5. Chemical synapses by the nature of the neurotransmitter divided into cholinergic (mediator - acetylcholine), adrenergic (norepinephrine), dopaminergic (dopamine), GABAergic (y-aminobutyric acid), etc. In the CNS, there are mainly chemical synapses, but there are also electrical excitatory synapses and electrochemical synapses.
B.Structural elements of a chemical synapse - presynaptic and postsynaptic membranes and synaptic cleft (Fig. 2.5).
At the presynaptic terminal there are synaptic vesicles (vesicles) with a diameter of about 40 nm, which are formed in the body of the neuron and, with the help of microtubules and microfilaments, are delivered to the presynaptic ending, where they are filled with a mediator and ATP. The mediator is formed in the nerve ending itself. The presynaptic ending contains several thousand vesicles, each of which contains from 1 to 10 thousand molecules of a chemical substance involved in the transmission of influence through the synapse and, therefore, called a mediator (mediator). The mitochondria of the presynaptic terminal provide energy for the process of synaptic transmission. The presynaptic membrane is the part of the membrane of the presynaptic terminal that limits the synaptic cleft.
synaptic cleft has a different width (20-50 nm), contains intercellular fluid and mucopolysaccharide dense
a substance in the form of strips, bridges, which provides a connection between the pre- and postsynaptic membranes and may contain enzymes.
The postsynaptic membrane this is a thickened part of the cell membrane of the innervated cell, containing protein receptors that have ion channels and are capable of binding mediator molecules. The postsynaptic membrane of the neuromuscular junction is also called the end plate.
AT.Excitation transfer mechanism in electric synapse similar to that in a nerve fiber: AP, which occurs on the presynaptic membrane, directly electrically irritates the postsynaptic membrane and provides its excitation. Electrical synapses, as it turned out, have a certain effect on the metabolism of contacting cells. There is evidence of the presence of inhibitory electrical synapses in the CNS, but they have not been studied enough.
G.Signal transmission in chemical synapses. An action potential (AP) received at the presynaptic ending of a chemical synapse causes depolarization of its membrane, which opens voltage-dependent Ca-channels. Ca 2+ ions enter the nerve ending according to the electrochemical gradient "provide the release of the mediator into the synaptic cleft through exocytosis. Transmitter molecules entering the synaptic cleft diffuse to the postsynaptic membrane and interact with its receptors. The action of mediator molecules leads to the opening of ion channels and the movement of Na + and K + ions according to the electrochemical gradient with a predominance of the current of Na + ions into the cell, which leads to its depolarization. This depolarization is called the excitatory postsynaptic potential (EPSP), which at the neuromuscular synapse is called the end plate potential (EPP) (Fig. 2.6).
The termination of the action of the mediator released into the synaptic cleft is carried out by means of its destruction by enzymes localized in the synaptic cleft and on the postsynaptic membrane, by diffusion of the mediator into the environment, and also by reuptake by the nerve ending.
D.Characteristics of the conduction of excitation in chemical synapses.
1 . Unilateral conduction of excitation - from the presynaptic ending towards the postsynaptic membrane. This is due to the fact that the mediator is released from the presynaptic ending, and the receptors interacting with it are localized only on the postsynaptic membrane.
Slow propagation of excitation in synapses compared with the nerve fiber, it is explained by the fact that it takes time for the release of the mediator from the presynaptic ending, the spread of the mediator in the synaptic cleft, and the action of the mediator on the postsynaptic membrane. The total delay in the transmission of excitation in the neuron reaches a value of the order of 2 ms, in the neuromuscular synapse 0.5-1.0 ms.
Low lability of chemical synapses. In the neuromuscular synapse, it is equal to 100-150 transmitted impulses per second, which is 5-6 times lower than the lability of the nerve fiber. In synapses, the central nervous system is very variable - it can be more or less. The reason for the low lability of the synapse is the synaptic delay.
4. Synaptic depression (fatigue of the synapse) -
weakening of the cell's response to afferent impulses, expressing
occurring in a decrease in postsynaptic potentials during a long
telny irritation or after it. It is explained by the cost
mediator, accumulation of metabolites, acidification of the environment
during prolonged excitation along the same lines -
crown chains.
E.electrical synapses have a gap an order of magnitude smaller than that of chemical synapses, conduct a signal in both directions without a synaptic delay, transmission is not blocked when Ca 2+ is removed, they are not very sensitive to pharmacological drugs and poisons, and are practically indefatigable, like a nerve fiber. The very low resistivity of the adjacent pre- and postsynaptic membranes ensures good electrical conductivity.
2.2. CHARACTERISTICS OF HORMONAL REGULATION
The reflex reaction may have a hormonal link, which is typical for the regulation of the functions of internal organs - vegetative functions, in contrast to somatic functions, the reflex regulation of which is carried out only by the nervous pathway (the activity of the musculoskeletal system). If the hormonal link is turned on, then this is due to the additional production of biologically active substances. For example, when strong stimuli (cold, heat, pain stimulus) act on exteroreceptors, a powerful stream of afferent impulses enters the central nervous system, while an additional amount of adrenaline and adrenal cortex hormones are released into the blood, playing an adaptive (protective) role.
Hormones (Greek pogtab - I excite) - biologically active substances produced by endocrine glands or specialized cells located in various organs (for example, in the pancreas, in the gastrointestinal tract). Hormones are also produced by nerve cells - neurohormones, for example, hormones of the hypothalamus (liberins and statins), which regulate the function of the pituitary gland. Biologically active substances are also produced by non-specialized cells - tissue hormones (paracrine hormones, hormones of local action, paracrine factors - parahormones). The action of hormones or parahormones directly on neighboring cells, bypassing the blood, is called paracrine action. By place of action to target organs or to other endocrine glands, hormones are divided into two groups: 1) effector hormones, acting on effector cells (for example, insulin, which regulates metabolism in the body, increases glycogen synthesis in liver cells, increases the transport of glucose and other substances through the cell membrane, increases the intensity of protein synthesis); 2) triple hormones (tropins), acting on other endocrine glands and regulating their functions (for example, ad-
pituitary renocorticotropic hormone - corticotropin (ACTH) - regulates the production of hormones by the adrenal cortex).
Types of hormone influences. Hormones have two types of influences on the organs, tissues and systems of the body: functional (play a very important role in the regulation of body functions) and morphogenetic (provide morphogenesis - growth, physical, sexual and mental development; for example, with a lack of thyroxine suffers from the development of the central nervous system, and consequently, mental development).
1. Functional influence of hormones there are three types.
Starting influence - this is the ability of the hormone to trigger the activity of the effector. For example, adrenaline triggers the breakdown of glycogen in the liver and the release of glucose into the blood, vasopressin (antidiuretic hormone - ADH) turns on the reabsorption of water from the collecting ducts of the nephron into the interstitium of the kidney.
The modulating effect of the hormone - change in the intensity of the flow of biochemical processes in organs and tissues. For example, activation of oxidative processes by thyroxin, which can take place without it; stimulation of the activity of the heart by adrenaline, which passes without adrenaline. The modulating effect of hormones is also a change in the sensitivity of tissue to the action of other hormones. For example, folliculin enhances the effect of progesterone on the uterine mucosa, thyroid hormones enhance the effects of catecholamines.
