motor synapse functions. The structure of the synapse: electrical and chemical synapses
Synapse(Greek σύναψις, from συνάπτειν - hug, clasp, shake hands) - the place of contact between two neurons or between and the effector cell receiving the signal. Serves for transmission between two cells, and during synaptic transmission, the amplitude and frequency of the signal can be regulated.
The term was introduced in 1897 by the English physiologist Charles Sherrington.
synapse structure
A typical synapse is an axo-dendritic chemical synapse. Such a synapse consists of two parts: presynaptic, formed by a club-shaped extension of the end of the maxon of the transmitting cell and postsynaptic, represented by the contact area of the cytolemma of the perceiving cell (in this case, the dendrite area). The synapse is a space separating the membranes of contacting cells, to which the nerve endings fit. The transmission of impulses is carried out chemically with the help of mediators or electrically through the passage of ions from one cell to another.
Between both parts there is a synaptic gap - a gap 10-50 nm wide between the postsynaptic and presynaptic membranes, the edges of which are reinforced with intercellular contacts.
The part of the axolemma of the club-shaped extension adjacent to the synaptic cleft is called presynaptic membrane. The section of the cytolemma of the perceiving cell that limits the synaptic cleft on the opposite side is called postsynaptic membrane, in chemical synapses it is relief and contains numerous.
In the synaptic extension there are small vesicles, the so-called synaptic vesicles containing either a mediator (transmission intermediary substance) or an enzyme that destroys this mediator. On the postsynaptic, and often on the presynaptic membranes, there are receptors for one or another mediator.
Synapse classification
Depending on the transmission mechanism nerve impulse distinguish
- chemical;
- electrical - cells are connected by highly permeable contacts using special connexons (each connexon consists of six protein subunits). The distance between cell membranes in an electrical synapse is 3.5 nm (usual intercellular is 20 nm)
Since the resistance of the extracellular fluid is small (in this case), the impulses pass without stopping through the synapse. Electrical synapses are usually excitatory.
Two release mechanisms have been discovered: with complete fusion of the vesicle with the plasmalemma and the so-called "kissed and ran away" (eng. kiss and run), when the vesicle connects to the membrane, and small molecules come out of it into the synaptic cleft, while large ones remain in the vesicle. The second mechanism, presumably, is faster than the first, with the help of which synaptic transmission occurs when high content calcium ions in the synaptic plaque.
The consequence of this structure of the synapse is the unilateral conduction of the nerve impulse. There is a so-called synaptic delay is the time it takes for a nerve impulse to be transmitted. Its duration is about - 0.5 ms.
The so-called "Dail principle" (one - one mediator) is recognized as erroneous. Or, as it is sometimes believed, it is refined: not one, but several mediators can be released from one end of a cell, and their set is constant for a given cell.
Discovery history
- In 1897, Sherrington formulated the concept of synapses.
- For research on the nervous system, including synaptic transmission, in 1906 the Nobel Prize was awarded to Golgi and Ramon y Cajal.
- In 1921, the Austrian scientist O. Loewi established the chemical nature of the transmission of excitation through synapses and the role of acetylcholine in it. Received the Nobel Prize in 1936 together with G. Dale (N. Dale).
- In 1933, the Soviet scientist A. V. Kibyakov established the role of adrenaline in synaptic transmission.
- 1970 - B. Katz (V. Katz, Great Britain), U. von Euler (U. v. Euler, Sweden) and J. Axelrod (J. Axelrod, USA) received the Nobel Prize for the discovery of rolinoradrenaline in synaptic transmission.
synapse structure
There are small vesicles in the synaptic extension, the so-called synaptic vesicles containing either a mediator (a mediator in the transfer of excitation), or an enzyme that destroys this mediator. On the postsynaptic, and often on the presynaptic membranes, there are receptors for one or another mediator.
Synapse classifications
Depending on the mechanism of transmission of a nerve impulse, there are
- electrical - cells are connected by highly permeable contacts using special connexons (each connexon consists of six protein subunits). The distance between cell membranes in an electrical synapse is 3.5 nm (usual intercellular is 20 nm)
Since the resistance of the extracellular fluid is small (in this case), the impulses pass without stopping through the synapse. Electrical synapses are usually excitatory.
For the nervous system of mammals, electrical synapses are less characteristic than chemical ones.
- mixed synapses: The presynaptic action potential creates a current that depolarizes the postsynaptic membrane of a typical chemical synapse, where the pre- and postsynaptic membranes do not fit tightly together. Thus, in these synapses, chemical transmission serves as a necessary reinforcing mechanism.
The most common chemical synapses.
Chemical synapses can be classified according to their location and belonging to the corresponding structures:
- peripheral
- neurosecretory (axo-vasal)
- receptor-neuronal
- central
- axo-dendritic- with dendrites, incl.
