SYNAPSE

From Big Medical Encyclopedia

SYNAPSE (Greek sinapsis connection, communication) — the specialized zone of contact between nervous cells or nervous cells and other excitable educations providing transfer of the coming information with preservation of its information importance.

S.'s studying as textural and functional features began at the end of the last century after the Spanish histologist S. Ramone-and-Kakhal established that all components of a nervous system are constructed of separate nervous cells of a various form and size (see. Nervous cell , Nervous system ). In 1897 the English physiologist Ch. Sherrington of a dlya.oboznacheniye of a point of contact of the termination of an axon with other nervous cell entered the concept «synapse». S.'s studying was closely connected with formation of idea of a chemical way of transfer of vozbuzhdeniye from a cell to a cell; adhered to it Elliott (Th. R. Elliott), O. Levi and G. Dale. Further progress in area of a research C. and ways of transfer of process excitement (see) it is connected with researches 3. Buck, J. Langley, W. Feldberg, J. H. Gaddum. The big contribution to studying of functions C. was made by the Soviet researchers A. F. Samoylov, A. V. Kibyakov, A. G. Ginetsinsky, X. S. Koshtoyants. Fiziol. analysis of functioning of the synoptic device of c. the N of page was carried out by J. Ekkls and P. G. Kostya a lump. A number of basic researches of S. A. Sarkisov (1948), A is devoted to structure of interneural bonds. D. Zurabashvili (1951), G. I. Polyakova (1973), etc.

Morphology

S. between two nervous cells consists of the presynaptic shoot belonging to an afferent neuron and a postsynaptic component, the Crimea can be a dendrite, a body or an axon of postsynaptic neuron, muscular or secretory tissue. The Sinaptichesky contact between nervous cells sometimes is called also synoptic plaque. The presynaptic nerves of an axon were called a trailer plate, S. between two nervous cells — interneural (neyronalny) bonds, or nervous Pages. The terminations of axons on muscle fibers having lines of specialization of S. call mioneyronalny bonds, presynaptic nerves on muscle fiber — a motor plate, their complex — neuromuscular connection, and synoptic contact of an axon with a ferruterous cell — neurosecretory Page. The new stage in development of the doctrine about S. is connected with the submicroscopy (see) which allowed to study S.'s ultrastructure and to show that it represents an elaborate complex of the interconnected structures.

Presynaptic nerves (a presynaptic shoot) and the site of postsynaptic neuron are divided by a synaptic gap. The presynaptic nerves contain the synoptic bubbles forming, as a rule, accumulations at an electronic and dense presynaptic membrane. The combination of synoptic bubbles and sites of the increased electron density of synaptic membranes received the name of active sites of contact.

Sinaptichesky bubbles can be light, electronic and transparent or have osmiofil-ny material (granular synoptic bubbles) in the center. Occasionally in a presynaptic shoot the synoptic bubbles covered with an additional cover meet. The sizes of synoptic bubbles fluctuate from 20 to 60 nanometers.

On a structure of synaptic membranes of S. it is possible to divide on symmetric (with uniform increase in electron density of synaptic membranes) and asymmetric (with preferential increase in electron density of a postsynaptic membrane).

In most cases the synaptic gap of asymmetric S. is much wider, than the distance between membranes of two next shoots of cells out of S. V of a synaptic gap of S. of this type is available a nek-swarm amount of electronic and dense material, to-ry can sometimes look as the dash line going between synaptic membranes parallel to them. Asymmetric S. are located on branches and spinules of dendrites more often, is more rare on large trunks of dendrites. Symmetric S. are, as a rule, localized on bodies of nervous cells and trunks of dendrites, is more rare on thin branchings of dendrites.

Their polymorphism is characteristic of interneural bonds that formed the basis for numerous classifications of Page. The fullest classification of S. based on svetooptichesky studying of interneural bonds (on character of branchings of axons around bodies of cells and their interposition with branches of dendrites), was offered S. Ramone-and-Kakhalem in 1954. According to this classification interneural bonds form network of branchings of one axon or a branch of an axon with insignificant number of collaterals, to-rye go straight to neuron and form with it S., network of the axons of several neurons contacting among themselves around a neurocyton or some site of a dendrite.

Diffraction patterns of aksodendritichesky and akso-axonal synapses (b) in bark big a vegetable pulp of a rat: 1 — a cytoplasmic membrane of presynaptic nerves; 2 — synoptic bubbles: 3 — mitochondrions; 4 — a cytoplasmic membrane of a dendrite; 5 — an active zone of synapses; 6 — cytoplasm of a dendrite; 7 — presynaptic nerves of axons; x 30 000.