Permissive effect of hormones - the ability of one hormone to ensure the implementation of the effect of another hormone. For example, insulin is necessary for the manifestation of the action of growth hormone, follitropin is necessary for the implementation of the effect of lutropin.
2. Morphogenetic influence of hormones(for growth, physical
and sexual development) is studied in detail by other disciplines
(histology, biochemistry) and only partially - in the course of physiology (see.
ch. 6). Both types of hormone influences (morphogenetic and functional
nal) are realized through the breakdown of metabolic processes,
launched through cellular enzyme systems.
2.3. REGULATION BY METABOLITES
AND TISSUE HORMONES.
MYOGENIC MECHANISM OF REGULATION.
REGULATORY FUNCTION OF BBB
Metabolites - products formed in the body during metabolism as a result of various biochemical reactions. These are amino acids, nucleotides, coenzymes, carbonic acid, mo-
local, pyruvic, adenylic acids, ionic shift, pH changes. Regulation by metabolites in the early stages of phylogenesis was the only one. Metabolites of one cell directly affected another, neighboring cell or group of cells, which in turn acted in the same way on the following cells. (contact regulation). With the advent of hemolymph and the vascular system, metabolites began to be transmitted to other cells of the body with moving hemolymph over long distances, and this became faster. Then the nervous system appeared as a regulatory system, and even later - the endocrine glands. Metabolites, although they act mainly as local regulators, can also affect to other organs and tissues, on the activity of nerve centers. For example, the accumulation of carbonic acid in the blood leads to excitation of the respiratory center and increased respiration. An example of local humoral regulation is the hyperemia of an intensively working skeletal muscle - the accumulating metabolites provide for the expansion of blood vessels, which increases the delivery of oxygen and nutrients to the muscle. Similar regulatory effects of metabolites occur in other actively working organs and tissues of the body.
tissue hormones: biogenic amines (histamine, serotonig), prostaglandins and kinins. They occupy an intermediate position between hormones and metabolites as humoral regulatory factors. These substances exert their regulatory effect on tissue cells by changing their biophysical properties (membrane permeability, their excitability), changing the intensity of metabolic processes, the sensitivity of cell receptors, and the formation of second mediators. As a result of this, the sensitivity of cells to nervous and humoral influences changes. Therefore, tissue hormones are called modules-tori regulatory signals - they have a modulating effect. Tissue hormones are formed by non-specialized cells, but they act through specialized cell receptors, for example, two types of receptors have been found for histamine - H (and H 2. Since tissue hormones affect the permeability of cell membranes, they regulate the entry into the cell and the exit from cells of various substances and ions that determine the membrane potential, and hence the development of the action potential.
Myogenic mechanism of regulation. With the development of the muscular system in the process of evolution, the myogenic mechanism of regulation of functions gradually becomes more and more noticeable. The human body is approximately 50% muscle. This is a skeletal muscle
ra (40% of body weight), cardiac muscle, circulatory smooth muscle and lymphatic vessels, walls of the gastrointestinal tract, gall bladder, bladder and other internal organs.
The essence of the myogenic mechanism of regulation is that preliminary moderate stretching of the skeletal or cardiac muscle increases the strength of their contractions. The contractile activity of a smooth muscle also depends on the degree of filling of the hollow muscular organ, and hence its stretching. With an increase in the filling of the organ, the tone of the smooth muscle first increases, and then returns to its original level (plasticity of the smooth muscle), which ensures the regulation of vascular tone and the filling of the internal hollow organs without a significant increase in pressure in them (up to a certain value). In addition, most smooth muscles are automatic, they are constantly in some degree of contraction under the influence of impulses that arise in themselves (for example, intestinal muscles, blood vessels). The impulses that come to them through the autonomic nerves have a modulating effect - they increase or decrease the tone of smooth muscle fibers.
Regulatory function of the BBB lies in the fact that it forms a special internal environment of the brain, providing an optimal mode of activity of nerve cells. It is believed that the barrier function in this case performs special structure of the walls of the capillaries of the brain. Their endothelium has very few pores, narrow gap-left junctions between cells contain almost no windows. An integral part of the barrier are also glial cells, which form a kind of cases around the capillaries, covering about 90% of their surface. The greatest contribution to the development of ideas about the blood-brain barrier was made by L. S. Stern and her collaborators. This barrier allows water, ions, glucose, amino acids, gases to pass through, retaining many physiologically active substances: adrenaline, serotonin, dopamine, insulin, thyroxine. However, there are “windows” in it, * through which the corresponding brain cells - chemoreceptors - receive direct information about the presence of hormones and other substances in the blood that do not penetrate the barrier; brain cells secrete their neurosecrets. The areas of the brain that do not have their own blood-brain barrier are the pituitary gland, the pineal gland, some parts of the hypothalamus and the medulla oblongata.
The BBB also has a protective function - prevents the entry of microbes, foreign or toxic substances of exogenous and endogenous nature into the intercellular spaces of the brain. The BBB does not allow many medicinal substances to pass through, which must be taken into account in medical practice.
2.4. SYSTEM PRINCIPLE OF REGULATION
The maintenance of indicators of the internal environment of the body is carried out with the help of the regulation of the activity of various organs and physiological systems, combined into a single functional system - the body. The concept of functional systems was developed by P.K. Anokhin (1898-1974). In recent years, the theory of functional systems has been successfully developed by K. V. Sudakov.
BUT.The structure of a functional system. A functional system is a dynamic combination of various organs and physiological systems of the body, which is formed to achieve a useful adaptive result. For example, in order to quickly run a distance, it is necessary to maximize the activity of the cardiovascular, respiratory, nervous systems and muscles. The functional system includes the following elements: 1) control device - nerve center, representing the union of the nuclei of various levels of the central nervous system; 2) him weekend channels(nerves and hormones); 3) executive bodies - effect-ry, ensuring in the course of physiological activity the maintenance of the regulated process (indicator) at some optimal level (a useful result of the activity of the functional system); four) result receptors(sensory receptors) - sensors that receive information about the parameters of the deviation of the controlled process (indicator) from the optimal level; 5) feedback channel(input channels), informing the nerve center with the help of impulses from the receptors of the result or with the help of the direct action of chemicals on the center - information about the sufficiency or insufficiency of effector efforts to maintain the regulated process (indicator) at an optimal level ( Fig. 2.7).
Afferent impulses from the receptors of the result through the feedback channels enter the nerve center that regulates one or another indicator, the center provides a change in the intensity of the work of the corresponding organ.
When changing the intensity of the effector, the metabolic rate, which also plays an important role in the regulation of the activity of the organs of a particular functional system (the humoral process of regulation).
B.Multiparametric principle of interaction of various functional systems - the principle that determines the generalized activity of functional systems (K. V. Sudakov). The relative stability of the indicators of the internal environment of the body is the result of the coordinated activity of many
functional systems. It turned out that various indicators of the internal environment of the body are interconnected. For example, excess water intake into the body is accompanied by an increase in the volume of circulating blood, an increase in blood pressure, and a decrease in the osmotic pressure of the blood plasma. In a functional system that maintains the optimal level of the gas composition of the blood, the interaction of pH, P CO2 and P 02 is simultaneously carried out. A change in one of these parameters immediately leads to a change in the quantitative characteristics of other parameters. To achieve any adaptive result, an appropriate functional system is formed.
