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 nervous system- 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 drug spinal cord: synapse between the terminal 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 small molecules(for example, 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.
V) 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 - specialized structures that provide the transfer of excitation from one excitable cell to another. The concept of SINAPSE was introduced into physiology by C. Sherrington (connection, contact). The synapse provides functional communication between individual cells. They are divided into neuronerve, neuromuscular and synapses of nerve cells with secretory cells (neuro-glandular). There are three functional divisions in a neuron: soma, dendrite, and axon. Therefore, there are all possible combinations of contacts between neurons. For example, axo-axonal, axo-somatic and axo-dendritic.
Classification.
1) by location and belonging to the relevant structures:
- peripheral(neuromuscular, neurosecretory, receptor-neuronal);
- central(axo-somatic, axo-dendritic, axo-axonal, somato-dendritic, somato-somatic);
2) mechanism of action - excitatory and inhibitory;
3) to a way of transmission of signals - chemical, electrical, mixed.
4) chemical are classified according to the mediator, with the help of which the transfer is carried out - cholinergic, adrenergic, serotonergic, glycinergic. etc.
Synapse structure.
The synapse consists of the following main elements:
Presynaptic membrane (in the neuromuscular synapse - this is the end plate):
postsynaptic membrane;
synaptic cleft. The synaptic cleft is filled with oligosaccharide-containing connective tissue, which plays the role of a supporting structure for both contacting cells.
The system of synthesis and release of the mediator.
its inactivation system.
In the neuromuscular synapse, the presynaptic membrane is part of the membrane of the nerve ending in the area of its contact with the muscle fiber, the postsynaptic membrane is part of the membrane of the muscle fiber.
The structure of the neuromuscular synapse.
1 - myelinated nerve fiber;
2 - nerve ending with mediator vesicles;
3 - subsynaptic membrane of the muscle fiber;
4 - synaptic cleft;
5-postsynaptic membrane of the muscle fiber;
6 - myofibrils;
7 - sarcoplasm;
8 - nerve fiber action potential;
9 - end plate potential (EPSP):
10 - the action potential of the muscle fiber.
The part of the postsynaptic membrane that is opposite the presynaptic is called the subsynaptic membrane. A feature of the subsynaptic membrane is the presence in it of special receptors that are sensitive to a certain mediator and the presence of chemodependent channels. In the postsynaptic membrane, outside the subsynaptic, there are voltage-gated channels.
The mechanism of excitation transmission in chemical excitatory synapses. In 1936, Dale proved that when a motor nerve is stimulated, acetylcholine is released in the skeletal muscle at its endings. In synapses with chemical transmission, excitation is transmitted with the help of mediators (intermediaries). Mediators are chemical substances that ensure the transmission of excitation in synapses. The mediator in the neuromuscular synapse is acetylcholine, in excitatory and inhibitory neuronerve synapses - acetylcholine, catecholamines - adrenaline, norepinephrine, dopamine; serotonin; neutral amino acids - glutamine, aspartic; acidic amino acids - glycine, gamma-aminobutyric acid; polypeptides: substance P, enkephalin, somatostatin; other substances: ATP, histamine, prostaglandins.
Mediators, depending on their nature, are divided into several groups:
Monoamines (acetylcholine, dopamine, norepinephrine, serotonin.);
Amino acids (gamma-aminobutyric acid - GABA, glutamic acid, glycine, etc.);
Neuropeptides (substance P, endorphins, neurotensin, ACTH, angiotensin, vasopressin, somatostatin, etc.).
The accumulation of the mediator in the presynaptic formation occurs due to its transport from the perinuclear region of the neuron with the help of a fast axstock; synthesis of a mediator occurring in synaptic terminals from its cleavage products; reuptake of the neurotransmitter from the synaptic cleft.
