with distance from the site of excitation. Local potentials are found in synapses excitatory postsynaptic potentials (e.r.z.r.) and inhibitory postsynaptic potentials (. r.s.r.)) and in sensory nerve endings receptor or generator potentials). Local potentials can be summed up, that is, they can increase with subsequent excitations, while action potentials do not have this ability and arise according to the all-or-nothing principle. Rice. 6. . a - scheme nerve fiber with synapse. Systems shown transport (ATraza) and three various systems passive transport. Right - chemoexcitable transport system, regulated by a non-transmitter molecule, for example, a channel in the postsynaptic membrane of a muscle end plate passing potassium ions and sodium on the left - separately K a + - and K + - channels in the axon membrane, controlled electric field and opened during depolarization biv - sodium conductivity gNg (b) and kalna ёk, (c), as well as incoming sodium /ka and outgoing potassium /k currents after depolarization (60 mV). Clearly differentiated kinetics two processes N3 and k implies the existence individual molecular structures for passive sodium and potassium transport. CI electrical discovery synapse by Vershpan and Potter occurred in 1959, when neural theory finally replaced the reticular. Electrical synapses are relatively rare, and their role in central nervous system higher organisms is still unclear. Vershpan and Potter discovered them in the abdominal nerve of a crab, and later they were found in numerous organisms of mollusks, arthropods and mammals. In contrast chemical synapse, Where impulse passage somewhat delayed due to the release and diffusion of the neurotransmitter, signal through the electrical synapse is transmitted rapidly. The physiological importance of such synapses may therefore be related to the need for rapid mating of specific cells. Worthy of attention is also particularly useful cell-line- cell line PC 12, cloned from pheochromocytoma - a tumor of the chromaffin tissue of the adrenal gland. PC 12 cells are similar chromaffin cells by their ability to synthesize, store and release catecholamines. Like not neuronal cells, they multiply, but under the action of NO they stop dividing, participate in neuritic processes and become very similar to sympathetic neurons. They acquire electrical excitability, respond to acetylcholine, and even form functional cholinergic synapses. PC 12 cells are used as model systems for studying neuron differentiation, hormonal actions And trophic factors, functions and hormone metabolism receptor (see p. 325). The basis of each NS constitute relatively simple, in most cases - the same type of elements (cells). In what follows, a neuron will mean artificial neuron, that is, the HC cell (Fig. 19.1). Each neuron has its own current state by analogy with brain nerve cells which may be excited or inhibited. It has a group of synapses - unidirectional input connections connected to exits of others neurons, and also has an axon - output connection of this neuron, from which the signal (excitation or inhibition) arrives at the synapses of the following neurons. Every synapse characterized by the value synaptic connection or its weight and which physical meaning equivalent to electrical conductivity. Signals carried by neurons are transmitted from one cell to another in special contact points called synapses (Fig. 18-3). Usually this transmission is carried out, oddly enough at first glance, indirectly. Cells electrically isolated from each other the presynaptic cell is separated from the postsynaptic gap synaptic cleft. Electrical change potential in the presynaptic cell leads to substance release, called a neurotransmitter (or neurotransmitter), which diffuses through synaptic cleft And causes a change electrophysiological state of the postsynaptic cell. Ta-

Rice. 18-3. Diagram of a typical synapse. eleggric signal, coming V axon ending cells A, leads to the release of synaptic cleft chemical mediator (ieromednatorX which causes electrical change in the dehydrite membrane of cell B. The wide arrow indicates the direction signal transmission, the axon of a single neuron, such as the one shown in Fig. 18-2, sometimes forms thousands of output synaptic connections with other cells. Conversely, a neuron can receive signals through thousands of input synaptic connections located on its dendrites and body.

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Most easy way signal transmission from neuron to neuron direct electric interaction through slot contacts. Such electric sand shishsy between neurons found in some areas nervous system many animals, including vertebrates. Main advantage of electrical synapses is that the signal is transmitted without delay. On the other hand, these synapses are not adapted to some functions and cannot be adjusted as finely as chemical synapses through which most links between neurons. electrical connection through slot contacts was discussed in chapter     skeletal muscle vertebrate fibers, like nerve cells, capable of being excited electric current, And neuromuscular connection (Fig. 18-24) can serve good model chemical synapse at all. On fig. 18-25 compared fine structure this synapse with a typical synapse between two neurons brain. The motor nerve and the muscle it innervates can be separated from the surrounding tissue and maintained in functioning state V environment of a certain composition. Exciting the nerve through external electrodes, it is possible to register the response of a single pulse using an intracellular microelectrode. muscle cell(Fig. 18-26). The microelectrode is relatively easy to insert into skeletal fiber muscle, as it is a very large cell (about 100 microns in diameter). Two simple observations show that for synaptic transmission an influx of Ca nons into axon ending. First, if there is no Ca in the extracellular environment, the mediator is not released and signal transmission not happening. Secondly, if Ca is artificially introduced into the cytoplasm nerve ending using a micropipette, the release of the neurotransmitter occurs even without electrical stimulation of the axon, the mouth is difficult to implement on neuromuscular junction because of small sizes axon ending therefore, such an experiment was carried out on a synapse between giant squid neurons.) These observations made it possible to reconstruct the last value events taking place in axon ending, which is described below.