AT. Systemogenesis. According to P.K. Anokhin, systemogenesis -selective maturation and development of functional systems in ante- and postnatal ontogenesis. At present, the term "systemogenesis" is used in a broader sense, while systemogenesis is understood not only as the processes of ontogenetic maturation of functional systems, but also the formation and transformation of functional systems in the course of the life of an organism.
system-forming factors of a functional system of any level are an adaptive result useful for the life of the organism, which is necessary at the moment, and the motivation that is formed at the same time. For example, to perform a high jump with a pole, the leading role is played by the muscles of the upper
of them limbs, in the long jump - the muscles of the lower extremities.
Heterochronism of maturation of functional systems. During antenatal ontogenesis, various structures of the body are laid down at different times and mature at different rates. Thus, the nerve center is grouped and usually matures earlier than the substrate innervated by it is laid down and matures. In ontogenesis, first of all, those functional systems mature, without which the further development of the organism is impossible. For example, of the three functional systems associated with the oral cavity, after birth, only the functional system of sucking is formed, later the functional system of chewing is formed, then the functional system of speech.
Consolidation of functional system components - integration into a functional system of individual fragments that develop in various parts of the body. Consolidation of fragments of a functional system is a critical point development of its physiological architecture. The central nervous system plays a leading role in this process. For example, the heart, blood vessels, respiratory apparatus, blood are combined into a functional system for maintaining the constancy of the gas composition of the internal environment based on the improvement of connections between various parts of the central nervous system, as well as on the basis of the development of innervation connections between the central nervous system and the corresponding peripheral structures.
All functional systems of different levels have the same architectonics(structure).
2.5. TYPES OF REGULATION OF BODY FUNCTIONS
1. Deviation control - a cyclic mechanism, in which any deviation from the optimal level of the regulated indicator mobilizes all the devices of the functional system to restore it at the previous level. Regulation by deviation implies the presence of a channel in the system complex negative feedback, providing a multidirectional influence: strengthening incentive management mechanisms in case of weakening process indicators or weakening incentive mechanisms in case of excessive strengthening of process indicators. For example, with an increase in blood pressure, regulatory mechanisms are activated that ensure a decrease in blood pressure, and with low blood pressure, opposite reactions are activated. Unlike negative feedback, positive
Feedback, which is rare in the body, has only a unidirectional, enhancing effect on the development of the process, which is under the control of the control complex. Therefore, positive feedback makes the system unstable, unable to ensure the stability of the regulated process within the physiological optimum. For example, if arterial pressure were regulated according to the principle of positive feedback, in the case of a decrease in arterial pressure, the action of regulatory mechanisms would lead to an even greater decrease, and in the case of an increase, to an even greater increase. An example of positive feedback is the increased secretion of digestive juices in the stomach after a meal, which is carried out with the help of hydrolysis products absorbed into the blood.
2. Lead control lies in the fact that the regulatory mechanisms are switched on before a real change in the parameter of the regulated process (indicator) based on information entering the nerve center of the functional system and signaling a possible change in the regulated process in the future. For example, thermoreceptors (temperature detectors) located inside the body provide temperature control of the internal regions of the body. Skin thermoreceptors mainly play the role of environmental temperature detectors. With significant deviations in the ambient temperature, prerequisites are created for a possible change in the temperature of the internal environment of the body. However, normally this does not happen, since the impulse from the thermoreceptors of the skin, continuously entering the hypothalamic thermoregulatory center, allows it to make changes in the work of the effectors of the system. until the moment of a real change in the temperature of the internal environment of the organism. Increased ventilation of the lungs during exercise begins before the increase in oxygen consumption and the accumulation of carbonic acid in human blood. This is carried out due to afferent impulses from the proprioreceptors of actively working muscles. Consequently, the impulsation of proprioreceptors acts as a factor organizing the restructuring of the functioning of the functional system, which maintains the optimal level of P 02 , P ss, 2 for metabolism and the pH of the internal environment ahead of time.
The advance control can be implemented using the mechanism conditioned reflex. It is shown that the conductors of freight trains in winter have a sharp increase in heat production as they move away from the departure station, where the conductor was in a warm room. On the way back, as we get closer
physical
Moscow Psychological and Social Institute (MPSI)
Abstract on the anatomy of the central nervous system on the topic:
SYNAPSE (structure, structure, functions).
1st year student of the Faculty of Psychology,
group 21/1-01 Logachev A.Yu.
Teacher:
Kholodova Marina Vladimirovna
year 2001.
Work plan:
1. Prologue.
2. Physiology of the neuron and its structure.
3. Structure and functions of the synapse.
4. Chemical synapse.
5. Isolation of the mediator.
6. Chemical mediators and their types.
7. Epilogue.
8. List of references.
PROLOGUE:
Our body is one big clockwork.
It consists of a huge number of tiny particles that are located in strict order and each of them performs certain functions, and has its own unique properties. This mechanism - the body, consists of cells, tissues and systems connecting them: all this as a whole is a single chain, a super-system of the body.
The greatest number of cellular elements could not work as a whole, if the body did not have a sophisticated mechanism of regulation. The nervous system plays a special role in regulation. All the complex work of the nervous system - regulation of the work of internal organs, control of movements, whether simple and unconscious movements (for example, breathing) or complex, movements of the human hands - all this, in essence, is based on the interaction of cells with each other.
All this, in essence, is based on the transmission of a signal from one cell to another. Moreover, each cell performs its work, and sometimes has several functions. The variety of functions is provided by two factors: the way the cells are connected to each other, and the way these connections are arranged.
NEURON PHYSIOLOGY AND ITS STRUCTURE:
The simplest reaction of the nervous system to an external stimulus is it's a reflex.
First of all, let's consider the structure and physiology of the structural elementary unit of the nervous tissue of animals and humans - neuron. The functional and basic properties of a neuron are determined by its ability to excite and self-excite.
The transmission of excitation is carried out along the processes of the neuron - axons and dendrites.
Axons are longer and wider processes. They have a number of specific properties: isolated conduction of excitation and bilateral conduction.
Nerve cells are able not only to perceive and process external excitation, but also to spontaneously issue impulses that are not caused by external irritation (self-excitation).
In response to stimulation, the neuron responds impulse of activity- action potential, the generation frequency of which ranges from 50-60 impulses per second (for motor neurons), to 600-800 impulses per second (for intercalary neurons of the brain). The axon ends in many thin branches called terminals.
From the terminals, the impulse passes to other cells, directly to their bodies, or more often to their processes, dendrites. The number of terminals in an axon can reach up to one thousand, which terminate in different cells. On the other hand, a typical vertebrate neuron has 1,000 to 10,000 terminals from other cells.
Dendrites are shorter and more numerous processes of neurons. They perceive excitation from neighboring neurons and conduct it to the cell body.
Distinguish between pulpy and non-pulmonic nerve cells and fibers.
Pulp fibers - are part of the sensory and motor nerves of the skeletal muscles and sensory organs. They are covered with a lipid myelin sheath.
Pulp fibers are more “fast-acting”: in such fibers with a diameter of 1-3.5 micromillimeters, excitation propagates at a speed of 3-18 m/s. This is due to the fact that the conduction of impulses along the myelinated nerve occurs spasmodically.
In this case, the action potential "jumps" through the area of the nerve covered with myelin and at the site of the interception of Ranvier (the exposed area of the nerve), passes to the sheath of the axial cylinder of the nerve fiber. The myelin sheath is a good insulator and excludes the transmission of excitation to the junction of parallel nerve fibers.