The presynaptic nerve ending contains structures for neurotransmitter synthesis. After synthesis, the neurotransmitter is packaged into vesicles. When stimulated, these synaptic vesicles fuse with the presynaptic membrane and the neurotransmitter is released into the synaptic cleft. It diffuses to the postsynaptic membrane and binds there to a specific receptor. As a result of the formation of the neurotransmitter-receptor complex, the postsynaptic membrane becomes permeable to cations and depolarizes. This results in an excitatory postsynaptic potential and then an action potential. The mediator is synthesized in the presynaptic terminal from the material supplied here by axonal transport. The mediator is "inactivated", i.e. is either cleaved or removed from the synaptic cleft by a reverse transport mechanism to the presynaptic terminal.
The value of calcium ions in the secretion of the mediator.
The secretion of the mediator is impossible without the participation of calcium ions in this process. Upon depolarization of the presynaptic membrane, calcium enters the presynaptic terminal through specific voltage-gated calcium channels in this membrane. The concentration of calcium in the axoplasm is 110 -7 M, with the entry of calcium and increasing its concentration to 110 - 4 M mediator secretion occurs. The concentration of calcium in the axoplasm after the end of excitation is reduced by the work of systems: active transport from the terminal, absorption by mitochondria, binding by intracellular buffer systems. At rest, irregular emptying of the vesicles occurs, with the release of not only single molecules of the mediator, but also the release of portions, quanta of the mediator. Quantum of acetylcholine includes approximately 10,000 molecules.
Federal Agency for Education
State educational institution
higher professional education
“Ryazan State University named after S.A. Yesenin"
Institute of Psychology, Pedagogy and Social Work
Control work on the discipline "Neurophysiology and the basics of GNI"
on the topic: “The concept of a synapse, the structure of a synapse.
Transmission of excitation in the synapse
Completed by student 13L group
1st year OZO (3) A.I. Sharova
Checked:
professor of medical sciences
O.A. Belova
Ryazan 2010
1. Introduction……………………………………………………………..3
2. Structure and functions of the synapse……………………………………...6
3. Transmission of excitation in the synapse………………………………….8
4. Chemical synapse……………………………………………………9
5. Isolation of mediator………………………………………………...10
6. Chemical mediators and their types………………………………..12
7. Conclusion…………………………………………………………… 15
8. References………………………………………………....17
Introduction.
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 supersystem 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. The transition (transmission) of excitation from the nerve fiber to the cell innervated by it (nerve, muscle, secretory) is carried out through a specialized formation, which is called the synapse.
Structure and functions of the 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 other nerve cell occurs through intercellular structural formations - synapses (from the 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 more more number components of the 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.
Synapse - is a complex structural formation, consisting of
presynaptic membrane - an electrogenic membrane in the axon terminal that forms a synapse on a muscle cell (most often this is the terminal branching of the axon)
postsynaptic membrane - an electrogenic membrane of an innervated cell on which a synapse is formed (most often this is a section of the body membrane or dendrite of another neuron)
synaptic cleft - the space between the presynaptic and postsynaptic membranes, filled with fluid, which in composition resembles blood plasma
Synapses can be between two neurons (interneuronal), between a neuron and a muscle fiber (neuromuscular), between receptor formations and processes of sensory neurons (receptor-neuronal), between neuron processes and other cells ( glandular).
There are several classifications of synapses.
1. By localization:
1) central synapses;
2) peripheral synapses.
Central synapses lie within the central nervous system and are also located in the ganglia of the autonomic nervous system.
central synapses- these are contacts between two nerve cells, and these contacts are heterogeneous and, depending on which structure the first neuron forms a synapse with the second neuron, they distinguish:
a) axosomatic, formed by the axon of one neuron and the body of another neuron;
b) axodendritic, formed by the axon of one neuron and the dendrite of another;
c) axoaxonal (the axon of the first neuron forms a synapse on the axon of the second neuron);
d) dendrodentritic (the dendrite of the first neuron forms a synapse on the dendrite of the second neuron).
