They form onion-like thickenings called synaptic plaques.
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 a presynaptic membrane. The dendrite membrane in the synapse area is also thickened and forms a postsynaptic membrane. These membranes are separated by a gap - a synaptic cleft 10 - 50 nm wide.
Since many ions are involved in the formation of the resting potential of the membrane, equilibrium can be disturbed by changes in the conductivity of various ions. So, for example, with an additional outgoing current of K + ions or with an incoming current of Cl- ions, the resting potential of the membrane can increase, which means that it hyperpolarizes. Membrane hyperpolarization is the opposite of excitation, i.e. certain chemical processes on the postsynaptic membrane can cause inhibition of a neuron. In this possibility, one can see a significant evolutionary advantage of chemical synapses over electrical synapses.
It is quite obvious that the chemical processes presented in this section very briefly can be modified by means of other, again chemical, substances. This happens with the help of independent connections - neuromodulators.
Chemical processes in the synapse open up wide possibilities for pharmacological regulation and are the subject of numerous studies in order to search for endogenous compounds capable of modifying synaptic transmission in a given direction. Indeed, the effect of many medications is based on the effect on synaptic conduction. This applies not only to psychotropic and narcotic substances. Many others, for example, blood pressure lowering (antihypertensive) drugs, also act indirectly through synapses. In addition, many poisons of plant and animal origin have a targeted effect on the chemical synapse.
Moscow Psychological and Social Institute (MPSI)
Abstract on the Anatomy of the central nervous system on the topic:
SYNAPSES (structure, structure, function).
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 a neuron and its structure.
3. The structure and function of the synapse.
4. Chemical synapse.
5. Isolation of a pick.
6. Chemical mediators and their types.
7 Epilogue.
8. List of references.
PROLOGUE:
Our body is one large 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, connecting them tissues and systems: all this as a whole is a single chain, a super-system of the body.
The greatest set of cellular elements could not work as a whole, if the body did not have a sophisticated regulation mechanism. The nervous system plays a special role in regulation. All hard work nervous systems s - regulation of work internal organs, control of movements, be it simple and unconscious movements (for example, breathing) or complex, human hand movements - 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 does its job, and sometimes has several functions. The variety of functions is provided by two factors: how cells are connected to each other, and how these connections are arranged.
PHYSIOLOGY OF THE NEURON AND ITS STRUCTURE:
The simplest reaction of the nervous system to an external stimulus is it is a reflex.
First of all, let us 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 excitement, but also to spontaneously emit impulses that are not caused by external stimulation (self-excitation).
In response to stimulation, the neuron responds impulse of activity- action potential, the generation frequency of which ranges from 50-60 pulses per second (for motoneurons), up to 600-800 pulses per second (for interneurons of the brain). The axon ends with many thin branches, which are called terminals.
From the terminals, the impulse passes to other cells, directly to their bodies or, more often, to their dendrites. The number of terminals at the axon can reach up to one thousand, which end 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 fleshy and non-fleshy nerve cells and fibers.
Fleshy fibers - are part of the sensory and motor nerves of the skeletal muscles and sensory organs They are covered with a lipid myelin sheath.
Fleshy fibers are more "fast-acting": in such fibers with a diameter of 1-3.5 micromillimeters, the excitement spreads at a speed of 3-18 m / s. This is due to the fact that the conduction of impulses along the myelinated nerve occurs abruptly.
In this case, the action potential "jumps" over the area of the nerve covered with myelin and, at the place of interception of Ranvier (the bare 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 neuroglia cells.
Cells play the role of insulators in non-fleshy fibers. neuroglia(nerve supporting tissue). Schwann cells - one of the varieties of glial cells. In addition to internal neurons that perceive and transform impulses from other neurons, there are neurons that perceive influences directly from environment- it receptors, as well as neurons that directly affect the executive organs - effectors, for example, muscles or glands.
If a neuron affects a muscle, it is called a motor neuron or motor neuron. 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 stress.
They are located in the organs of hearing, balance. Tactile cells are also mechanoreceptors. Some mechanoreceptors are located in the muscles and measure the extent of their stretching.
— chemoreceptors - they 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 a change in temperature or to its level - cold and heat receptors,
— electroreceptors react to current impulses, and are found in some fish, amphibians and mammals, for example, 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 merge into one another.
However, in 1875, an Italian scientist, professor of histology at the University of Pavia, came up with a new way of staining cells - silvering. When one of the thousands of adjacent cells is silvering, only it is stained - the only one, but completely, with all its processes.
Golgi method greatly helped the study of the structure of nerve cells. Its use has shown that, despite the fact that the cells in the brain are located extremely close to each other, and their processes are entangled, yet each cell is clearly separated. That is, the brain, like other tissues, consists of separate cells that are not united into a common network. This conclusion was made by a Spanish histologist WITH.
Ramon y Cajal, who thereby extended the cellular theory to the nervous system. The rejection of the idea of a united network meant that in the nervous system pulse passes from cell to cell not through direct electrical contact, but through break.