Non-fleshy fibers - make up the bulk of the sympathetic nerves.
They do not have a myelin sheath and are separated from each other by neuroglial cells.
In non-fleshy fibers, the role of insulators is played by cells neuroglia(nerve support tissue). Schwann cells - one of the types of glial cells. In addition to internal neurons that perceive and convert impulses coming from other neurons, there are neurons that perceive influences directly from the environment - these are receptors as well as neurons that directly affect the executive organs - effectors, for example, muscles or glands.
If a neuron acts on a muscle, it is called a motor neuron or motoneuron. Among neuroreceptors, 5 types of cells are distinguished, depending on the type of pathogen:
— photoreceptors, which are excited under the influence of light and ensure the functioning of the organs of vision,
— mechanoreceptors, those receptors that respond to mechanical influences.
They are located in the organs of hearing, balance. Tactile cells are also mechanoreceptors. Some mechanoreceptors are located in the muscles and measure the degree of their stretching.
— chemoreceptors - selectively react to the presence or change in the concentration of various chemicals, the work of the organs of smell and taste is based on them,
— thermoreceptors, react to changes in temperature or to its level - cold and heat receptors,
— electroreceptors respond to current impulses, and are present in some fish, amphibians, and mammals, such as the platypus.
Based on the foregoing, I would like to note that for a long time among biologists who studied the nervous system, there was an opinion that nerve cells form long complex networks that continuously pass one into another.
However, in 1875, an Italian scientist, professor of histology at the University of Pavia, came up with a new way to stain cells - silvering. When one of the thousands of nearby cells is silvered, only it is stained - the only one, but completely, with all its processes.
Golgi Method greatly contributed to the study of the structure of nerve cells. Its use has shown that, despite the fact that the cells in the brain are located extremely close to each other, and their processes are mixed up, yet each cell is clearly separated. That is, the brain, like other tissues, consists of separate cells that are not united in a common network. This conclusion was made by a Spanish histologist FROM.
Ramon y Cajal, who thereby extended the cellular theory to the nervous system. The rejection of the concept of a unified network meant that in the nervous system pulse passes from cell to cell not through direct electrical contact, but through gap.
When did the electron microscope come into use in biology, which was invented in 1931 M. Knolem and E. Ruska, these ideas about the presence of a gap have received direct confirmation.
STRUCTURE AND FUNCTIONS OF SYNAPSE:
Every multicellular organism, every tissue consisting of cells, needs mechanisms that provide intercellular interactions.
Let's take a look at how it's done interneuronalinteractions. The nerve cell carries information in the form action potentials. The transfer of excitation from axon terminals to an innervated organ or another nerve cell occurs through intercellular structural formations - synapses(from Greek.
"Synapsis" connection, connection). The concept of synapse was introduced by an English physiologist Ch. Sherrington in 1897, to denote functional contact between neurons. It should be noted that in the 1960s THEM.
Sechenov emphasized that without intercellular communication it is impossible to explain the origin of even the most nervous elementary process. The more complex the nervous system is, and the greater the number of constituent nerve brain elements, the more important the value of synaptic contacts becomes.
Different synaptic contacts are different from each other.
However, with all the variety of synapses, there are certain common properties of their structure and function. Therefore, we first describe the general principles of their functioning.
A synapse is a complex structural formation consisting of a presynaptic membrane (most often this is the terminal branching of an axon), a postsynaptic membrane (most often this is a section of the body membrane or a dendrite of another neuron), as well as a synaptic cleft.
The mechanism of transmission through the synapse remained unclear for a long time, although it was obvious that the transmission of signals in the synaptic region differs sharply from the process of conducting an action potential along the axon.
However, at the beginning of the 20th century, a hypothesis was formulated that synaptic transmission occurs or electric or chemical way. The electrical theory of synaptic transmission in the CNS enjoyed recognition until the early 1950s, but it lost ground significantly after the chemical synapse was demonstrated in a number of peripheral synapses. For example, A.V. Kibyakov, having conducted an experiment on the nerve ganglion, as well as the use of microelectrode technology for intracellular registration of synaptic potentials
neurons of the CNS led to the conclusion about the chemical nature of the transmission in the interneuronal synapses of the spinal cord.
Microelectrode studies of recent years have shown that an electrical transmission mechanism exists in certain interneuronal synapses.
It has now become apparent that there are synapses, both with a chemical transmission mechanism and with an electrical one. Moreover, in some synaptic structures, both electrical and chemical transmission mechanisms function together - these are the so-called mixed synapses.
Synapse: structure, functions
Synapse(Greek synapsis - association) provides unidirectional transmission of nerve impulses. Synapses are sites of functional contact between neurons or between neurons and other effector cells (eg, muscle and glandular).
Function synapse consists in converting an electrical signal (impulse) transmitted by the presynaptic cell into a chemical signal that acts on another cell, known as the postsynaptic cell.
Most synapses transmit information by releasing neurotransmitters during the signal propagation process.
neurotransmitters- These are chemical compounds that, by binding to a receptor protein, open or close ion channels or trigger cascades of the second mediator. Neuromodulators are chemical messengers that do not directly act on synapses, but change (modify) the sensitivity of a neuron to synaptic stimulation or to synaptic inhibition.
Some neuromodulators are neuropeptides or steroids and are produced in the nervous tissue, others are circulating steroids in the blood. The synapse itself includes an axon terminal (presynaptic terminal), which brings a signal, a site on the surface of another cell in which a new signal is generated (postsynaptic terminal), and a narrow intercellular space - the synaptic cleft.
If the axon terminates on the cell body, this is an axosomatic synapse, if it ends on a dendrite, then such a synapse is known as axodendritic, and if it forms a synapse on an axon, it is an axoaxonal synapse.
Most of synapses- chemical synapses, since they use chemical mediators, however, individual synapses transmit ionic signals through gap junctions that penetrate the pre- and postsynaptic membranes, thereby providing direct transmission of neuronal signals.
Such contacts are known as electrical synapses.
presynaptic terminal always contains synaptic vesicles with neurotransmitters and numerous mitochondria.
neurotransmitters usually synthesized in the cell body; further they are stored in vesicles in the presynaptic part of the synapse. During nerve impulse transmission, they are released into the synaptic cleft through a process known as exocytosis.
5. The mechanism of information transmission in synapses
Endocytosis promotes the return of excess membrane that accumulates in the presynaptic part as a result of exocytosis of synaptic vesicles.
returned membrane fuses with the agranular endoplasmic reticulum (aER) of the presynaptic compartment and is reused to form new synaptic vesicles.
Some neurotransmitters are synthesized in the presynaptic compartment using enzymes and precursors that are delivered by the axonal transport mechanism.
The first described neurotransmitters were acetylcholine and norepinephrine. The axon terminal that releases norepinephrine is shown in the figure.
Most neurotransmitters are amines, amino acids, or small peptides (neuropeptides). Some inorganic substances, such as nitric oxide, may also act as neurotransmitters. Individual peptides that play the role of neurotransmitters are used in other parts of the body, for example, as hormones in the digestive tract.
Neuropeptides are very important in the regulation of sensations and urges such as pain, pleasure, hunger, thirst and sex drive.
Sequence of events during signal transmission in a chemical synapse
Phenomena occurring during transmission signal in a chemical synapse are illustrated in the figure.