There are several types peripheral synapses:
a) myoneural (neuromuscular), formed by the axon of a motor neuron and a muscle cell;
b) neuro-epithelial, formed by the axon of the neuron and the secretory cell.
2. Functional classification of synapses:
1) excitatory synapses;
2) inhibitory synapses.
synapse excitatory- a synapse in which the postsynaptic membrane is excited; an excitatory postsynaptic potential arises in it and the excitation that has come to the synapse spreads further.
synapse inhibitory- A. A synapse, on the postsynaptic membrane of which an inhibitory postsynaptic potential arises, and the excitation that has come to the synapse does not spread further; B. excitatory axo-axonal synapse, causing presynaptic inhibition.
3. According to the mechanisms of excitation transmission in synapses:
1) chemical;
2) electrical;
3) mixed
Peculiarity chemical synapses is that the transfer of excitation is carried out with the help of a special group of chemicals - mediators. It is more specialized than the electrical synapse.
There are several types chemical synapses, depending on the nature of the mediator:
a) cholinergic.
b) adrenergic.
c) dopaminergic. They transmit excitation with the help of dopamine;
d) histaminergic. In them, the transfer of excitation occurs with the help of histamine;
e) GABAergic. In them, excitation is transferred with the help of gamma-aminobutyric acid, i.e., the process of inhibition develops.
Synapse adrenergic - synapse, the mediator in which is norepinephrine. It is the transfer of excitation with the help of three catecholamines; distinguish a1-, b1-, and b2 - adrenergic synapses. They form neuroorgan synapses of the sympathetic nervous system and synapses of the central nervous system. Excitation of a-adrenergic synapses causes vasoconstriction, uterine contraction; b1- adrenoreactive synapses - strengthening the work of the heart; b2 - adrenoreactive - bronchial dilatation.
Synapse cholinergic - its mediator is acetylcholine. They are divided into n-cholinergic and m-cholinergic synapses.
In m-cholinergic At the synapse, the postsynaptic membrane is sensitive to muscarine. These synapses form neuroorgan synapses of the parasympathetic system and synapses of the central nervous system.
In n-cholinergic synapse, the postsynaptic membrane is sensitive to nicotine. This type of synapses is formed by neuromuscular synapses of the somatic nervous system, ganglionic synapses, synapses of the sympathetic and parasympathetic nervous systems, synapses of the central nervous system.
Synapse electrical- in it, excitation from the pre- to the postsynaptic membrane is transmitted electrically, i.e. epaptic transmission of excitation occurs - the action potential reaches the presynaptic ending and then spreads through the intercellular channels, causing depolarization of the postsynaptic membrane. In an electrical synapse, the mediator is not produced, the synaptic cleft is small (2 - 4 nm) and it contains protein bridges-channels, 1 - 2 nm wide, along which ions and small molecules move. This contributes to the low resistance of the postsynaptic membrane. This type of synapses is much less common than chemical synapses and differs from them in a higher rate of transmission of excitation, high reliability, and the possibility of two-way conduction of excitation.
Synapses have a number of physiological properties :
1) valvular property of synapses, i.e., the ability to transmit excitation in only one direction from the presynaptic membrane to the postsynaptic one;
2) synaptic delay property associated with the fact that the rate of transmission of excitation is reduced;
3) potentiation property(each subsequent impulse will be conducted with a smaller postsynaptic delay). This is due to the fact that the mediator from the conduction of the previous impulse remains on the presynaptic and postsynaptic membrane;
4) low synapse lability(100–150 pulses per second).
Transmission of excitation in the synapse.
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 recording of the synaptic potential of CNS neurons, led to the conclusion about the chemical nature of transmission in the interneuronal synapses of the spinal cord.
Microelectrode research recent years showed that in certain interneuronal synapses there is an electrical transmission mechanism. 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.