When the electron microscope, which was invented in 1931, began to be used in biology M. Knoll and E. Ruska, these ideas about the presence of a gap have received direct confirmation.
SYNAPSE STRUCTURE AND FUNCTIONS:
Every multicellular organism, every tissue, consisting of cells, needs mechanisms that ensure intercellular interactions.
Consider how interneuronalinteractions. Information spreads along the nerve cell in the form action potentials. The transmission of excitation from axonal terminals to an innervated organ or other nerve cell occurs through intercellular structural formations - synapses(from the Greek.
"Synapsis"-connection, communication). The concept of synapse was introduced by an English physiologist C. Sherrington in 1897, to indicate functional contact between neurons. It should be noted that back in the 60s of the last century THEM.
Sechenov emphasized that outside the intercellular communication it is impossible to explain the ways of origin of even the most elementary nervous process. The more complex the nervous system is, and the greater the number of constituent nervous brain elements, the more important the importance of synaptic contacts becomes.
The various synaptic contacts differ from each other.
However, with all the variety of synapses, there are certain general properties of their structure and function. Therefore, we first describe the general principles of their functioning.
Synapse - is a complex structural formation consisting of a presynaptic membrane (most often it is the terminal branching of an axon), a postsynaptic membrane (most often this is a section of the body membrane or 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 is sharply different from the process of conducting an action potential along an axon.
However, at the beginning of the 20th century, a hypothesis was formulated that synaptic transmission is carried out or electric or chemically. The electrical theory of synaptic transmission in the central nervous system was recognized until the early 50s, but it significantly lost ground after the chemical synapse was demonstrated in a number of peripheral synapses. For example, A.V. Kibyakov, conducting an experiment on the nerve ganglion, as well as the use of microelectrode technology for intracellular registration of synaptic potentials
neurons of the central nervous system made it possible to draw a conclusion about the chemical nature of transmission in the interneuronal synapses of the spinal cord.
Microelectrode studies in recent years have shown that there is an electrical transmission mechanism at 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, function
Synapse(Greek synapsis - union) 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 cells).
Function synapse consists in converting an electrical signal (impulse) transmitted by a presynaptic cell into a chemical signal that acts on another cell, known as a postsynaptic cell.
Most synapses transmit information by releasing neurotransmitters during signal propagation.
Neurotransmitters are chemical compounds that, by binding to a receptor protein, open or close ion channels or trigger cascades of a second messenger. Neuromodulators are chemical messengers that do not directly act on synapses, but alter (modify) the sensitivity of a neuron to synaptic stimulation or synaptic inhibition.
Some neuromodulators are neuropeptides or steroids and are produced in the nervous tissue, others are steroids circulating 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 ends 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 messengers, however, individual synapses transmit ionic signals through gap junctions that permeate the pre- and postsynaptic membranes, thereby providing direct transmission of neuronal signals.
These contacts are known as electrical synapses.
Presynaptic terminal always contains synaptic vesicles with neurotransmitters and numerous mitochondria.
Neurotransmitters usually synthesized in the cellular body; then they are stored in vesicles in the presynaptic part of the synapse. During transmission of a nerve impulse, they are released into the synaptic cleft through a process known as exocytosis.
5. Mechanism of information transfer in synapses
Endocytosis contributes to the return of excess membrane, which accumulates in the presynaptic part as a result of exocytosis of synaptic vesicles.
Returned membrane fuses with the agranular endoplasmic reticulum (aEPS) 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 there were acetylcholine and norepinephrine. The norepinephrine-releasing axonal terminal is shown in the figure.
Most neurotransmitters are amines, amino acids, or small peptides (neuropeptides). Some inorganic substances, such as nitric oxide, can also act as neurotransmitters. Certain peptides that act as neurotransmitters are used elsewhere in the body, such 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 in signal transmission at a chemical synapse
Phenomena during transmission signal at a chemical synapse are illustrated in the figure.
Nerve impulses that quickly (within milliseconds) run across the cell membrane cause explosive electrical activity (depolarization) that spreads across the cell membrane.
Such impulses briefly open calcium channels in the presynaptic region, providing calcium influx, which triggers exocytosis of synaptic vesicles.
In areas of exopitosis, neurotransmitters that 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 neurotransmitter-receptor interaction gives opposite effect- hyperpolarization occurs, and there is no transmission of a nerve impulse. These synapses are known as inhibitory synapses. Thus, synapses can either enhance or inhibit the transmission of impulses, thereby they are able to regulate neural activity.
After use neurotransmitters are quickly removed due to enzymatic destruction, diffusion or endocytosis mediated by specific receptors on the presynaptic membrane. This removal of neurotransmitters is of great functional importance because it prevents unwanted prolonged stimulation of the postsynaptic neuron.