- axo-spiky- with dendritic spines, outgrowths on dendrites;
- axo-somatic- with the bodies of neurons;
- axo-axonal- between axons;
- dendro-dendritic- between dendrites;
- axo-dendritic- with dendrites, incl.
There are two types of inhibitory synapses: 1) a synapse, in the presynaptic endings of which a mediator is released that hyperpolarizes the postsynaptic membrane and causes the appearance of an inhibitory postsynaptic potential; 2) axo-axonal synapse, providing presynaptic inhibition. Cholinergic synapse (s. cholinergica) - a synapse in which acetylcholine is a mediator.
Some synapses have postsynaptic compaction- an electron-dense zone consisting of proteins. Synapses are distinguished by its presence or absence. asymmetrical and symmetrical. It is known that all glutamatergic synapses are asymmetric, while GABAergic synapses are symmetrical.
In cases where several synaptic extensions are in contact with the postsynaptic membrane, they form multiple synapses.
To special forms synapses include spine devices, in which short single or multiple protrusions of the postsynaptic membrane of the dendrite are in contact with the synaptic expansion. Spiny apparatus significantly increase the number of synaptic contacts on the neuron and, consequently, the amount of information processed. "Non-spiky" synapses are called "sessile". For example, all GABAergic synapses are sessile.
The mechanism of functioning of the chemical synapse
When the presynaptic terminal is depolarized, voltage-sensitive calcium channels open, calcium ions enter the presynaptic terminal and trigger the mechanism for fusion of synaptic vesicles with the membrane. As a result, the mediator enters the synaptic cleft and attaches to the receptor proteins of the postsynaptic membrane, which are divided into metabotropic and ionotropic. The former are associated with a G-protein and trigger a cascade of intracellular signal transduction reactions. The latter are associated with ion channels, which open when a neurotransmitter binds to them, which leads to a change in the membrane potential. The mediator acts for a very short time, after which it is destroyed by a specific enzyme. For example, in cholinergic synapses, the enzyme that destroys the mediator in the synaptic cleft is acetylcholinesterase. At the same time, part of the mediator can move with the help of carrier proteins through the postsynaptic membrane (direct capture) and in the opposite direction through the presynaptic membrane (reverse capture). In some cases, the neurotransmitter is also taken up by neighboring neuroglial cells.
Two release mechanisms have been discovered: with complete fusion of the vesicle with the plasmalemma and the so-called "kissed and ran away" (eng. kiss and run), when the vesicle connects to the membrane, and small molecules come out of it into the synaptic cleft, while large ones remain in the vesicle. The second mechanism, presumably, is faster than the first, with the help of which synaptic transmission occurs at a high content of calcium ions in the synaptic plaque.
The consequence of this structure of the synapse is the unilateral conduction of the nerve impulse. There is a so-called synaptic delay is the time it takes for a nerve impulse to be transmitted. Its duration is about - 0.5 ms.
PNS: Schwann cells Neurolemma Noxus of Ranvier/Internodal segment Myelin notch
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Synonyms:See what "Synapse" is in other dictionaries:
- (from the Greek. synapsis connection) the area of \u200b\u200bcontact (connection) of nerve cells (neurons) with each other and with the cells of the executive organs. Interneuronal synapses are usually formed by branching of the axon of one nerve cell and body, dendrites or axon... Big Encyclopedic Dictionary
In neural networks, communication between formal neurons. The output signal from a neuron enters the synapse, which transmits it to another neuron. Complex synapses can have memory. See also: Neural networks Financial Dictionary Finam ... Financial vocabulary
synapse- A specialized contact zone between neurons (interneuronal synapse) or between neurons and other excitable formations (organ synapse), which ensures the transmission of excitation with the preservation, change or disappearance of its information ... ... Technical Translator's Handbook
3.6. synapses
Neurons in the CNS are interconnected in the most complex neural circuits through synapses. Synapse– area (zone) of contact between neurons or a neuron and a working organ. Synapses are classified according to several criteria:
by location and accessories to the corresponding cells - central(axosomatic, axodendritic,
axoaxonal) and peripheral(neuromuscular, neurosecretory)
by functional value - excitatory and inhibitory;
according to the method of information transfer- chemical, electrical, mixed.
3.6.1. Synapse structure. Conducting impulses across the synapse
The axon, approaching other neurons or cells of the working organ, loses its myelin sheath, branches, becomes thinner. Each branching of the axon ends with a thickening that contacts with bodies, dendrites, axons of neighboring neurons, organ cells (1 axon can form up to 10,000 synapses). The presynaptic compartment contains a large number of vesicle(bubbles)
which contain mediators – chemical substances(intermediaries) that have excitatory or inhibitory effects depending on their chemical structure. The membrane covering the presynaptic ending in the contact area is somewhat thickened and is called presynaptic membrane (Fig. 8, 8.1).