If electrical synapses are characteristic of the nervous system of more primitive animals (nervous diffusion system of coelenterates, some synapses of cancer and annelids, synapses of the nervous system of fish), although they are found in the brain of mammals. In all the above cases, impulses are transmitted through depolarizing the action of an electric current that is generated in the presynaptic element. I would also like to note that in the case of electrical synapses, the transmission of impulses is possible both in one and in two directions. Also, in lower animals, contact between presynaptic And postsynaptic element is carried out through only one synapse - monosynaptic form of communication, however, in the process of phylogenesis, there is a transition to polysynaptic form of communication, that is, when the above contact is made through a greater number of synapses.
However, in this work, I would like to dwell in more detail on synapses with a chemical transmission mechanism, which make up a large part of the synaptic apparatus of the CNS of higher animals and humans. Thus, chemical synapses, in my opinion, are of particular interest, since they provide very complex cell interactions, and are also associated with a number of pathological processes and change their properties under the influence of certain drugs.
Synapse is a structural and functional formation that provides transmission
chu excitation from a neuron to the cell innervated by it (nervous, glandular, muscle-
nuyu). Synapses can be divided into the following types:
1) according to the method of excitation transfer - electrical, chemical;
2) by localization - central, peripheral;
3) by functional feature – excitatory, inhibitory;
4) according to the structural and functional features of the postsynaptic receptors
membranes - cholinergic, adrenergic, serotonergic, etc..
2. The structure of the myoneural synapse
The myoneural synapse consists of:
a) presynaptic membrane;
b) postsynaptic membrane;
c) synaptic cleft.
The presynaptic membrane is the electrogenic membrane of the presynaptic
sky terminals (nerve fiber endings). in presynaptic terminals
mediators (transmitters) are formed and accumulate in vesicles (vesicles)
acetylcholine, norepinephrine, histamine, serotonin, gamma-aminobutyric acid
and others.
The postsynaptic membrane is part of the innervated cell membrane.
ki, in which chemosensitive ion channels are located. In addition, on
postsynaptic membrane localized receptors for a particular mediato-
ru and enzymes that destroy them, for example, cholinergic receptors and cholinesterase.
Synaptic cleft - filled with intercellular fluid, located
between the pre- and postsynaptic membranes.
3. Mechanism of conducting excitation through the myoneural synapse
The myoneural synapse is formed by the axon of a motor neuron on the striated
muscle fibre. Excitation through the myoneural synapse is transmitted with the help of
acetylcholine. Under the influence of nerve impulses, the presynaptic membrane depolarizes
zuyutsya. Acetylcholine is released from the vesicles and enters the synaptic cleft.
The release of the mediator occurs in portions - quanta. Acetylcholine diffuses
through the synaptic cleft to the postsynaptic membrane. On the postsynaptic mem-
Brane mediator interacts with the cholinergic receptor. As a result, it increases
permeability to sodium and potassium ions and end plate potential occurs
(EPP) or excitatory postsynaptic potential (EPSP). According to the mechanism of the circle
output currents under its influence, an action potential arises in sections of the muscle membrane
leg fiber adjacent to the postsynaptic membrane.
The connection of acetylcholine with the cholinergic receptor is fragile. The mediator is destroyed by the holi-
nesterase. The electrical state of the postsynaptic membrane is then restored
pours.
4. Physiological properties synapses
Synapses have the following physiological properties:
a) unilateral conduction of excitation (valve property) - due to
structural features of the synapse;
b) synaptic delay - due to the fact that it takes a certain time to
conduction of excitation through the synapse;
c) potentiation (facilitation) of subsequent nerve impulses -
occurs because more metal is released for each subsequent impulse.
d) low lability - due to the peculiarities of exchange and physical
chemical processes;
e) relatively easy onset of braking and fast development tired
nia - due to low lability.
f) desensitization - a decrease in the sensitivity of the cholinergic receptor to acetylcholine
The spinal cord, features of its structure. Types of neurons. Functional difference between the anterior and posterior roots of the spinal cord. Bell-Magendie law. Physiological significance of the spinal cord. "Laws" of the reflex activity of the spinal cord.