Instructional video - synapse structure
- The body of the nerve cell - neuron: structure, histology
- Dendrites of nerve cells: structure, histology
- Axons of nerve cells: structure, histology
- Membrane potentials of nerve cells.
Physiology
- Synapse: structure, function
- Glial cells: oligodendrocytes, Schwann cells, astrocytes, ependyma cells
- Microglia: structure, histology
- Central nervous system (CNS): structure, histology
- Histology of the meninges. Structure
- Blood-brain barrier: structure, histology
Synapse structureLet us consider the structure of the synapse using the axosomatic one as an example. The synapse consists of three parts: the presynaptic terminal, the synaptic cleft, and the postsynaptic membrane (Fig. 9). The presynaptic ending is filled with vesicles (vesicles) and mitochondria. The vesicles contain biologically active substances - mediators. Mediators are synthesized in the soma and transported by microtubules to the presynaptic terminal. The most common mediators are adrenaline, norepinephrine, acetylcholine, serotonin, gamma-aminobutyric acid (GABA), glycine, and others. Usually a synapse contains one of the neurotransmitters in a larger amount than other neurotransmitters. By the type of mediator, it is customary to designate synapses: adrenergic, cholinergic, serotoninergic, etc. The synaptic cleft is filled with intercellular fluid, which contains enzymes that contribute to the destruction of mediators. In electrical synapses, the interaction of two neurons is carried out through biocurrents. Chemical synapseNerve fiber PD (PD - action potential) 9. Diagram of the structure of the synapse. chemical synapses predominate in the nervous system. The influence of excitatory and inhibitory synapses on the excitability of the postsynaptic neuron is summarized, 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 further away the synapses are from the axonal mound (for example, at the end of the dendrites), the less effective they are. Thus, the synapses located on the soma and axonal knoll affect the excitability of the neuron quickly and efficiently, and the effect of distant synapses is slow and smooth. Ampmsch iipinl system This neural network is called local. In addition, the network can include neurons that are distant from each other, from different areas of the brain. Most high level the organization of connections of neurons reflects the connection of several areas of the central nervous system. This neural network is called a path or system. Distinguish between descending and ascending paths. Information is transmitted along the ascending pathways from the lower regions of the brain to the higher ones (for example, from the spinal cord to the cerebral cortex). Descending pathways connect the cerebral cortex with the spinal cord. Some neural networks provide convergence (convergence) of impulses on a limited number of neurons. Nerve networks can also be built according to the type of divergence (divergence). Such networks provide for the transmission of information over long distances. In addition, neural networks provide integration (summation or generalization) of various kinds of information (Fig. 10). |
What is a synapse? A synapse is a special structure that provides signal transmission from fibers of a nerve cell to another cell or a fiber from a contact cell. What requires the presence of 2 nerve cells. In this case, the synapse is represented in 3 functional areas (presynaptic fragment, synaptic cleft and postsynaptic fragment) of nerve cells and is located in the area where the cell contacts the muscles and glands of the human body.
The system of neural synapses is carried out according to their localization, the type of activity and the method of transit of the available signal data. With regard to localization, synapses are distinguished: neuroneuronal, neuromuscular... Neuroneuronal to axosomatic, dendrosomatic, axodendritic, axoaxonal.
By the type of activity for perception, it is customary to distinguish synapses: excitatory and equally important inhibitory ones. With regard to the information signal transit method, they are classified into:
- Electric type.
- Chemical type.
- Mixed type.
Etiology of neuronal contact comes down to the type of this docking, which can be distant, contact, as well as borderline. The connection of the distant property is carried out by means of 2 neurons located in many parts of the body.
So, in the tissues of the human brain, neurohormones and neuropeptide substances are generated that affect the neurons present in the body of another location. The contact connection is reduced to special docking of membrane films of typical neurons that make up chemical synapses, as well as those that make up electrical properties.
Adjacent (borderline) work of neurons is performed during the time during which the films-membranes of neurons are blocked only by the synaptic cleft. As a rule, such a fusion is observed if between 2 special membrane films no glial tissue... This contiguity is characteristic of parallel fibers of the cerebellum, axons of a special nerve for olfactory purposes, and so on.
There is an opinion that adjacent contact provokes the work of adjacent neurons in the product of a common function. This is due to the fact that the metabolites, the fruits of the action of the human neuron, penetrating into the cavity located between the cells, affect the close-localized active neurons. Moreover, the boundary connection can often transmit data of an electrical nature from 1 working neuron to 2 participants in the process.
Electrical and chemical synapses
The action of film-membrane fusion is considered to be electrical synapses... In conditions when the necessary synaptic cleft is intermittent with intervals of monolithic septa. These partitions form an alternating structure of the synapse compartments, while the compartments are separated by fragments of approximate membranes, the gap between which in the synapses of the usual warehouse is 0.15-0.20 nm in representatives of mammals. At the junction of the films-membranes, there are pathways through which part of the fruits are exchanged.