Nerve impulses traveling rapidly (within milliseconds) across the cell membrane cause explosive electrical activity (depolarization) that propagates across the cell membrane.
Such impulses briefly open calcium channels in the presynaptic region, providing an influx of calcium that triggers synaptic vesicle exocytosis.
In areas of exopytosis, neurotransmitters, which react with receptors located on the postsynaptic site, causing transient electrical activity (depolarization) of the postsynaptic membrane.
Such synapses are known as excitatory because their activity promotes impulses in the postsynaptic cell membrane. In some synapses, the interaction of the neurotransmitter - the receptor has the opposite effect - hyperpolarization occurs, and there is no transmission of the nerve impulse. These synapses are known as inhibitory synapses. Thus, synapses can either enhance or inhibit the transmission of impulses, thus they are able to regulate nerve activity.
After use neurotransmitters are rapidly removed by enzymatic degradation, diffusion, or endocytosis mediated by specific receptors on the presynaptic membrane. This removal of neurotransmitters is of important functional importance, since it prevents unwanted prolonged stimulation of the postsynaptic neuron.
Educational video - the structure of the synapse
- The body of a nerve cell - a neuron: structure, histology
- Dendrites of nerve cells: structure, histology
- Axons of nerve cells: structure, histology
- Membrane potentials of nerve cells.
Physiology
- Synapse: structure, functions
- Glial cells: oligodendrocytes, Schwann cells, astrocytes, ependymal cells
- Microglia: structure, histology
- Central nervous system (CNS): structure, histology
- Histology of the meninges. Structure
- Blood-brain barrier: structure, histology
The structure of the synapseLet us consider the structure of the synapse on the example of an axosomatic synapse. The synapse consists of three parts: the presynaptic ending, the synaptic cleft, and the postsynaptic membrane (Fig. 9). The presynaptic ending is filled with vesicles (vesicles) and mitochondria. Vesicles contain biologically active substances - mediators. Mediators are synthesized in the soma and transported via microtubules to the presynaptic ending. Most often, adrenaline, noradrenaline, acetylcholine, serotonin, gamma-aminobutyric acid (GABA), glycine and others act as a mediator. Usually, the synapse contains one of the mediators in a larger amount compared to other mediators. According to the type of mediator, it is customary to designate synapses: adrenoergic, cholinergic, serotonergic, etc. The synaptic cleft is filled with intercellular fluid, which contains enzymes that contribute to the destruction of neurotransmitters. In electrical synapses, the interaction of two neurons is carried out through biocurrents. chemical synapsePD nerve fiber (AP - action potential) 9. Scheme of the structure of the synapse. The central nervous system is dominated by chemical synapses. The influence of excitatory and inhibitory synapses on the excitability of the postsynaptic neuron is summed up, and the effect depends on the location of the synapse. The closer the synapses are to the axonal hillock, the more efficient they are. On the contrary, the farther the synapses are located from the axonal hillock (for example, at the end of the dendrites), the less effective they are. Thus, synapses located on the soma and axonal hillock affect neuron excitability quickly and efficiently, while the influence of distant synapses is slow and smooth. Ampmsch iipinl system Such a neural network is called local. In addition, neurons remote from each other, from different areas of the brain, can be combined into a network. The highest level of organization of neuron connections reflects the connection of several areas of the central nervous system. Such a neural network is called a path, or a system. There are descending and ascending paths. Information is transmitted along ascending pathways from the underlying areas of the brain to the overlying ones (for example, from the spinal cord to the cerebral cortex). Descending tracts connect the cerebral cortex with the spinal cord. Some neural networks provide convergence (convergence) of impulses on a limited number of neurons. Neural networks can also be built according to the type of divergence (divergence). Such networks cause the transmission of information over considerable distances. In addition, neural networks provide integration (summation or generalization) of various kinds of information (Fig. 10). |
The synapse is the site of functional rather than physical contact between neurons; it transmits information from one cell to another. Synapses are usually found between the terminal branches of the axon of one neuron and dendrites ( axodendritic synapses) or body ( axosomatic synapses) of another neuron. The number of synapses is usually very large, which provides a large area for information transfer. For example, there are more than 1000 synapses on the dendrites and bodies of individual motor neurons of the spinal cord. Some brain cells can have up to 10,000 synapses (Figure 16.8).
There are two types of synapses - electrical and chemical- depending on the nature of the signals passing through them. Between the endings of the motor neuron and the surface of the muscle fiber there is neuromuscular junction, which differs in structure from interneuronal synapses, but is functionally similar to them. Structural and physiological differences between a normal synapse and a neuromuscular junction will be described later.
The structure of a chemical synapse
Chemical synapses are the most common type of synapse in vertebrates. These are bulbous thickenings of nerve endings called synaptic plaques and located in close proximity to the end of the dendrite. The cytoplasm of the synaptic plaque contains mitochondria, smooth endoplasmic reticulum, microfilaments, and numerous synaptic vesicles. Each bubble is about 50 nm in diameter and contains mediator A substance that transmits nerve signals across the synapse. The membrane of the synaptic plaque in the area of the synapse itself is thickened as a result of the compaction of the cytoplasm and forms presynaptic membrane. The dendrite membrane in the area of the synapse is also thickened and forms postsynaptic membrane. These membranes are separated by a gap - synaptic cleft about 20 nm wide. The presynaptic membrane is designed in such a way that synaptic vesicles can attach to it and neurotransmitters can be released into the synaptic cleft. The postsynaptic membrane contains large protein molecules that act as receptors mediators, and numerous channels and pores(usually closed), through which ions can enter the postsynaptic neuron (see Fig. 16.10, A).
Synaptic vesicles contain a neurotransmitter that is formed either in the body of the neuron (and enters the synaptic plaque, having passed through the entire axon), or directly in the synaptic plaque. In both cases, the synthesis of the mediator requires enzymes that are formed in the cell body on ribosomes. In the synaptic plaque, the neurotransmitter molecules are "packed" into vesicles, in which they are stored until they are released. The main mediators of the nervous system of vertebrates - acetylcholine and norepinephrine, but there are other mediators that will be discussed later.
Acetylcholine is an ammonium derivative whose formula is shown in fig. 16.9. This is the first known mediator; in 1920, Otto Levi isolated it from the terminals of the parasympathetic neurons of the vagus nerve in the frog heart (section 16.2). The structure of norepinephrine is discussed in detail in Sec. 16.6.6. Neurons that release acetylcholine are called cholinergic, and releasing norepinephrine - adrenergic.
Mechanisms of synaptic transmission
It is believed that the arrival of a nerve impulse in the synaptic plaque causes depolarization of the presynaptic membrane and an increase in its permeability for Ca 2+ ions. The Ca 2+ ions entering the synaptic plaque cause the fusion of synaptic vesicles with the presynaptic membrane and the release of their contents from the cell. (exocytosis), causing it to enter the synaptic cleft. This whole process is called electrosecretory conjugation. After release of the mediator, the vesicle material is used to form new vesicles filled with mediator molecules. Each vial contains about 3,000 molecules of acetylcholine.