The spinal cord contains: 1. motor neurons(effector, motor nervous
cells, out of 3%), 2. intercalary neurons(interneurons, intermediate, 97% of them).
Motor neurons are divided into three types:
1) α - motor neurons, innervate skeletal muscles;
2) γ - motor neurons, innervate muscle proprioceptors;
3) neurons of the autonomic nervous system, the axons of which innervate the nerve
nye cells located in the vegetative ganglia, and through them internal
organs, vessels and glands.
2. Functional significance of the anterior and posterior roots of the spinal cord
(Bella-Magendie law)
Bell-Magendie's law: "All afferent nerve impulses enter the spinal
noah brain through the posterior roots (sensitive), and all efferent nerve impulses
leave (exit) the spinal cord through the anterior (motor) roots.
3. Functions of the spinal cord
The spinal cord performs two functions: 1) reflex, 2) conductive.
Due to the reflex activity of the spinal cord, a number of simple and
complex unconditioned reflexes. Simple reflexes have two-neuron reflex
nye arcs, complex - three or more neural reflex arcs.
The reflex activity of the spinal cord can be studied in "spinal animals"
nyh" - animals in which the brain is removed and the spinal cord is preserved.
4. Nerve centers of the spinal cord.
The lumbosacral region of the spinal cord contains: 1. center of urination
nia, 2. defecation center, 3. reflex centers of sexual activity.
In the lateral horns of the thoracic and lumbar spinal cord are:
1) spinal vasomotor centers, 2) spinal sweat centers.
In the anterior horns of the spinal cord are located at different levels movement centers
breath reflexes(centers of extero- and proprioceptive reflexes).
5. Pathways of the spinal cord
There are the following pathways of the spinal cord: 1) ascending(affe-
rental) and 2) descending(efferent).
Ascending pathways connect the body's receptors (proprio-, tactile, pain-
vye) with different parts of the brain.
Descending pathways of the spinal cord: 1) pyramidal, 2) extrapyramidal. Pira-
mid way - from the neurons of the anterior central gyrus of the cerebral cortex to
spinal cord is not interrupted. Extrapyramidal path - also starts from the neuro-
new anterior central gyrus and ends in the spinal cord. This path is many
neural, it is interrupted in: 1) subcortical nuclei; 2) diencephalon;
3) midbrain; 4) medulla oblongata.
regulation of vascular tone. Local regulation (autoregulation). Nervous regulation vascular tone (vasoconstrictor and vasodilator nerves). Humoral regulation of vascular tone. Indicators of blood pressure in children.
There are two types of vascular tone:
Basal (myogenic);
Neurogenic.
Basal tone.
If the vessel is denervated and the sources of humoral influences are eliminated, the basal vascular tone can be detected.
Distinguish:
A) electrogenic component- due to spontaneous electrical activity of the myocytes of the vascular wall. The greatest automation is in precapillary sphincters and arterioles;
b) non-electrogenic component (plastic)- due to stretching of the muscle wall due to blood pressure on it.
Shown, that The automation of smooth muscle cells is enhanced under the influence of their stretching. Their mechanical (contractile) activity also increases (i.e., there is a positive Feedback: between BP and vascular tone).
Local humoral regulation.
1. Vasodilators:
A) non-specific metabolites - are continuously formed in the tissues, and at the place of formation they always prevent vasoconstriction, and also cause their expansion (metabolic regulation).
These include - CO2, carbonic acid, H+, lactic acid, acidification (accumulation of acidic products), decrease in O2 tension, increase in osmotic pressure due to accumulation of low molecular weight products, nitric oxide (N0) (an incretion product of vascular endothelium).
b) BAS (when acting at the site of release) - are formed by specialized cells that are part of the vascular environment.