On S.'s localization on neurons interneural bonds divide into 3 basic groups: aksodendritichesky (fig., a), aksosomatichesky and akso-axonal (fig., b). In turn, aksodendritichesky S. divide into 3 subgroups: Page on trunks of dendrites, on spinules of dendrites and on thin trailer branches of dendrites; less often dend-ro-dentritic, dendrosomati-chesky and somatodendritichesky synapses meet.

More irregular, complex shapes of interneural contacts are described. The reciprocal S. located between an axon and a dendrite (the aksodendritichesky and dendroaksonalny S. which are located with a row) and serial S., i.e. the akso-aksodendritichesky or akso-aksosomatichesky contacts going one by one concern to them. Peculiar forms of interneural contacts are glomerula, in to-rykh a complex of interneural bonds is as if separated from surrounding tissue of a brain by shoots of glial cells, and also the synoptic fields which are characterized by difficult coherence of the terminations of axons of various systems of fibers.

In c. N of page axons form interneural bonds or the trailer branches, or on the course of the advance. The pages formed on the course of advance serve, apparently, for transfer of low-specific information.

An essential role in the organization of interneural contact is played by the area of an active zone of contacting S.'s membranes though there is no rigid dependence between the area of an active zone of contact and contact area of presynaptic and postsynaptic sites. On the basis of a research of ultrastructure of synapses of H. N. Bogolepov offered the term «informational content» of interneural contact, edges is defined by extent of impact of an axon on a dendrite in the area C. and (from the morphological point of view) depends on the area of active zones C. in the field of contact between a presynaptic shoot and postsynaptic neuron. With increase in the area of an active zone C. its informational content increases.

Pages are one of the most plastic and vulnerable components of neurons. However not all S. have equal plastic opportunities; on degree of plasticity they can be divided on stable and dynamic. Stable S. in the course of ontogenesis ripen earlier, than dynamic.

The research of ontogenetic features of maturing of interneural contacts shows that in ultrastructure of each of them a certain way of development is reflected. It begins with the unripe desmosovidny contact having increase in electron density, small on the area, of the contacting membranes and only 2 — 3 synoptic bubbles in a presynaptic shoot and gradually turns into S. typical for an adult organism. In the course of ontogenesis irregularity of development of interneural bonds of various type is also observed. Nair., in visual area of bark of a great brain of S. are formed first of all on large trunks of dendrites, then on their main branchings and on bodies of nervous cells. Later S. on spinules and thin branches of the dendrites formed by trailer branches of axons form, still later there is a formation akso-axonal S.

Neravnomernost of S.'s maturing creates a basis for plastic changes of interneural bonds at this or that individual as a result of features of an ontogeny, training, trainings and DR-

of Feature of the structurally functional organization C. find the reflection and in patterns of their destructive changes at damage. At section of axons of various systems of neurons there are different forms of destructive changes of S. Osnovnymi among them so-called dark, light and filamentarny degenerations are. Besides, the focal degeneration S. Naiboley is described the dark degeneration which is characterized by increase in an osmiofiliya of cytoplasm of a presynaptic shoot is widespread. At the same time the degenerating fibers and their terminations compress, get the irregular, ugly shape. Increase in electron density of a presynaptic shoot happens due to accumulation in it of melkogranulyarny material in the beginning. Then cytoplasm is homogenized and becomes so osmiofilny that on its background only hardly it is possible to distinguish any discrete structures.

The filamentarny degeneration is similar to a dark degeneration on the development. It is characterized by emergence in a presynaptic shoot of a set of the filaments with a diameter of 6 — 10 nanometers which are quite often forming ring-shaped structure. At electronic microscopic examination filamentarny reaction comes to light on 3 — the 7th day after section of the corresponding nerve fibril. Further increase in quantity of filaments is supplemented with emergence of melkogranulyarny material in a presynaptic shoot and the subsequent course of process differs from a dark degeneration of Page a little.

Hypostasis of presynaptic nerves, increase in its sizes, an enlightenment of cytoplasm and reduction of number of synoptic bubbles are characteristic of a light degeneration of S. The light degeneration arises in the same terms, as dark, in nek-ry cases slightly earlier. The light degeneration is also a typiform of pathology of S. at hypoxias (see).

At a focal degeneration of S. along with an enlightenment of a presynaptic shoot partial disintegration of synaptic membranes and gross violation of contact in this site is observed.