The membrane of the body, axon, dendrite, cells of the working organs is called postsynaptic membrane. It contains receptors
having high sensitivity and specificity to mediators (figuratively, the mediator is the key, the receptor is the lock). Different synapses contain various mediators - acetylcholine, norepinephrine, dopamine, serotonin, etc.) In neuromuscular synapses, the postsynaptic membrane has a folded structure, which increases its surface.
Between the presynaptic and postsynaptic membranes is a synaptic cleft (20 to 50 nanometers in size) filled with extracellular fluid.
Thus, the synapse includes 3 parts:
presynaptic membrane
postsynaptic membrane
synaptic cleft
Conduction of excitation through the synapse. Conduction of excitation through chemical synapse is a complex physiological process that proceeds in stages with the participation of mediators. In many central, neuromuscular, and parasympathetic synapses, the mediator is acetylcholine. The action potential along the axon reaches the plaque and causes a change in the permeability of the presynaptic membrane for calcium ions, which enter the plaque from the synaptic cleft, which leads to rupture of the vesicles and the release of acetylcholine from them into the synaptic cleft. It diffuses to the postsynaptic membrane, interacts with membrane receptors, which increases its excitability, changes the permeability for sodium ions, as a result, excitation occurs on the membrane, which extends to another neuron or cells of the working organ. The neurotransmitter is released into the synaptic cleft more than is necessary for the conduction of nerve impulses (manifestation of the principle of biological reliability). Excess mediators are hydrolyzed by enzymes in the extracellular fluid of the synaptic cleft.
Inhibitory synapses according to the structure and conduction of excitation
do not differ from excitatory
synapses, the only difference is
in the nature of mediators and receptors postsynaptic membrane. Mediators of inhibitory synapses in the spinal cord are glycine, brain - gamma-aminobutyric acid(GABA). The inhibitory mediator, interacting with the receptors of the postsynaptic membrane, causes decrease in her excitability, which leads to blocking of nerve impulses on the postsynaptic membrane,
and excitation does not spread to other neurons.
E electrical synapses are found in small amounts in the CNS and smooth muscles. At these synapses, the presynaptic
and postsynaptic membranes are closely adjacent to each other, the synaptic cleft is very narrow (5 nanometers), transverse (from cell to cell) channels formed by protein molecules pass through it. Through this gap junction, the action potential easily passes from the presynaptic ending to the postsynaptic membrane.
Sometimes meet mixed synapses: in one part - chemical, in the other - electrical mechanisms for the transmission of nerve impulses.
Physiological properties of synapses
All synapses are characterized by a number of common properties:
1) unilateral excitation;
2) delayed (delay) conduction of excitation (in electrical synapses, the delay is shorter);
3) low excitability and lability;
4) the ability to sum up excitations;
5) tendency to fatigue.
3.6.2. Features of the functioning of synapses in children
The synaptic delay in the conduction of nerve impulses in children is longer than in adults (in newborns, about 20 impulses per second pass through the synapse, in adults - 100–150 impulses per second).
In children, the presynaptic section of the synapse contains a smaller number of mediators, their synthesis occurs more slowly, therefore, fatigue occurs faster in synapses and nerve centers with prolonged excitation, the younger the child, the more pronounced it is. In the process of growth, children form a large number of new synapses, which contributes to the development of the brain, learning processes, and memory.
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) the 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.
The area of contact between two neurons is called synapse.
Internal structure axodendritic synapse.a) electrical synapses. electrical synapses in nervous system mammals are rare. They are formed by slit-like junctions (nexuses) between the dendrites or somas of adjoining neurons, which are connected via cytoplasmic channels 1.5 nm in diameter. The process of signal transmission occurs without synaptic delay and without the participation of mediators.
Through electrical synapses, it is possible to spread electrotonic potentials from one neuron to another. Due to the close synaptic contact, signal conduction modulation is impossible. The task of these synapses is the simultaneous excitation of neurons that perform the same function. An example is the neurons of the respiratory center of the medulla oblongata, which synchronously generate impulses during inspiration. In addition, the neural circuits that control saccades, in which the fixation point of the gaze moves from one object of attention to another, can serve as an example.
b) Chemical synapses. Most synapses in the nervous system are chemical. The functioning of such synapses depends on the release of neurotransmitters. The classical chemical synapse is represented by the presynaptic membrane, the synaptic cleft, and the postsynaptic membrane. The presynaptic membrane is part of the club-shaped extension of the nerve ending of the cell that transmits the signal, and the postsynaptic membrane is the part of the cell that receives the signal.