1. Vasodilator biologically active substances (at the site of release) -
acetylcholine, histamine, bradykinin, some prostaglandins, prostacyclin secreted by the endothelium can mediate their effect through nitric oxide.
2. Vasoconstrictive biologically active substances (when acting at the site of release) - are formed by specialized cells that are part of the vascular environment - catecholamines, serotonin, some prostaglandins, endothelial 1-peptide, 21-amino acid, a product of vascular endothelial incretion, as well as thromboxane A2, secreted by platelets during aggregation.
The role of biologically active substances in the distant regulation of vascular tone.
Along with nervous influences An important role in the regulation of vascular tone is played by various biologically active substances that have a distant, vasomotor effect:
Hormones (vasopressin, adrenaline); parahormones (serotonin, bradykinin, angiotensin, histamine, opiate peptides), endorphins and enkephalins.
Basically, these biologically active substances have a direct effect, since the majority of smooth muscle vessels have specific receptors for these biologically active substances.
Some biologically active substances cause an increase in vascular tone, while others reduce it.
Functions of the endothelium of small blood vessels and their role in the regulation of hemodynamic processes, hemostasis, immunity:
1. Self-sustaining structure (self-regulation of cell growth and recovery).
2. Formation of vasoactive substances, as well as activation and inactivation of biologically active substances circulating in the blood.
3. Local regulation of smooth muscle tone: synthesis and secretion of prostaglandins, prostacyclin, endothelins and NO.
4. Transmission of vasomotor signals from capillaries and arterioles to larger vessels (creator connections).
5. Maintaining the anticoagulant properties of the surface (releasing substances that prevent various types of hemostasis, ensuring the mirror surface, its non-wetting).
6. Implementation of protective (phagocytosis) and immune (binding of immune complexes) reactions.
7. Formation of vasoactive substances, as well as activation and inactivation of biologically active substances circulating in the blood.
8. Local regulation of smooth muscle tone: synthesis and secretion of prostaglandins, prostacyclin, endothelins and NO.
9. Transmission of vasomotor signals from capillaries and arterioles to larger vessels (creator connections).
10. Maintaining the anticoagulant properties of the surface (releasing substances that prevent various types of hemostasis, ensuring the mirror surface, its non-wetting).
11. Implementation of protective (phagocytosis) and immune (binding of immune complexes) reactions.
Neurogenic tone is due to activity vasomotor center(SDC) in the medulla oblongata, at the bottom of the IV ventricle (V.F. Ovsyannikov, 1871, discovered by cutting the brain stem at various levels), represented by two departments(pressor and depressor).
A synapse is a certain contact zone of the processes of nerve cells and other non-excitable and excitable cells that provide the transmission of an information signal. The synapse is morphologically formed by contacting membranes of 2 cells. The membrane related to the process is called the presynaptic membrane of the cell into which the signal enters, its second name is postsynaptic. Together with belonging to the postsynaptic membrane, the synapse can be interneuronal, neuromuscular and neurosecretory. The word synapse was introduced in 1897 by Charles Sherrington (English physiologist).
What is a synapse?
A synapse is a special structure that ensures the transmission of a nerve impulse from a nerve fiber to another nerve fiber or nerve cell, and in order for the nerve fiber to be affected by a receptor cell (the area where nerve cells and another nerve fiber come into contact with each other), two nerve cells are required.
A synapse is a small section at the end of a neuron. With its help, information is transmitted from the first neuron to the second. The synapse is located in three areas of nerve cells. Also, synapses are located in the place where the nerve cell comes into contact with various glands or muscles of the body.
What is a synapse made of?
The structure of the synapse has a simple scheme. It is formed from 3 parts, in each of which certain functions are carried out during the transmission of information. Thus, such a structure of the synapse can be called suitable for transmission. Two main cells directly affect the process: the perceiving and transmitting. At the end of the axon of the transmitting cell is the presynaptic ending (the initial part of the synapse). It can affect the launch of neurotransmitters in the cell (this word has several meanings: mediators, mediators or neurotransmitters) - determined by which an electrical signal is transmitted between 2 neurons.