In addition to separate types of synapses, there are necessary electrical typical synapses in the form of a single synaptic cleft, the total perimeter of which extends over 1000 microns. So, a similar synaptic phenomenon is presented in the neurons of the ciliary ganglion.
Electrical synapses are capable of conducting high-quality excitation unilaterally. This fact is noted when fixing the electrical reserve of the synaptic component. For example, at the moment when the afferent tubules are touched, the synaptic membrane-membrane is depolarized, when, when the efferent particles of the fibers are touched, it hyperpolarizes. It is believed that synapses of acting neurons with common responsibilities can carry out the required excitation (between 2 skipping sections) in both directions.
On the contrary, the synapses of the neurons present with a different list of actions (motor and sensory) carry out the act of excitement unilaterally... The main work of synaptic components is due to the production of urgent reactions of the body. The electrical synapse is subject to an insignificant proportion of fatigue, has a significant percentage of resistance to internal-external factors.
Chemical synapses have the appearance of a presynaptic segment, a functional synaptic cleft with a fragment of the postsynaptic component. The presynaptic fragment is formed by an increase in the size of an axon within its own tubule or towards its completion. This fragment contains granular as well as agranular special sacs containing a mediator.
Presynaptic augmentation observes the localization of active mitochondria, generating particles of the substance-glycogen, as well as required mediator production and other. In conditions of frequent contact with the presynaptic field, the reserve of the mediator in the existing sacs is lost.
It is believed that small granular vesicles have a substance such as norepinephrine, and large ones - catecholamines. Moreover, in the agranular cavities (vesicles), acetylchonin is located. In addition, substances formed according to the type of produced aspartic or no less significant glutamine acid are considered to be mediators of increased excitement.
Active synapse contacts are often located between:
- Dendrite and axon.
- Catfish and axon.
- Dendrites.
- Axons.
- Soma cells and dendrites.
Influence of the produced mediator relative to the postsynaptic film-membrane present is due to the excessive penetration of its sodium particles. The generation of powerful outpourings of sodium particles from the working synaptic cleft through the postsynaptic film-membrane forms its depolarization, forming the excitation of the postsynaptic reserve. The transit of the chemical direction of the synapse data is characterized by synaptic suspension of excitation for a time equal to 0.5 ms with the development of postsynaptic reserve, as a reaction to the presynaptic flow.
This possibility at the moment of excitation is represented in the depolarization of the postsynaptic film-membrane, and at the moment of suspension in its hyperpolarization. Because of what the suspended postsynaptic reserve... As a rule, during strong arousal, the level of permeability of the postsynaptic film-membrane increases.
The required excitatory property is fixed inside neurons if norepinephrine, a dopamine substance, acetylcholine, important serotonin, substance P and glutamine acid work in typical synapses.
The restraining potential is formed during the influence on synapses from gamma-aminobutyric acid and glycine.
Mental performance of children
A person's working capacity directly determines his age, when all values increase simultaneously with the development and physical growth of children.
The accuracy and speed of mental actions with age is carried out unevenly, depending on other factors that fix the development and physical growth of the body. Students of any age who health abnormalities are present, the working capacity of a low value relative to the surrounding strong children is characteristic.
In healthy first-graders with a reduced readiness of the body for a constant learning process, according to some indicators, the ability to act is low, which complicates the struggle with emerging problems in the learning process.
The speed of onset of weakening is determined by the initial state of the child's system of sensitive nervous genesis, the working pace and the volume of the load. At the same time, children are prone to overwork during prolonged immobility and when the child is not interested in the performed actions. After the break, the working capacity becomes the same or becomes higher than the previous one, and it is better to do the rest not passive, but active, switching to an activity different from this.
First part educational process in ordinary children of primary grades, it is accompanied by excellent working capacity, but by the end of 3 lessons they have there is a decrease in concentration of attention:
- They look out the window.
- They listen inattentively to the teacher's words.
- Change the position of their body.
- They start talking.
- Get up from their place.
The values of working capacity are especially high among senior pupils studying in the second shift. It is especially important to pay attention to the fact that a sufficiently short time for preparation for classes before the time of the beginning of the educational activity in the classroom does not guarantee complete disposal of harmful changes in the central nervous system. Mental activity is quickly depleted in the first hours of lessons, which is clearly noted in negative behavior.
Therefore, qualitative shifts in working capacity are observed in students of the junior block in lessons from 1 - 3, and in blocks of the middle-senior level in 4 - 5 lessons. In turn, 6th lesson takes place in conditions of a particularly reduced ability to act. At the same time, the duration of the lesson for 2-11 graders is 45 minutes, which weakens the condition of the children. Therefore, it is recommended to periodically change the type of work, and take an active pause in the middle of the lesson.
Chemical synapses.