Transmitter molecules diffuse through the synaptic cleft (this process takes about 0.5 ms) and bind to receptors located on the postsynaptic membrane that can recognize the molecular structure of acetylcholine. When a receptor molecule binds to a mediator, its configuration changes, which leads to the opening of ion channels and the entry of ions into the postsynaptic cell, causing depolarization or hyperpolarization(Fig. 16.4, A) its membranes, depending on the nature of the released mediator and the structure of the receptor molecule. The mediator molecules that caused a change in the permeability of the postsynaptic membrane are immediately removed from the synaptic cleft either by their reabsorption by the presynaptic membrane, or by diffusion from the cleft or enzymatic hydrolysis. When cholinergic synapses, acetylcholine located in the synaptic cleft is hydrolyzed by the enzyme acetylcholinesterase located on the postsynaptic membrane. As a result of hydrolysis, choline is formed, it is absorbed back into the synaptic plaque and again converted there into acetylcholine, which is stored in the vesicles (Fig. 16.10).
AT exciting In synapses, under the action of acetylcholine, specific sodium and potassium channels open, and Na + ions enter the cell, and K + ions leave it in accordance with their concentration gradients. The result is depolarization of the postsynaptic membrane. This depolarization is called excitatory postsynaptic potential(VPSP). The amplitude of the EPSP is usually small, but its duration is longer than that of the action potential. The amplitude of the EPSP changes in a stepwise manner, and this suggests that the neurotransmitter is released in portions, or "quanta", and not in the form of individual molecules. Apparently, each quantum corresponds to the release of a mediator from one synaptic vesicle. A single EPSP is usually unable to induce the threshold depolarization required for an action potential to occur. But the depolarizing effects of several EPSPs add up, and this phenomenon is called summation. Two or more EPSPs occurring simultaneously at different synapses of the same neuron can collectively induce depolarization sufficient to excite an action potential in a postsynaptic neuron. It's called spatial summation. The rapidly repeated release of the mediator from the vesicles of the same synaptic plaque under the influence of an intense stimulus causes separate EPSPs that follow so often one after another in time that their effects also add up and evoke an action potential in the postsynaptic neuron. It is called temporary summation. Thus, impulses can occur in a single postsynaptic neuron, either as a result of weak stimulation of several presynaptic neurons associated with it, or as a result of repeated stimulation of one of its presynaptic neurons. AT brake synapses, the release of the mediator increases the permeability of the postsynaptic membrane by opening specific channels for K + and Cl - ions. Moving along concentration gradients, these ions cause membrane hyperpolarization, called inhibitory postsynaptic potential(TPSP).
Mediators themselves do not have excitatory or inhibitory properties. For example, acetylcholine has an excitatory effect at most neuromuscular junctions and other synapses, but causes inhibition at the neuromuscular junctions of the heart and visceral muscles. These opposite effects are due to the events that unfold on the postsynaptic membrane. The molecular properties of the receptor determine which ions will enter the postsynaptic neuron, and these ions, in turn, determine the nature of the change in postsynaptic potentials, as described above.
electrical synapses
In many animals, including coelenterates and vertebrates, the transmission of impulses through some synapses is carried out by passing an electric current between pre- and postsynaptic neurons. The width of the gap between these neurons is only 2 nm, and the total resistance to current from the side of the membranes and the fluid filling the gap is very small. Impulses pass through the synapses without delay, and their transmission is not affected by drugs or other chemicals.
neuromuscular junction
The neuromuscular junction is a specialized type of synapse between the endings of a motor neuron (motoneuron) and endomysium muscle fibers (section 17.4.2). Each muscle fiber has a specialized area - motor end plate, where the axon of a motor neuron (motoneuron) branches, forming unmyelinated branches about 100 nm thick, passing in shallow grooves along the surface of the muscle membrane. The membrane of the muscle cell - the sarcolemma - forms many deep folds called postsynaptic folds (Fig. 16.11). The cytoplasm of motor neuron endings is similar to the contents of a synaptic plaque and releases acetylcholine during stimulation using the same mechanism as mentioned above. Changes in the configuration of receptor molecules located on the surface of the sarcolemma lead to a change in its permeability for Na + and K +, and as a result, local depolarization occurs, called end plate potential(PKP). This depolarization is quite sufficient in magnitude for the occurrence of an action potential, which propagates along the sarcolemma deep into the fiber along the system of transverse tubules ( T-system) (section 17.4.7) and causes the muscle to contract.
Functions of synapses and neuromuscular junctions
The main function of interneuronal synapses and neuromuscular junctions is to transmit a signal from receptors to effectors. In addition, the structure and organization of these sites of chemical secretion determine a number of important features of the conduction of a nerve impulse, which can be summarized as follows:
1. Unidirectional transmission. The release of the mediator from the presynaptic membrane and the localization of receptors on the postsynaptic membrane allow the transmission of nerve signals along this pathway in only one direction, which ensures the reliability of the nervous system.
2. Gain. Each nerve impulse causes enough acetylcholine to be released at the neuromuscular junction to cause a propagating response in the muscle fiber. Due to this, the nerve impulses coming to the neuromuscular junction, no matter how weak, can cause an effector response, and this increases the sensitivity of the system.
3. adaptation or accommodation. With continuous stimulation, the amount of mediator released in the synapse gradually decreases until the stores of the mediator are depleted; then they say that the synapse is tired, and further transmission of signals to them is inhibited. The adaptive value of fatigue is that it prevents damage to the effector due to overexcitation. Adaptation also takes place at the receptor level. (See description in section 16.4.2.)
4. Integration. A postsynaptic neuron can receive signals from a large number of excitatory and inhibitory presynaptic neurons (synaptic convergence); in this case, the postsynaptic neuron is able to sum up the signals from all presynaptic neurons. Due to spatial summation, the neuron integrates signals from many sources and produces a coordinated response. In some synapses, facilitation occurs, consisting in the fact that after each stimulus the synapse becomes more sensitive to the next stimulus. Therefore, successive weak stimuli can cause a response, and this phenomenon is used to increase the sensitivity of certain synapses. The facilitation cannot be considered as a temporary summation: there is a chemical change in the postsynaptic membrane, and not an electrical summation of the postsynaptic membrane potentials.
5. Discrimination. Temporal summation at the synapse allows weak background impulses to be filtered out before they reach the brain. For example, the exteroceptors of the skin, eyes, and ears constantly receive signals from the environment that are not of particular importance to the nervous system: only changes stimulus intensities leading to an increase in the frequency of impulses, which ensures their transmission through the synapse and the proper response.
6. Braking. Signaling across synapses and neuromuscular junctions can be inhibited by certain blocking agents that act on the postsynaptic membrane (see below). Presynaptic inhibition is also possible, if at the end of the axon just above this synapse, another axon ends, forming here an inhibitory synapse. When such an inhibitory synapse is stimulated, the number of synaptic vesicles that are discharged in the first, excitatory synapse decreases. Such a device allows you to change the impact of a given presynaptic neuron using signals coming from another neuron.
Chemical effects on the synapse and neuromuscular junction
Chemicals perform many different functions in the nervous system. The effects of some substances are widespread and well understood (such as the excitatory effects of acetylcholine and adrenaline), while the effects of others are local and not yet clear enough. Some substances and their functions are given in Table. 16.2.
Some drugs used for mental disorders such as anxiety and depression are thought to interfere with chemical transmission at synapses. Many tranquilizers and sedatives (tricyclic antidepressant imipramine, reserpine, monoamine oxidase inhibitors, etc.) exert their therapeutic effect by interacting with mediators, their receptors or individual enzymes. For example, monoamine oxidase inhibitors inhibit the enzyme involved in the breakdown of adrenaline and norepinephrine, and most likely exert their therapeutic effect in depression by increasing the duration of these mediators. Type hallucinogens lysergic acid diethylamide and mescaline, reproduce the action of some natural mediators of the brain or suppress the action of other mediators.