Physiology

Achievements in a research of a structure of cellular membranes gave a vozkhmozhnost to study more deeply not only structure, but also function C. Assume that emergence of intercellular contacts in phylogenesis was that key moment, to-ry led to a possibility of interaction of cells among themselves, i.e. to education from unicells multicellular, to emergence of fabric systems, including a nervous system. The page as contact of a nervous cell with other excitable educations represents a specialized type of intercellular contact. As S. represent the elements combining separate neurons in neural systems and providing a possibility of the regulating influence of c. the N of page on other excitable educations (muscles, glands, etc.), becomes clear their big role in activity of all nervous system in general.

On a way of transfer of excitement of S. subdivide into three groups. S. concern to the first group, in to-rykh transfer of excitement it is carried out by means of chemical substances transmitters (see. Mediators ), to the second — synapses with transfer of excitement without participation of mediators, only due to direct transition of an electric signal with pre-on postsynaptic structures (see. Efaps ), to the third — «the mixed synapses», in to-rykh transfer it is carried out both chemical, and electric in the ways.

On change of potential of a postsynaptic membrane distinguish brake and exciting S. Besides, all S. subdivide on central (being in a head and spinal cord) and peripheral (neuromuscular, neurosecretory, and also S. vegetative gangliyev).

Akso-aksonalnye S. are found in the most various parts of the central and peripheral nervous system. Most of researchers consider that by means of akso-axonal S. presynaptic is carried out braking (see).

In S. with chemical transfer a mediator, being allocated from presynaptic nerves, gets to a synaptic gap, edges is direct continuation of intercellular space. Contents of a synaptic gap have properties of polisakharidny gel, glikozaminoglikana are its part. In presynaptic area mitochondrions, granules of a glycogen, a granule with fine-grained contents, different filamentozny structures are localized. Assume that spiral-shaped filamentozny threads in the absence of excitement of presynaptic nerves are located parallel to a subsynaptic membrane and serve as a peculiar barrier to interaction of synoptic bubbles with active zones of a presynaptic membrane. In that case when nervous impulse reaches presynaptic nerves, filamentozny threads break up to separate subunits, promoting thereby interaction of contents of a synoptic bubble with active zones of a synapse. Thus, allocation of a mediator has discontinuous character in the form of separate portions, or quanta. The majority of the researches devoted to studying of quantum nature of emission of mediators was executed on the synoptic bubbles containing acetylcholine. So, for S. of mammals the number of the molecules of a mediator which are contained in one synoptic bubble and at the same time streaming in a synaptic gap (i.e. quantum of acetylcholine), fluctuates over a wide range and makes from 4 * 102 to 4-104 molecules.

Between synoptic bubbles and allocation of quanta of a mediator there is a functional linkage. So, at dlitelnokhm irritation in presynaptic nerves the quantity of synoptic bubbles considerably decreases that is followed by decrease in efficiency of transfer through S. of the extending excitement.

In the course of synoptic transfer the mediator from presynaptic nerves gets to a synaptic gap in two ways: at contact of a synoptic bubble with an inner surface of a presynaptic membrane or at allocation of the free not vesicular transmitter. At rest (i.e. in the absence of depolarization of presynaptic nerves) there is an accidental collision of synoptic bubbles with a presynaptic membrane. As a result of an exocytosis a small amount of a mediator gets to a synaptic gap, to-ry, interacting with cellular receptors of a postsynaptic membrane, leads to emergence of the tiny postsynaptic potential (MPP). MPP arise aperiodicly, randomly, their size much less threshold of excitement of a postsynaptic membrane. At depolarization of a presynaptic membrane the number of quanta of the mediator which is allocated in a synaptic gap increases several times therefore there is an excitement of a postsynaptic membrane and emergence of the extending excitement.

The effect of depolarization of presynaptic nerves and respectively increase in allocation of quanta of a mediator affects a condition of excitability of a postsynaptic membrane not at once. The noticeable time lag from the moment of depolarization of a presynaptic membrane before emergence of an impulse in a postsynaptic membrane — the so-called synaptic delay (SD) is noted. It is caused by a number of factors: time of release of a mediator from a synoptic bubble at an exocytosis, diffusion of a mediator through a synaptic gap, interaction of a mediator with cellular receptors of a postsynaptic membrane and formation of the extending process of excitement in postsynaptic structures. In particular, at motor-neurons of a spinal cord duration of SZ makes 0,5 ms, and in nek-ry cases can reach 2 — 2,5 ms. Duration of SZ is caused by many factors: temperature, maintenance of various ions, pH of the environment, etc.