The chemical synapse is characterized by:
1. Synaptic delay, lasting at least 0.5 s;
2. Lack of electric current from the pre- to postsynaptic membrane.
3. Postsynaptic potential as a result the functioning of the chemical synapse. The postsynaptic potential (PSP) is the target of the functioning of the chemical synapse and can be excitatory (EPSP) or inhibitory (PSP). The terms EPSP and TPSP are used more often to synapses formed by neurons on neurons. At the neuromuscular synapse, the target of the synaptic transmission is the formation of an action potential coupled with subsequent muscle contraction.
4. An increase in the conductivity of the postsynaptic membrane during the implementation of synapse functions (PSP in the form of TPSP or EPSP is caused by the movement of ions through ion channels in the membrane).
5. Synaptic vesicles, or vesicles present in the presynaptic endings, a specific staining characteristic of the postsynaptic membrane.
6. Dependence of the release process, or release of the mediator, on the entrance to the presynaptic ending of Ca ++ ions.
Excitatory chemical synapses
The chemical synapse is characterized by the presynaptic region, the synaptic cleft, and the postsynaptic region.
The synaptic cleft has a lumen of 20 to 50 nm at chemical synapses. In the presynaptic region, there are always vesicles containing mediator (transmitter, neurotransmitter, neurotransmitter) .
In the considered type of synapse, due to the high resistance of synaptic membranes and a wide synaptic cleft, the electrotonic potential and AP are not able to move to the postsynaptic region using the cable properties of the membrane. The transfer coefficient in this case is less than thousandths, and the extracellular shunt has a low resistance and "takes away" the charge. The axon transport mechanism (400 mm / day) moves enzyme systems and precursors for the synthesis of mediators and vesicles along the presynaptic fiber. In the synaptic terminal there is always a certain supply of a transmitter ready for secretion, packed in vesicles.
The synthesis of mediators is carried out using enzymes, for example, acetylcholine AX is synthesized by choline acetyltransferase, which transfers the acetyl group from acetyl coenzyme A to choline. Approximately 85% of the finished mediator is stored in vesicles. The process of synthesis and decay of AX occurs constantly.
The release of the mediator from the terminal also occurs continuously, this is the so-called non-quantum releasing, its intensity can exceed the effective, quantum one by tens of times, but it has no electrogenic consequences (it has a trophic effect on the object of innervation), and the ACh is destroyed without changing the permeability of the postsynaptic membrane.
The AX quantum output has electrically significant consequences. The initiation of quantum release is set by the arrival of an action potential along the axon, which depolarizes its membrane in the presynaptic terminal that has lost myelin, which leads to the opening of voltage-sensitive Ca ++ channels. Due to the high electrochemical and concentration gradient, Ca ++ ions enter the presynaptic terminal. Calcium is necessary so that vesicles with a mediator can connect to the outer membrane and release a portion (quantum) of the mediator into the synaptic cleft by exocytosis. At the same time, up to hundreds of vesicles can be emptied in a synapse. A quantum contains from 10 2 to 10 5 AX molecules.
The target of ACh in the cholinergic synapse is a complex protein molecule cholinergic receptor ... Cholinergic receptors sensitive to nicotine, belong to the type of H-cholinergic receptors, to muscarine- M-cholinergic receptors (metabotropic). H-cholinergic receptors are located (expressed) on the membranes of muscle fibers of skeletal muscles, neurons of the central nervous system and sympathetic ganglia.
H-cholinergic receptor, ionotropic , consists of 5 (sometimes 7) protein subunits, one of which is duplicated (bvbgd). The total size (11 × 8.5 nm) of the molecule is twice the membrane thickness. The amino acid sequence of proteins of all subunits was established; it turned out to be species-specific, although the differences in closely related animal species are insignificant. Duplicated b-subunits are ligand-sensitive. The cholinergic receptor can be considered as an ion channel, since, as an integral membrane protein, it penetrates the cell membrane and has a central pore. There are 2 known states of the cholinergic receptor molecule - closed and open. In the open state, the central pore of cholinergic receptors has a size of about 0.7 nm, which is sufficient for the penetration of monovalent cations, mainly Na + and K +, through it.
After binding of ACh to the H-cholinergic receptor and opening the pore, an ionic current flows through the postsynaptic membrane due to the movement of Na + and K + ions along electrochemical and concentration gradients. Since the gradient for sodium is directed inside the cell, and for potassium - outward, with their counter movement, the total current turns out to be able to locally displace the membrane potential to KUD in the neuromuscular synapse or cause significant depolarization of the neuron membrane in the neuro-neuronal synapse. The local response in the form of depolarization in this case is called PSP - postsynaptic potential, or EPSP, which excites postsynaptic potential. Previously, the term potential of the end plate (EPP) was often used for the neuromuscular synapse.
The local response in the form of EPSP obeys the laws of potential conduction through the membrane and can be extended over a short distance due to the limitations imposed by the capacitive and resistive properties of the membrane - constant time and constant length. Since there are many synapses on the membrane of a neuron or muscle fiber, the cell's response always consists of the activity of individual synaptic inputs.