The mediator is released from the club-shaped expansion by exocytosis, passes through the synaptic cleft, and binds to receptors on the postsynaptic membrane. Under the postsynaptic membrane there is a subsynaptic active zone, in which, after the activation of the receptors of the postsynaptic membrane, various biochemical processes occur.
The club-shaped extension contains synaptic vesicles containing neurotransmitters, as well as a large number of mitochondria and cisternae of the smooth endoplasmic reticulum. The use of traditional methods of fixation in the study of cells makes it possible to distinguish presynaptic seals on the presynaptic membrane, limiting the active zones of the synapse, to which synaptic vesicles are directed by means of microtubules.
axodendritic synapse. Section of the spinal cord preparation: synapse between the end section of the dendrite and, presumably, a motor neuron.
The presence of rounded synaptic vesicles and postsynaptic compaction is characteristic of excitatory synapses.
The section of the dendrite is drawn in the transverse direction, as evidenced by the presence of many microtubules.
In addition, some neurofilaments are visible. The site of the synapse is surrounded by a protoplasmic astrocyte.
Processes occurring in the nerve endings of two types. (A) Synaptic transmission of small molecules (eg, glutamate).
(1) Transport vesicles containing the membrane proteins of the synaptic vesicles are guided along the microtubules to the clubbed plasma membrane.
At the same time, enzyme and glutamate molecules are transferred by slow transport.
(2) Vesicle membrane proteins exit the plasma membrane and form synaptic vesicles.
(3) Glutamate sinks into synaptic vesicles; mediator accumulation occurs.
(4) Vesicles containing glutamate approach the presynaptic membrane.
(5) Depolarization results in mediator exocytosis from partially destroyed vesicles.
(6) The released neurotransmitter spreads diffusely in the area of the synaptic cleft and activates specific receptors on the postsynaptic membrane.
(7) Synaptic vesicle membranes are transported back into the cell by endocytosis.
(8) Partial reuptake of glutamate into the cell for reuse occurs.
(B) Transmission of neuropeptides (eg, substance P) occurring simultaneously with synaptic transmission (eg, glutamate).
The joint transmission of these substances occurs in the central nerve endings of unipolar neurons, which provide pain sensitivity.
(1) Synthesized in the Golgi complex (at the perikaryon) vesicles and peptide precursors (propeptides) 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 electrical tone is transmitted to the soma in the form of an electrotonic potential that decays as it spreads, due to which a change in the resting potential occurs in the initial segment of the axon.
Ionic processes are described in detail in a separate article on the site. With the predominance of excitatory postsynaptic potentials, the initial segment of the axon depolarizes to a threshold level and generates an action potential.
The most common excitatory CNS mediator is glutamate, and the inhibitory one is gamma-aminobutyric acid (GABA). In the peripheral nervous system, acetylcholine serves as a mediator for motor neurons of striated muscles, and glutamate for sensory neurons.
The sequence of processes occurring in glutamatergic synapses is shown in the figure below. When glutamate is transferred together with other peptides, the release of peptides is carried out extrasynaptically.
Most sensitive neurons, in addition to glutamate, also secrete other peptides (one or more) that are released in different parts of the neuron; however, the main function of these peptides is to modulate (increase or decrease) the efficiency of synaptic glutamate transmission.
In addition, neurotransmission can occur through diffuse extrasynaptic signaling characteristic of monoaminergic neurons (neurons that use biogenic amines to mediate neurotransmission). There are two types of monoaminergic neurons. In some neurons, catecholamines (norepinephrine or dopamine) are synthesized from the amino acid tyrosine, while in others, serotonin is synthesized from the amino acid tryptophan. For example, dopamine is released both in the synaptic region and from axon varicose thickenings, in which this neurotransmitter is also synthesized.
Dopamine penetrates into the intercellular fluid of the CNS and, until degradation, is able to activate specific receptors at a distance of up to 100 microns. Monoaminergic neurons are present in many CNS structures; disruption of impulse transmission by these neurons leads to various diseases among which are Parkinson's disease, schizophrenia and major depression.
Nitric oxide (a gaseous molecule) is also involved in diffuse neurotransmission in the glutamatergic system of neurons. Excessive influence of nitric oxide has a cytotoxic effect, especially in those areas whose blood supply is impaired due to arterial thrombosis. Glutamate is also a potentially cytotoxic neurotransmitter.
In contrast to diffuse neurotransmission, traditional synaptic signal transmission is called “conductive” due to its relative stability.
in) Summary. Multipolar CNS neurons consist of a soma, dendrites, and an axon; the axon forms collateral and terminal branches. In the soma there are 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, medicine can play the role of a "spare key" that performs a similar function 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 process of disinhibition - important feature 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