The synaptic cleft is the middle part of the synapse - this is the gap between 2 interacting nerve cells. Through this gap, an electrical impulse comes from the transmitting cell. The final part of the synapse is considered to be the perceiving part of the cell, which is the postsynaptic ending (the contacting fragment of the cell with different sensitive receptors in its structure).
Synapse mediators
Mediator (from the Latin Media - transmitter, intermediary or middle). Such synapse mediators are very important in the transmission process.
The morphological difference between inhibitory and excitatory synapses is that they do not have a mediator release mechanism. The mediator in the inhibitory synapse, motor neuron, and other inhibitory synapses is considered to be the amino acid glycine. But the inhibitory or excitatory nature of the synapse is determined not by their mediators, but by the property of the postsynaptic membrane. For example, acetylcholine gives an excitatory effect in the neuromuscular synapse of the terminals (vagus nerves in the myocardium).
Acetylcholine serves as an excitatory mediator in cholinergic synapses (the end of the spinal cord of a motor neuron plays the presynaptic membrane in it), in a synapse on Ranshaw cells, in the presynaptic terminal of the sweat glands, the adrenal medulla, in the intestinal synapse and in the ganglia of the sympathetic nervous system. Acetylcholinesterase and acetylcholine were also found in fractions of different parts of the brain, sometimes in in large numbers, but apart from the cholinergic synapse on Ranshaw cells, they have not yet been able to identify other cholinergic synapses. According to scientists, the mediator excitatory function of acetylcholine in the central nervous system is very likely.
The catelchomines (dopamine, norepinephrine, and epinephrine) are considered adrenergic neurotransmitters. Adrenaline and norepinephrine are synthesized at the end of the sympathetic nerve, in the cell of the head substance of the adrenal gland, spinal cord and brain. Amino acids (tyrosine and L-phenylalanine) are considered the starting material, and adrenaline is the final product of the synthesis. The intermediate substance, which includes norepinephrine and dopamine, also function as mediators in the synapse created at the endings of sympathetic nerves. This function can be either inhibitory (intestinal secretory glands, several sphincters, and smooth muscle of the bronchi and intestines) or excitatory (smooth muscles of certain sphincters and blood vessels, norepinephrine in the myocardial synapse, dopamine in the subcutaneous nuclei of the brain).
When the synaptic mediators complete their function, catecholamine is absorbed by the presynaptic nerve ending, and transmembrane transport is switched on. During the absorption of neurotransmitters, the synapses are protected from premature depletion of the supply during a long and rhythmic work.
Synapse: main types and functions
Langley in 1892 suggested that synaptic transmission in the vegetative ganglion of mammals is not of an electrical nature, but of a chemical one. After 10 years, Eliott found out that adrenaline is obtained from the adrenal glands from the same effect as stimulation of the sympathetic nerves.
After that, it was suggested that adrenaline is able to be secreted by neurons and, when excited, be released by the nerve ending. But in 1921, Levi made an experiment in which he established the chemical nature of transmission in the autonomic synapse among the heart and vagus nerves. He filled the vessels with saline and stimulated the vagus nerve, creating a slow heart rate. When the fluid was transferred from the inhibited stimulation of the heart to the unstimulated heart, it beat more slowly. It is clear that stimulation of the vagus nerve caused the release of the inhibitory substance into the solution. Acetylcholine fully reproduced the effect of this substance. In 1930, the role in the synaptic transmission of acetylcholine in the ganglion was finally established by Feldberg and his collaborators.