The summation of EPPs leads to a state when the membrane potential is displaced by depolarization to KUD, the generation of AP occurs. Calcium enters the cell through voltage-gated calcium channels; it participates in the mechanism of muscle contraction.
After ACh has played the role of a signaling molecule and launched the conformation of the cholinergic receptor from a closed to an open state, it is necessary to prepare the system for receiving the next signal. Therefore, the postsynaptic membrane has a mechanism for inactivation of the mediator. In the cholinergic synapse, ACh inactivation is achieved by its enzymatic cleavage by acetylcholinesterase. In other types of synapses, inactivation takes place in a different way, for example, norepinephrine in the adrenergic synapse undergoes reverse uptake (capture) in the presynaptic terminal.
Acetylcholinesterase can be blocked, in which case the cholinergic receptor channels are constantly open and muscle control is impaired. Insecticidal preparations such as "Prima", "Diclofos" have such a principle of action, therefore, they are dangerous not only for household insect pests, but also for warm-blooded animals.
Stages of the functioning of chemical synaptic transmission
1. Synthesis, storage and transport of mediator in vesicles.
2. Secretion of a mediator during depolarization of the presynaptic membrane and the entry of calcium ions into the terminal.
3. Reaction of the postsynaptic membrane in the form of binding of a mediator to a receptor and a change in the permeability of the postsynaptic membrane for cations.
4. Generation of postsynaptic potentials.
5. Inactivation of the mediator.
Excitatory chemical synapses formed on neurons are very numerous, interspersed with inhibitory ones, and never ensure that the membrane reaches KUD alone. The neuron is capable integrate synaptic signals and output at the output, in the most excitable part of the cell, for example, if it is a motor neuron, in axonal mound, AP after the analysis of PSP received by synaptic inputs.
In neuro-neuronal synapses, not only ACh can be a mediator, most often the excitatory amino acids glutamate and aspartate, norepinephrine, neuropeptides, ATP and NO perform the functions of mediators.
Glutamate excitatory synaptic neurotransmission is most common in the central nervous system. The reception of glutamate in synapses is carried out by NMDA and AMPA (ionotropic) receptors, the synaptic mechanisms in them are very complex and not fully understood.
Due to the fact that the processes of release and destruction of a transmitter in synapses have long time implementation, there is a synaptic delay in the functioning of neural networks. Therefore, the chemical synapse is said to act as a frequency filter and has low lability.
Since signals from individual synapses can add up and determine the total charge of the membrane, the phenomena of tetanic synaptic relief and depression are possible.
Chemical synapse properties
1. Slow signal transmission rate, large synaptic delay.
2. Unilateral signal conduction from the pre- to the postsynaptic membrane, but not vice versa.
3. High transmission reliability under normal operating conditions.
4. The existence of trace processes (trace depolarization and hyperpolarization, which increases the possibilities of integrating signals by the neuron).
5. Chemical synapses by the nature of the neurotransmitter are divided into choline-ergic (mediator - acetylcholine), adrenergic (noradrenaline), dopaminergic (dopamine), GABA-ergic (y-aminobutyric acid), etc. The central nervous system has 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).
In the presynaptic ending there are synaptic vesicles (vesicles) with a diameter of about 40 nm, which are formed in the body of a neuron and, with the help of microtubules and microfilaments, are delivered to the presynthetic ending, where they are filled with a mediator and ATP. The mediator is formed at the very nerve endings. The presynaptic ending contains several thousand vesicles, each of which contains from 1 to 10 thousand molecules of a chemical that is involved in the transmission of influence through the synapse and is 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 different widths (20-50 nm), contains intercellular fluid and mucopolysaccharide dense
a substance in the form of stripes, bridges, which provides a connection between the pre- and postsynaptic membranes and may contain enzymes.
Postsynaptic membrane - This is a thickened part of the cell membrane of an innervated cell, containing protein receptors with ion channels and capable of binding mediator molecules. The postsynaptic membrane of the neuromuscular synapse is also called the end plate.
V.Mechanism of transmission of excitation in electric synapses is similar to that in a nerve fiber: PD, 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 central nervous system, but they have not been studied enough.
G.Signal transmission at chemical synapses. The action potential (AP), which arrives at the presynaptic end of a chemical synapse, causes depolarization of its membrane, which opens voltage-gated Ca-channels. Ca 2+ ions enter the nerve endings according to the electrochemical gradient "provide the release of the mediator into the synaptic cleft through exocytosis. Mediator molecules entering the synaptic cleft diffuse to the postsynaptic membrane and interact with its receptors. The action of the mediator molecules leads to the opening of ion channels and the movement of Na + and K + ions according to an 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 in the neuromuscular synapse is called the end plate potential (EPP) (Figure 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, as well as by reuptake by the nerve ending.
D.Characteristics of the conduction of excitation in chemical synapses.
1 . Unilateral conduct of arousal - from the presynaptic terminal towards the postsynaptic membrane. This is due to the fact that the mediator is released from the presynaptic terminal, and the receptors interacting with it are localized only on the postsynaptic membrane.