A recent study on the effects of certain painkillers, opiates, heroin and morphine- showed that in the brain of mammals there are natural (endogenous) substances that cause a similar effect. All these substances that interact with opiate receptors are collectively called endorphins. To date, many such compounds have been discovered; of these, the group of relatively small peptides called enkephalins(meth-enkephalin, β-endorphin, etc.). They are believed to suppress pain, affect emotions and are related to some mental illnesses.
All of this has opened up new avenues for studying brain functions and the biochemical mechanisms underlying pain management and treatment through methods as diverse as suggestion, hypno? and acupuncture. Many other endorphin-type substances remain to be isolated, their structure and functions to be established. With their help, it will be possible to get a more complete picture of the work of the brain, and this is only a matter of time, since methods for isolating and analyzing substances present in such small quantities are constantly being improved.
The area of contact between two neurons is called synapse.
The internal structure of the axodendritic synapse.a) electrical synapses. Electrical synapses are rare in the mammalian nervous system. They are formed by slit-like junctions (nexuses) between the dendrites or somas of adjoining neurons, which are connected via cytoplasmic channels 1.5 nm in diameter. The process of signal transmission occurs without synaptic delay and without the participation of mediators.
Through electrical synapses, it is possible to spread electrotonic potentials from one neuron to another. Due to the close synaptic contact, signal conduction modulation is impossible. The task of these synapses is the simultaneous excitation of neurons that perform the same function. An example is the neurons of the respiratory center of the medulla oblongata, which synchronously generate impulses during inspiration. In addition, the neural circuits that control saccades, in which the fixation point of the gaze moves from one object of attention to another, can serve as an example.
b) Chemical synapses. Most synapses in the nervous system are chemical. The functioning of such synapses depends on the release of neurotransmitters. The classical chemical synapse is represented by the presynaptic membrane, the synaptic cleft, and the postsynaptic membrane. The presynaptic membrane is part of the club-shaped extension of the nerve ending of the cell that transmits the signal, and the postsynaptic membrane is the part of the cell that receives the signal.
The mediator is released from the club-shaped expansion by exocytosis, passes through the synaptic cleft, and binds to receptors on the postsynaptic membrane. Under the postsynaptic membrane there is a subsynaptic active zone, in which, after the activation of the receptors of the postsynaptic membrane, various biochemical processes occur.
The club-shaped extension contains synaptic vesicles containing neurotransmitters, as well as a large number of mitochondria and cisternae of the smooth endoplasmic reticulum. The use of traditional methods of fixation in the study of cells makes it possible to distinguish presynaptic seals on the presynaptic membrane, limiting the active zones of the synapse, to which synaptic vesicles are directed by means of microtubules.
axodendritic synapse. Section of the spinal cord preparation: synapse between the end section of the dendrite and, presumably, a motor neuron.
The presence of rounded synaptic vesicles and postsynaptic compaction is characteristic of excitatory synapses.
The section of the dendrite is drawn in the transverse direction, as evidenced by the presence of many microtubules.
In addition, some neurofilaments are visible. The site of the synapse is surrounded by a protoplasmic astrocyte.
Processes occurring in the nerve endings of two types. (A) Synaptic transmission of small molecules (eg, glutamate).
(1) Transport vesicles containing the membrane proteins of the synaptic vesicles are guided along the microtubules to the clubbed plasma membrane.
At the same time, enzyme and glutamate molecules are transferred by slow transport.
(2) Vesicle membrane proteins exit the plasma membrane and form synaptic vesicles.
(3) Glutamate sinks into synaptic vesicles; mediator accumulation occurs.
(4) Vesicles containing glutamate approach the presynaptic membrane.
(5) Depolarization results in mediator exocytosis from partially destroyed vesicles.
(6) The released neurotransmitter spreads diffusely in the area of the synaptic cleft and activates specific receptors on the postsynaptic membrane.
(7) Synaptic vesicle membranes are transported back into the cell by endocytosis.
(8) Partial reuptake of glutamate into the cell for reuse occurs.
(B) Transmission of neuropeptides (eg, substance P) occurring simultaneously with synaptic transmission (eg, glutamate).
The joint transmission of these substances occurs in the central nerve endings of unipolar neurons, which provide pain sensitivity.
(1) The vesicles and peptide precursors (propeptides) synthesized in the Golgi complex (in the perikaryon region) are transported to the club-shaped extension by rapid transport.
(2) When they enter the region of the club-shaped thickening, the process of formation of the peptide molecule is completed, and the bubbles are transported to the plasma membrane.
(3) Membrane depolarization and transport of vesicle contents into the extracellular space by exocytosis.
(4) At the same time, glutamate is released.
1. Receptor activation. Transmitter molecules pass through the synaptic cleft and activate receptor proteins located in pairs on the postsynaptic membrane. Receptor activation triggers ionic processes that lead to depolarization of the postsynaptic membrane (excitatory postsynaptic action) or hyperpolarization of the postsynaptic membrane (inhibitory postsynaptic action). The change in the electrotonus is transmitted to the soma in the form of an electrotonic potential that decays as it spreads, due to which a change in the resting potential occurs in the initial segment of the axon.
Ionic processes are described in detail in a separate article on the site. With the predominance of excitatory postsynaptic potentials, the initial segment of the axon depolarizes to a threshold level and generates an action potential.
The most common excitatory CNS mediator is glutamate, and the inhibitory one is gamma-aminobutyric acid (GABA). In the peripheral nervous system, acetylcholine serves as a mediator for motor neurons of striated muscles, and glutamate for sensory neurons.
The sequence of processes occurring in glutamatergic synapses is shown in the figure below. When glutamate is transferred together with other peptides, the release of peptides is carried out extrasynaptically.
Most sensitive neurons, in addition to glutamate, also secrete other peptides (one or more) that are released in different parts of the neuron; however, the main function of these peptides is to modulate (increase or decrease) the efficiency of synaptic glutamate transmission.
In addition, neurotransmission can occur through diffuse extrasynaptic signaling characteristic of monoaminergic neurons (neurons that use biogenic amines to mediate neurotransmission). There are two types of monoaminergic neurons. In some neurons, catecholamines (norepinephrine or dopamine) are synthesized from the amino acid tyrosine, while in others, serotonin is synthesized from the amino acid tryptophan. For example, dopamine is released both in the synaptic region and from axon varicose thickenings, in which this neurotransmitter is also synthesized.
Dopamine penetrates into the intercellular fluid of the CNS and, until degradation, is able to activate specific receptors at a distance of up to 100 microns. Monoaminergic neurons are present in many CNS structures; disruption of impulse transmission by these neurons leads to various diseases, among which are Parkinson's disease, schizophrenia and major depression.
Nitric oxide (a gaseous molecule) is also involved in diffuse neurotransmission in the glutamatergic system of neurons. Excessive influence of nitric oxide has a cytotoxic effect, especially in those areas whose blood supply is impaired due to arterial thrombosis. Glutamate is also a potentially cytotoxic neurotransmitter.