Postsynaptic potential(PSP) is a temporary change in the potential of the postsynaptic membrane in response to a signal received from the presynaptic neuron. Distinguish:

excitatory postsynaptic potential (EPSP), which provides depolarization of the postsynaptic membrane, and

inhibitory postsynaptic potential (IPSP), which provides hyperpolarization of the postsynaptic membrane.

EPSP brings the cell potential closer to the threshold value and facilitates the occurrence of an action potential, while IPSP, on the contrary, makes it difficult to generate an action potential. Conventionally, the probability of triggering an action potential can be described as a resting potential + the sum of all excitatory postsynaptic potentials - the sum of all inhibitory postsynaptic potentials > threshold for triggering an action potential.

Individual PSPs are usually small in amplitude and do not cause action potentials in the postsynaptic cell; however, unlike action potentials, they are gradual and can be summed up. There are two summation options:

temporal - combining the signals that came through one channel (when a new impulse arrives before the previous one fades)

spatial - superposition of EPSPs of neighboring synapses

What is a synapse? A synapse is a special structure that provides signal transmission from the fibers of a nerve cell to another cell or fiber from a contact cell. What does it take to have 2 nerve cells? In this case, the synapse is represented in 3 functional areas (presynaptic fragment, synaptic cleft and postsynaptic fragment) of nerve cells and is located in the area where the cell contacts the muscles and glands of the human body.

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

According to the type of activity for perception, synapses are usually distinguished: 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 reduced to the type of this docking, which can be distant, contact, and also borderline. The connection of the distant property is carried out by means of 2 neurons located in many parts of the body.

So, in the tissues of the human brain, neurohormones and neuropeptide substances are generated that affect the neurons present in the body of a different location. The contact connection is reduced to special joints of membrane-films of typical neurons that make up the synapses of the chemical direction, as well as the components of the electrical property.

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

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

Synapses of electrical and chemical direction

The action of film-membrane fusion is considered to be electrical synapses. In conditions where the necessary synaptic cleft is discontinuous with intervals of septa of a monolithic connection. These partitions form an alternating structure of the synapse compartments, while the compartments are separated by fragments of approximate membranes, the gap between which in the synapses of the usual warehouse is 0.15 - 0.20 nm in representatives of mammalian creatures. At the junction of the film-membranes, there are ways through which the exchange of part of the fruit occurs.

In addition to separate 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 microns. Thus, a similar synaptic phenomenon is represented in the neurons of the ciliary ganglion.

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 depolarizes, when, with the touch of the efferent particles of the fibers, it becomes hyperpolarized. It is believed that the synapses of acting neurons with common responsibilities can carry out the required excitation (between 2 passing areas) in both directions.

On the contrary, the synapses of the neurons present with a different list of actions (motor and sensory) carry out the act of excitation 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 degree of fatigue, 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 inside its own tubule or towards its completion. This fragment contains granular as well as agranular special sacs containing the neurotransmitter.

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

There is an opinion that small granular vesicles have a substance such as norepinephrine, and large ones - catecholamines. Moreover, acetylchonin is located in agranular cavities (vesicles). In addition, mediators of increased excitation are substances formed according to the type of aspartic or no less significant acid glutamine produced.

Active synapse contacts are often located between:

• Dendrite and axon.
• Soma and axon.
• Dendrites.
• axons.
• cell soma and dendrites.

Influence of the developed mediator relative to the present postsynaptic film-membrane is due to excessive penetration of its sodium particles. The generation of powerful outpourings of sodium particles from the working synaptic cleft through the postsynaptic film-membrane forms its depolarization, forming the excitation of the postsynaptic reserve. The transit of the chemical direction of synapse data is characterized by a synaptic suspension of excitation in time equal to 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. Because of what there is a suspended postsynaptic reserve. As a rule, during a strong excitation, the level of permeability of the postsynaptic film-membrane increases.

The required excitatory property is fixed inside neurons if norepinephrine, the substance dopamine, acetylcholine, the 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

The working capacity of a person directly determines his age, when all values ​​increase simultaneously with the development and physical growth of children.

The accuracy and speed of mental actions with age is carried out unevenly, depending on other factors that fix the development and physical growth of the body. Students of all ages who have there are health problems, the performance of a low value relative to the surrounding strong children is characteristic.

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

The speed of the onset of weakness is determined by the initial state of the child's system of sensitive nervous genesis, the working pace and the volume of the load. At the same time, children are prone to overwork during prolonged immobility and when the actions performed by the child are not interesting. After a break, the working capacity becomes the same or becomes higher than the previous one, and it is better to make the rest not passive, but active, switching to a different activity.

The first part of the educational process in 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.
• Listen carefully to the words of the teacher.
• Change the position of their body.
• They start talking.
• They get up from their place.

The values ​​of working capacity are especially high in high school students studying in the 2nd shift. It is especially important to pay attention to the fact that the time to prepare for classes is short enough before the start of the learning activity in the classroom and does not guarantee a complete elimination of harmful changes in the central nervous system. mental activity is quickly depleted in the first hours of lessons, which is clearly noted in negative behavior.

Therefore, qualitative shifts in working capacity are observed in students of the younger block in lessons from 1 to 3, and blocks of the middle-senior link in 4-5 lessons. In turn, the 6th lesson takes place in conditions of a particularly reduced ability to act. At the same time, the duration of the lesson for 2-11 graders is 45 minutes, which weakens the condition of the children. Therefore, it is recommended to periodically change the type of work, and in the middle of the lesson to hold an active pause.