The interrelation between depolarization of a presynaptic mekhmbrana and release of a mediator is provided by calcium ions (see. Membranes biological ). Calcium carries out a role of the factor interfacing process of change of size of membrane potential of a presynaptic membrane to strengthening of allocation of a mediator in a synaptic gap. Because the entering current in a presynaptic membrane is connected with increase in permeability of natrium channels, and process of secretion of a mediator — with increase in permeability of calcium channels, there was an opportunity to separate processes of depolarization of a presynaptic membrane and secretion of a mediator. So, at effect of inhibitors of synoptic transfer, the pas of the ave. of a tetrodotoksin — a blocker of conductivity of natrium channels of a membrane, can be absent change of potential of a presynaptic membrane, but secretion of a mediator continues. Blockade of secretion of a mediator can be caused by means of the factors connecting calcium ions (e.g., to - they are ethylene diamine tetraacetic EDTA), or by removal of calcium ions from external environment (see. Permeability ). Secretion of a mediator can be strengthened action of specific calcic ionophores (see). The mechanism of secretion of a mediator is adjusted by a number of biologically active agents, including and mediators, and also cyclic nucleotides (see. Nucleic acids ), prostaglandins (see) and number of neuropeptids.

In processes of change of a condition of a postsynaptic membrane the system adenylatecyclase — cyclic AMF is of great importance. Assume that interaction of a mediator with the receptor site of adenylatecyclase leads to activation of its catalytic center, to formation of cyclic AMF with the subsequent increase in activity of protein kinases of cytoplasm and a kernel of a cell. It in turn intensifies phosphorylation of the proteins which are directly participating in change of permeability of a postsynaptic membrane and leads to emergence in postsynaptic structures of the extending process of excitement. At spontaneous allocation of quanta of a mediator (e.g., in neuromuscular S.) in a postsynaptic membrane is registered l8kalnoye change of level of the membrane potential called the miniature potential of a trailer plate (MPTP).

In that case when allocation of quanta of a mediator is not enough for emergence of action potential, in a postsynaptic membrane local depolarization or hyperpolarization is registered. In neural S. this reaction is called the exciting postsynaptic potential (VPSP) and brake postsynaptic potential (TPSP), and in neuromuscular S. — the potential of a trailer plate (PTP).

Emergence of VPSP is connected with functioning in synaptic membranes of special channels for such cations as Na + and K+. Anions do not take part in generation of VPSP in overwhelming number of cases. Emergence of VPSP in motor-neurons results from simultaneous increase in sodium and potassium conductivity of a postsynaptic membrane, however also selective increase in its permeability to ions of sodium is possible. Dependence between the size of membrane potential and VPSP value is found. This dependence demonstrates that the movement of ions on channels of a postsynaptic membrane is carried out by passive diffusion, but not by means of active transport (see. Bioelectric potential ).

Transfer of excitement in electric S. is carried out only by means of electric currents. It should be noted that electric S. meet considerably less than S. chemical transfer. Electric synapses (see Efaps) in comparison with chemical differ in bigger speed of signal transmission, high reliability of transfer, a possibility of bilateral transfer of excitement. At the same time the chemical S. using the difficult multifunctional neurochemical device are capable to provide in immeasurably bigger degree preservation of the information importance of signals coded by biologically active agents and metabolic processes proceeding in postsi-naptnchesky structures.

Thus, the main stages of synoptic transfer can be schematically presented by the following processes: arrival of excitement to a pre-sinagkhtichesky membrane and its depolarization, penetration in a presynaptic membrane of calcium ions, interaction of synoptic bubbles with active sites of a presynaptic membrane, an exocytosis and allocation of quanta of a mediator in a synaptic gap, diffusion of a mediator to a postsinapti-chevky membrane, interaction of a mediator with cellular receptors of a postsynaptic membrane and as a result of it change of permeability of the last to certain ions, formation of postsynaptic potentials and emergence of the extending process of excitement. Emergence of TPSP completely prevents an opportunity poyavlenrsh excitement in a postsynaptic membrane and is connected with change of permeability of a membrane to ions of chlorine or potassium.

The main methods of a research of synoptic processes are microelectrode technics (see. Microelectrode method of a research ), gistoavtoradiografichesky methods (see. Autoradiography ), submicroscopy (see), biochemical methods of a research (see) and neuropharmacological analysis.



Bibliography: Tank 3. M. Chemical transfer of nervous impulse, the lane with fr., M., 1977; Bogolepov H. N. Ultrastruktura of synapses is normal also of pathology, M., 1975; it, Ultrastructure of a brain at a hypoxia, M., 1978; Glebov R. N. and Kryzhanov-s to and y G. N. Functional biochemistry of synapses, M., 1978, bibliogr.; To and - I to about in A. V. Chemical transfer of nervous excitement, M. — JI., 1964; Sarkisov S.A. Some features of a structure of neyronalny bonds of bark of a great brain, M., 1948.


S. A. Osipovsky; H. N. Bogolepov (morphology).

Яндекс.Метрика