Slow propagation of excitation in synapses in comparison with the nerve fiber is explained by the fact that it takes time for the release of the mediator from the presynaptic terminal, the propagation 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 synaptic delay.
4. Synaptic depression (synapse fatigue) -
weakening of the cell's response to afferent impulses, expressing
reducing postsynaptic potentials during
irritation or after it. It is explained by the expense
mediator formation, accumulation of metabolites, acidification of the environment
with prolonged excitation for the same
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 contiguous pre- and postsynaptic membranes provides good electrical conductivity.
2.2. CHARACTERISTICS OF HORMONAL REGULATION
The reflex reaction may have a hormonal response, 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 way (the activity of the musculoskeletal system). If the hormonal link is turned on, then this is carried out due to the additional production of biologically active substances. For example, when strong stimuli (cold, heat, painful stimulus) act on exteroreceptors, a powerful flow of afferent impulses arises, entering the central nervous system, while additional amounts of adrenaline and adrenal cortex hormones are released into the blood, playing adaptive (protective) role.
Hormones (Greek pogtab - I excite) - biologically active substances produced by the 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, hypothalamic hormones (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, local hormones, paracrine factors - parahormones). The action of hormones or parahormones directly on neighboring cells, bypassing the blood, is called paracrine action. By place of action hormones are divided into target organs or other endocrine glands into two groups: 1) effector hormones, acting on effector cells (for example, insulin, which regulates metabolism in the body, increases the synthesis of glycogen in liver cells, increases the transport of glucose and other substances through the cell membrane, increases the intensity of protein synthesis); 2) triple hormones (pathways), 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 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 the development of the central nervous system suffers, therefore, and mental development).
1. Functional influence of hormones there are three types.
Starting influence - it 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) activates 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 biochemical processes in organs and tissues. For example, activation of oxidative processes by thyroxine, which can take place without it; stimulation of the activity of the heart with adrenaline, which passes without adrenaline. The modulating effect of hormones is also a change in the sensitivity of the tissue to the action of other hormones. For example, folliculin enhances the effect of progesterone on the uterine lining, thyroid hormones enhance the effects of catecholamines.
The permissive influence 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 realization 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 implemented by the breakdown of metabolic processes,
launched by means of cellular enzyme systems.
2.3. REGULATION WITH METABOLITES
AND TISSUE HORMONES.
MYOGENIC MECHANISM OF REGULATION.
REGULATING FUNCTION OF THE BBB
Metabolites - products formed in the body during the metabolic process as a result of various biochemical reactions. These are amino acids, nucleotides, coenzymes, carbonic acid, mo-
lactic, pyruvic, adenylic acids, ion shift, changes in pH. Regulation by metabolites on early stages phylogenesis was the only one. The metabolites of one cell directly influenced another, neighboring cell or a 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 transferred to other cells of the body with a moving hemolymph over long distances, and this became faster. Then the nervous system appeared as a regulatory system, and even later - the endocrine glands. Although metabolites act mainly as local regulators, they can also influence to other bodies and tissue, on the activity of nerve centers. For example, the accumulation of carbonic acid in the blood leads to the excitation of the respiratory center and increased respiration. An example of local humoral regulation is hyperemia of an intensively working skeletal muscle - accumulating metabolites ensure 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, serotonigg), prostaglandins and kinins. They occupy an intermediate position between hormones and metabolites as humoral factors of regulation. 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 called modul tori regulatory signals - they have a modulating effect. Tissue hormones are produced by non-specialized cells, but they act through specialized cellular receptors, for example, two types of receptors have been found for histamine - H (and H 2. Since tissue hormones affect the permeability cell membranes, they regulate the entry into the cell and the exit from the cell of various substances and ions that determine the membrane potential, and hence the development of the action potential.
Myogenic regulation mechanism. With the development of the muscular system in the process of evolution, the myogenic mechanism of regulation of functions is gradually becoming more and more noticeable. The human body is approximately 50% muscle. This is skeletal muscle-
ra (40% of body weight), heart muscle, smooth muscles of the blood and lymphatic vessels, walls of the gastrointestinal tract, gallbladder, bladder and other internal organs.
The essence of the myogenic regulation mechanism is that preliminary moderate stretching of the skeletal or cardiac muscle increases the strength of their contractions. The contractile activity of the 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 the initial 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 them (for example, the muscles of the intestines, blood vessels). The impulses coming to them along the autonomic nerves have a modulating effect - they increase or decrease the tone of smooth muscle fibers.
The 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 fulfills special structure of the walls of the capillaries of the brain. Their endothelium has very few pores, and the narrow gap junctions between cells contain almost no windows. Glial cells are also an integral part of the barrier, which form a kind of sheaths 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 colleagues. This barrier allows water, ions, glucose, amino acids, gases to pass through, trapping 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 in the blood and other substances that do not penetrate the barrier; brain cells also secrete their own neurosecrets. Areas of the brain that do not have their own blood-brain barrier are the pituitary gland, pineal gland, some parts of the hypothalamus and medulla oblongata.
