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Synapse structure: electrical and chemical synapses. What is Synapse? The concept of a synapse and its structure

The area of ​​contact between two neurons is called synapse.

Internal structure axodendritic synapse.

A) Electrical synapses. Electrical synapses are rare in the mammalian nervous system. They are formed by gap junctions (nexuses) between the dendrites or somata of adjacent neurons, which are connected by cytoplasmic channels with a diameter of 1.5 nm. The signal transmission process occurs without synaptic delay and without the participation of mediators.

Through electrical synapses, electrotonic potentials can spread from one neuron to another. Due to the close synaptic contact, modulation of signal transmission is impossible. The task of these synapses is to simultaneously excite neurons that perform the same function. An example is the neurons of the respiratory center of the medulla oblongata, which synchronously generate impulses during inhalation. In addition, an example is the neural circuits that control saccades, in which the point of fixation of the gaze moves from one object of attention to another.

b) Chemical synapses. Most synapses in the nervous system are chemical. The functioning of such synapses depends on the release of transmitters. The classic chemical synapse is represented by a presynaptic membrane, a synaptic cleft, and a postsynaptic membrane. The presynaptic membrane is the 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 transmitter is released from the clavate extension 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 activation of the receptors of the postsynaptic membrane, various biochemical processes occur.

The club-shaped extension contains synaptic vesicles containing mediators, as well as a large number of mitochondria and cisterns of the smooth endoplasmic reticulum. The use of traditional fixation techniques 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 with the help of microtubules.


Axodendritic synapse.
Section of the spinal cord specimen: synapse between the terminal portion of the dendrite and, presumably, a motor neuron.
The presence of round synaptic vesicles and postsynaptic compaction is characteristic of excitatory synapses.
The dendrite was cut in the transverse direction, as evidenced by the presence of many microtubules.
In addition, some neurofilaments are visible. The synapse site is surrounded by a protoplasmic astrocyte.
Processes occurring in two types of nerve endings.
(A) Synaptic transmission of small molecules (eg, glutamate).
(1) Transport vesicles containing membrane proteins of synaptic vesicles are directed along microtubules to the plasma membrane of the club-shaped thickening.
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 is loaded into synaptic vesicles; mediator accumulation occurs.
(4) Vesicles containing glutamate approach the presynaptic membrane.
(5) As a result of depolarization, exocytosis of the mediator occurs from partially destroyed vesicles.
(6) The released transmitter spreads diffusely in the region 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 occurs for reuse.
(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) 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 area of ​​the club-shaped thickening, the process of formation of the peptide molecule is completed, and the vesicles are transported to the plasma membrane.
(3) Depolarization of the membrane and transfer of vesicle contents into the intercellular 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. Activation of receptors 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 electrotonicity is transmitted to the soma in the form of an electrotonic potential that decays as it spreads, due to which the resting potential in the initial segment of the axon changes.

Ionic processes are described in detail in a separate article on the website. When excitatory postsynaptic potentials predominate, the initial segment of the axon is depolarized to a threshold level and generates an action potential.

The most common excitatory neurotransmitter of the central nervous system is glutamate, and the inhibitory one is gamma-aminobutyric acid (GABA). In the peripheral nervous system, acetylcholine serves as a transmitter for motor neurons of striated muscles, and glutamate for sensory neurons.

The sequence of processes occurring at glutamatergic synapses is shown in the figure below. When glutamate is transferred together with other peptides, the release of peptides occurs via extrasynaptic pathways.

Most sensory neurons, in addition to glutamate, also release other peptides (one or more) that are released in various areas 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 signal transmission, 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, and in others, serotonin is synthesized from the amino acid tryptophan. For example, dopamine is released both in the synaptic region and from axonal varicosities, in which the synthesis of this neurotransmitter also occurs.

Dopamine penetrates into the intercellular fluid of the central nervous system and, before degradation, is able to activate specific receptors at a distance of up to 100 microns. Monoaminergic neurons are present in many structures of the central nervous system; disruption of impulse transmission by these neurons leads to various diseases, including Parkinson's disease, schizophrenia and major depression.

Nitric oxide (a gaseous molecule) is also involved in diffuse neurotransmission in the glutamatergic neuronal system. Excessive nitric oxide has a cytotoxic effect, especially in those areas where the 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 “conductor” due to its relative stability.

V) Resume. Multipolar neurons of the CNS consist of soma, dendrites and axon; the axon forms collateral and terminal branches. The soma contains smooth and rough endoplasmic reticulum, Golgi complexes, neurofilaments and microtubules. Microtubules permeate the entire neuron, take part in the process of anterograde transport of synaptic vesicles, mitochondria and membrane-building substances, 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 anatomical structure into axodendritic, axosomatic, axoaxonal and dendro-dendritic. The synapse is represented by pre- and postsynaptic membranes, a synaptic cleft and a subsynaptic active zone.

Electrical synapses ensure the simultaneous activation of entire groups, forming electrical connections between them due to gap-like contacts (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 dopamine receptors of the dendritic trunk and capillary pericyte walls.
Disinhibition.
(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 to a lock, and the mediator can be compared to a key that matches it. If the process of mediator release is disrupted with age or as a result of any disease, medicine can play the role of a “spare key”, performing a function similar to a mediator. This drug is called an agonist. At the same time, in case of excessive production, the mediator can be “intercepted” by a receptor blocker - a “fake key”, which will contact the “lock” receptor, but will not cause its activation.