Synapse chemical
The chemical synapse is fundamentally different in the transmission of irritation with the help of a mediator from the presynapse to the postsynapse. Therefore, differences are formed in the morphology of the chemical synapse. The chemical synapse is more common in the vertebral CNS. It is now known that a neuron is capable of isolating and synthesizing a pair of mediators (coexisting mediators). Neurons also have neurotransmitter plasticity - the ability to change the main mediator during development.
neuromuscular junction
This synapse carries out the transmission of excitation, but this connection can be destroyed various factors. The transmission ends during the blockade of the ejection of acetylcholine into the synaptic cleft, as well as during an excess of its content in the zone of postsynaptic membranes. Many poisons and drugs affect the capture, output, which is associated with the cholinergic receptors of the postsynaptic membrane, then the muscle synapse blocks the transmission of excitation. The body dies during suffocation and stops the contraction of the respiratory muscles.
Botulinus is a microbial toxin in the synapse; it blocks the transmission of excitation by destroying the syntaxin protein in the presynaptic terminal, which is controlled by the release of acetylcholine into the synaptic cleft. Several poisonous warfare agents, pharmacological drugs (neostigmine and prozerin), and insecticides block the conduction of excitation to the neuromuscular junction by inactivating acetylcholinesterase, an enzyme that destroys acetylcholine. Therefore, acetylcholine accumulates in the zone of the postsynaptic membrane, the sensitivity to the mediator decreases, the postsynaptic membranes are released and the receptor block is immersed in the cytosol. The acetylcholine will be ineffective and the synapse will be blocked.
Synapse nerve: features and components
A synapse is a connection between a contact point between two cells. Moreover, each of them is enclosed in its own electrogenic membrane. nerve synapse consists of three main components: postsynaptic membrane, synaptic cleft and presynaptic membrane. The postsynaptic membrane is a nerve ending that passes to the muscle and descends into the muscle tissue. In the presynaptic region there are vesicles - these are closed cavities that have a neurotransmitter. They are always on the move.
Approaching the membrane of nerve endings, the vesicles merge with it, and the neurotransmitter enters the synaptic cleft. One vesicle contains a quantum of the mediator and mitochondria (they are needed for the synthesis of the mediator - the main source of energy), then acetylcholine is synthesized from choline and, under the influence of the enzyme acetylcholine transferase, is processed into acetylCoA).
Synaptic cleft among post- and presynaptic membranes
In different synapses, the size of the gap is different. filled with intercellular fluid, which contains a neurotransmitter. The postsynaptic membrane covers the site of contact of the nerve ending with the innervated cell in the myoneural synapse. In certain synapses, the postsynaptic membrane creates a fold, the contact area increases.
Additional substances that make up the postsynaptic membrane
The following substances are present in the zone of the postsynaptic membrane:
Receptor (cholinergic receptor in myoneural synapse).
Lipoprotein (has a great similarity with acetylcholine). This protein has an electrophilic end and an ionic head. The head enters the synaptic cleft and interacts with the cationic head of acetylcholine. Because of this interaction, the postsynaptic membrane changes, then depolarization occurs, and potentially dependent Na-channels open. Membrane depolarization is not considered a self-reinforcing process;
Gradual, its potential on the postsynaptic membrane depends on the number of mediators, that is, the potential is characterized by the property of local excitations.
Cholinesterase - is considered a protein that has an enzymatic function. In structure, it is similar to the cholinergic receptor and has similar properties with acetylcholine. Cholinesterase destroys acetylcholine, initially the one that is associated with the cholinergic receptor. Under the action of cholinesterase, the cholinergic receptor removes acetylcholine, repolarization of the postsynaptic membrane is formed. Acetylcholine breaks down to acetic acid and choline, necessary for the trophism of muscle tissue.
With the help of the existing transport, choline is displayed on the presynaptic membrane, it is used to synthesize a new mediator. Under the influence of the mediator, the permeability in the postsynaptic membrane changes, and under cholinesterase, the sensitivity and permeability returns to the initial value. Chemoreceptors are able to interact with new mediators.