In contrast to diffuse neurotransmission, traditional synaptic signal transmission is called “conductive” due to its relative stability.
in) Summary. Multipolar CNS neurons consist of a soma, dendrites, and an axon; the axon forms collateral and terminal branches. The soma contains smooth and rough endoplasmic reticulum, Golgi complexes, neurofilaments and microtubules. Microtubules penetrate the neuron throughout, take part in the process of anterograde transport of synaptic vesicles, mitochondria and substances for building membranes, and also provide retrograde transport of "marker" molecules and destroyed organelles.
There are three types of chemical interneuronal interactions: synaptic (eg, glutamatergic), extrasynaptic (peptidergic), and diffuse (eg, monoaminergic, serotonergic).
Chemical synapses are classified according to their anatomical structure into axodendritic, axosomatic, axoaxonal, and dendro-dendritic. The synapse is represented by pre- and postsynaptic membranes, the synaptic cleft and the subsynaptic active zone.
Electrical synapses provide simultaneous activation of entire groups, forming electrical connections between them due to slot-like junctions (nexuses).
Diffuse neurotransmission in the brain. Axons of glutamatergic (1) and dopaminergic (2) neurons form tight synaptic contacts with the process of the stellate neuron (3) of the striatum.
Dopamine is released not only from the presynaptic region, but also from the varicose thickening of the axon, from where it diffuses into the intercellular space and activates the dopamine receptors of the dendritic trunk and the capillary pericyte wall.
Release. (A) Excitatory neuron 1 activates inhibitory neuron 2, which in turn inhibits neuron 3.
(B) The appearance of the second inhibitory neuron (2b) has the opposite effect on neuron 3, since neuron 2b is inhibited.
Spontaneously active neuron 3 generates signals in the absence of inhibitory influences.
2. Medicines - "keys" and "locks". The receptor can be compared with a lock, and the mediator - with a key that fits it. In the event that the mediator release process is impaired with age or as a result of any disease, the drug can play the role of a “spare key” that performs a function similar to the mediator. Such a drug is called an agonist. At the same time, in case of excessive production, the mediator can be "intercepted" by the receptor blocker - a "false key", which will contact the "lock" receptor, but will not cause its activation.
3. Braking and releasing. The functioning of spontaneously active neurons is inhibited under the influence of inhibitory neurons (usually GABAergic). The activity of inhibitory neurons, in turn, can be inhibited by other inhibitory neurons acting on them, resulting in disinhibition of the target cell. The disinhibition process is an important feature of neuronal activity in the basal ganglia.
4. Rare types of chemical synapses. There are two types of axoaxonal synapses. In both cases, the club-shaped thickening forms an inhibitory neuron. Synapses of the first type are formed in the region of the initial segment of the axon and transmit a powerful inhibitory effect of the inhibitory neuron. Synapses of the second type are formed between the club-shaped thickening of the inhibitory neuron and the club-shaped thickening of excitatory neurons, which leads to inhibition of the release of mediators. This process is called presynaptic inhibition. In this regard, the traditional synapse provides postsynaptic inhibition.
Dendro-dendritic (D-D) synapses are formed between the dendritic spines of the dendrites of adjacent spiny neurons. Their task is not to generate a nerve impulse, but to change the electrical tone of the target cell. In successive D-D synapses, synaptic vesicles are located only in one dendritic spine, and in the reciprocal D-D synapse, in both. Excitatory D-D synapses are shown in the figure below. Inhibitory D-D synapses are widely represented in the switching nuclei of the thalamus.
In addition, a few somato-dendritic and somato-somatic synapses are distinguished.
Axoaxonal synapses of the cerebral cortex. The arrows indicate the direction of the impulses.
(1) Presynaptic and (2) postsynaptic inhibition of a spinal neuron traveling to the brain. The arrows indicate the direction of impulse conduction (possibly inhibition of the switching neuron under the action of inhibitory influences).
Excitatory dendro-dendritic synapses. The dendrites of three neurons are shown. Reciprocal synapse (right). The arrows indicate the direction of propagation of electrotonic waves.
Educational video - the structure of the synapse
Synapse(Greek σύναψις, from συνάπτειν - hug, clasp, shake hands) - the place of contact between two neurons or between and the effector cell receiving the signal. Serves for transmission between two cells, and during synaptic transmission, the amplitude and frequency of the signal can be regulated.
The term was introduced in 1897 by the English physiologist Charles Sherrington.
synapse structure
A typical synapse is an axo-dendritic chemical synapse. Such a synapse consists of two parts: presynaptic, formed by a club-shaped extension of the end of the maxon of the transmitting cell and postsynaptic, represented by the contact area of the cytolemma of the perceiving cell (in this case, the dendrite area). The synapse is a space separating the membranes of contacting cells, to which the nerve endings fit. The transmission of impulses is carried out chemically with the help of mediators or electrically through the passage of ions from one cell to another.
Between both parts there is a synaptic gap - a gap 10-50 nm wide between the postsynaptic and presynaptic membranes, the edges of which are reinforced with intercellular contacts.
The part of the axolemma of the club-shaped extension adjacent to the synaptic cleft is called presynaptic membrane. The section of the cytolemma of the perceiving cell that limits the synaptic cleft on the opposite side is called postsynaptic membrane, in chemical synapses it is relief and contains numerous.
In the synaptic extension there are small vesicles, the so-called synaptic vesicles containing either a mediator (transmission intermediary substance) or an enzyme that destroys this mediator. On the postsynaptic, and often on the presynaptic membranes, there are receptors for one or another mediator.
Synapse classification
Depending on the mechanism of transmission of a nerve impulse, there are
- chemical;
- electrical - cells are connected by highly permeable contacts using special connexons (each connexon consists of six protein subunits). The distance between cell membranes in an electrical synapse is 3.5 nm (usual intercellular is 20 nm)
Since the resistance of the extracellular fluid is small (in this case), the impulses pass without stopping through the synapse. Electrical synapses are usually excitatory.
Two release mechanisms have been discovered: with complete fusion of the vesicle with the plasmalemma and the so-called "kissed and ran away" (Eng. kiss and run), when the vesicle connects to the membrane, and small molecules come out of it into the synaptic cleft, while large ones remain in the vesicle. The second mechanism, presumably, is faster than the first, with the help of which synaptic transmission occurs at a high content of calcium ions in the synaptic plaque.
The consequence of this structure of the synapse is the unilateral conduction of the nerve impulse. There is a so-called synaptic delay is the time it takes for a nerve impulse to be transmitted. Its duration is about - 0.5 ms.
The so-called "Dail principle" (one - one mediator) is recognized as erroneous. Or, as it is sometimes believed, it is refined: not one, but several mediators can be released from one end of a cell, and their set is constant for a given cell.
Discovery history
- In 1897, Sherrington formulated the concept of synapses.
- For research on the nervous system, including synaptic transmission, in 1906 the Nobel Prize was awarded to Golgi and Ramon y Cajal.
- In 1921, the Austrian scientist O. Loewi established the chemical nature of the transmission of excitation through synapses and the role of acetylcholine in it. Received the Nobel Prize in 1936 together with G. Dale (N. Dale).
- In 1933, the Soviet scientist A. V. Kibyakov established the role of adrenaline in synaptic transmission.
- 1970 - B. Katz (V. Katz, Great Britain), U. von Euler (U. v. Euler, Sweden) and J. Axelrod (J. Axelrod, USA) received the Nobel Prize for the discovery of rolinoradrenaline in synaptic transmission.