The BBB also performs a protective function - prevents the ingress of microbes, foreign or toxic substances of exo- 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
Maintaining the indicators of the internal environment of the body is carried out by regulating 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.
A.The structure of the functional system. A functional system is a dynamic set 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 - the nerve center, representing the union of the nuclei of different levels of the central nervous system; 2) him weekend channels(nerves and hormones); 3) executive bodies - effectivery, providing in the course of physiological activity support regulated process(indicator) at some optimal level (useful result of the activity of the functional system); 4) result receptors(sensory receptors) - sensors that perceive information about the parameters of the deviation of the controlled process (indicator) from the optimal level; 5) channel feedback (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 the effector efforts to maintain the controlled process (indicator) at the optimal level ( fig. 2.7).
Afferent impulses from the receptors of the result through the feedback channels enter the nerve center, which regulates this or that indicator, the center provides a change in the intensity of the work of the corresponding organ.
When the intensity of the effector changes, the metabolic rate, which also plays an important role in the regulation of the activity of the organs of a particular functional system (humoral regulation process).
B.Multiparameter principle of interaction of various functional systems - the principle that determines the generalized activity of functional systems (K.V. Sudakov). The relative stability of indicators of the internal environment of the body is the result of the coordinated activities of many
functional systems. It turned out that various indicators of the internal environment of the body are interconnected. For example, an excessive intake of water 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 blood plasma. In a functional system that maintains an optimal level of blood gas composition, 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.
V. Systemogenesis. According to P.K.Anokhin, systemogenesis -selective maturation and development of functional systems in ante- and postnatal ontogenesis. Currently, the term "system genesis" is used in a broader sense, while under the system genesis is understood not only the processes of ontogenetic maturation of functional systems, but also the formation and transformation of functional systems during the life of the organism.
Backbone factors a functional system of any level is an adaptive result useful for the life of the body, necessary at a given moment, and the resulting motivation. For example, to perform a high jump with a pole, the muscles of the upper
their 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 embedded in different time and ripen at different rates. So, the nerve center is grouped and matures usually earlier than the substrate innervated by it is laid and matures. In ontogenesis, first of all, those functional systems ripen, without which it is impossible further development organism. For example, of the three functional systems associated with the oral cavity, after birth, only a functional sucking system is formed, later a functional chewing system is formed, then a functional speech system.
Consolidation of functional system components - combining into a functional system of individual fragments developing in different parts organism. Consolidation of fragments of a functional system is a critical point development of its physiological architecture. The leading role in this process is played by the central nervous system. 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 on the basis of improving connections between different 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 FUNCTIONS OF THE BODY
1. Deviation regulation - a cyclical mechanism, in which any deviation from the optimal level of the regulated indicator mobilizes all apparatuses of the functional system to restore it to the previous level. Regulation by deviation assumes the presence of a channel in the system complex negative feedback, providing multidirectional influence: strengthening of incentive control mechanisms in case of weakening of process indicators or weakening of 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 a low blood pressure, opposite reactions are turned on. Unlike negative feedback, positive
Feedback, occurring in the body rarely, it has only a unidirectional, amplifying 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 blood pressure were regulated according to the principle of positive feedback, in the case of a decrease in blood 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 a positive feedback is an increase in the onset of 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. Advance regulation lies in the fact that the regulatory mechanisms are turned 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 control over the temperature of the interior of the body. Skin thermoreceptors mainly play the role of ambient temperature detectors. With significant deviations in the ambient temperature, the prerequisites for a possible change in the temperature of the internal environment of the body are created. However, normally this does not happen, since the impulse from the skin thermoreceptors, continuously entering the hypothalamic thermoregulatory center, allows it to make changes in the work of the effectors of the system until the moment of real change in the temperature of the internal environment of the body. Strengthening ventilation of the lungs with physical activity begins earlier than the increase in oxygen consumption and the accumulation of carbonic acid in the human blood. This is done due to afferent impulses from the proprioceptors of actively working muscles. Consequently, the impulse of proprioceptors acts as a factor organizing the restructuring of the functioning of the functional system, which maintains the level of Р 02, Рсс, 2 and the pH of the internal environment, optimal for metabolism, ahead of time.
Advance regulation can be implemented using the mechanism conditioned reflex. It is shown that the conductors of commodity trains in winter time The production of heat sharply increases with distance from the departure station, where the conductor was in a warm room. On the way back as you get closer
physical
How to understand: will the kitten be fluffy?
What light alcohol can pregnant women drink: consequences of consumption
Why do legs swell in the ankles and ankles of feet in pregnant women: causes and methods of treatment
The wedding of Prince Harry and Meghan Markle: scandalous and secret details of the marriage (photo) Future marriage of Prince Harry year NTV
How to close white plums for the winter