3. Braking and disinhibition. The functioning of spontaneously active neurons is inhibited by 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 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 transmitters. 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 electrotonus of the target cell. In successive D-D synapses, synaptic vesicles are located in only one dendritic spine, and in reciprocal D-D synapses, 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, there are a few somato-dendritic and somato-somatic synapses.

Axoaxonal synapses of the cerebral cortex.
The arrows indicate the direction of the impulses.
(1) Presynaptic and (2) postsynaptic inhibition of the spinal neuron traveling to the brain.
The arrows indicate the direction of impulse conduction (inhibition of the switching neuron under the influence of inhibitory influences is possible).
Excitatory dendro-dendritic synapses. The dendrites of three neurons are depicted.
Reciprocal synapse (right). The arrows indicate the direction of propagation of electrotonic waves.

Educational video - structure of a synapse

A synapse is a site of functional rather than physical contact between neurons; it transmits information from one cell to another. Usually there are synapses between the terminal branches of the axon of one neuron and dendrites ( axodendritic synapses) or body ( axosomatic synapses) of another neuron. The number of synapses is usually very large, which provides a large area for information transfer. For example, there are over 1000 synapses on the dendrites and cell bodies of individual motor neurons in the spinal cord. Some brain cells can have up to 10,000 synapses (Figure 16.8).

There are two types of synapses - electric And chemical- depending on the nature of the signals passing through them. Between the terminals of the motor neuron and the surface of the muscle fiber there is neuromuscular junction, different in structure from interneuron synapses, but similar to them in functional terms. The structural and physiological differences between a normal synapse and a neuromuscular junction will be described a little later.

The structure of a chemical synapse

Chemical synapses are the most common type of synapse in vertebrates. These are bulbous thickenings of nerve endings called synaptic plaques and located in close proximity to the end of the dendrite. The cytoplasm of the synaptic plaque contains mitochondria, smooth endoplasmic reticulum, microfilaments and numerous synaptic vesicles. Each vesicle is about 50 nm in diameter and contains mediator- a substance with which a nerve signal is transmitted across a synapse. The membrane of the synaptic plaque in the area of ​​the synapse itself is thickened as a result of compaction of the cytoplasm and forms presynaptic membrane. The dendrite membrane in the synapse area is also thickened and forms postsynaptic membrane. These membranes are separated by a gap - synaptic cleft about 20 nm wide. The presynaptic membrane is designed in such a way that synaptic vesicles can attach to it and mediators can be released into the synaptic cleft. The postsynaptic membrane contains large protein molecules that act as receptors mediators, and numerous channels And pores(usually closed), through which ions can enter the postsynaptic neuron (see Fig. 16.10, A).

Synaptic vesicles contain a transmitter that is formed either in the body of the neuron (and enters the synaptic plaque, passing through the entire axon), or directly in the synaptic plaque. In both cases, the synthesis of the mediator requires enzymes formed in the cell body on ribosomes. In a synaptic plaque, transmitter molecules are “packed” into vesicles in which they are stored until released. The main mediators of the vertebrate nervous system are acetylcholine And norepinephrine, but there are other mediators that will be discussed later.

Acetylcholine is an ammonium derivative, the formula of which is shown in Fig. 16.9. This is the first known mediator; in 1920, Otto Lewy isolated it from the endings of parasympathetic neurons of the vagus nerve in the heart of the frog (section 16.2). The structure of norepinephrine is discussed in detail in section. 16.6.6. Neurons that release acetylcholine are called cholinergic, and those releasing norepinephrine - adrenergic.

Mechanisms of synaptic transmission

It is believed that the arrival of a nerve impulse at the synaptic plaque causes depolarization of the presynaptic membrane and an increase in its permeability to Ca 2+ ions. Ca 2+ ions entering the synaptic plaque cause the fusion of synaptic vesicles with the presynaptic membrane and the release of their contents from the cell (exocytosis), as a result of which it enters the synaptic cleft. This whole process is called electrosecretory coupling. Once the mediator is released, the vesicle material is used to form new vesicles that are filled with mediator molecules. Each vial contains about 3000 molecules of acetylcholine.

The mediator molecules diffuse through the synaptic cleft (this process takes about 0.5 ms) and bind to receptors located on the postsynaptic membrane that are capable of recognizing the molecular structure of acetylcholine. When a receptor molecule binds to a transmitter, its configuration changes, which leads to the opening of ion channels and the entry of ions into the postsynaptic cell, causing depolarization or hyperpolarization(Fig. 16.4, A) its membrane, depending on the nature of the released mediator and the structure of the receptor molecule. Transmitter molecules that cause a change in the permeability of the postsynaptic membrane are immediately removed from the synaptic cleft either by reabsorption by the presynaptic membrane, or by diffusion from the cleft or enzymatic hydrolysis. In case cholinergic synapses, acetylcholine located in the synaptic cleft is hydrolyzed by the enzyme acetylcholinesterase, localized on the postsynaptic membrane. As a result of hydrolysis, choline is formed, it is absorbed back into the synaptic plaque and again converted there into acetylcholine, which is stored in vesicles (Fig. 16.10).

IN stimulating At synapses, under the influence of acetylcholine, specific sodium and potassium channels open, and Na + ions enter the cell, and K + ions leave it in accordance with their concentration gradients. As a result, depolarization of the postsynaptic membrane occurs. This depolarization is called excitatory postsynaptic potential(EPSP). The amplitude of the EPSP is usually small, but its duration is longer than that of the action potential. The amplitude of the EPSP changes in a stepwise manner, suggesting that the transmitter is released in portions, or “quanta,” rather than in the form of individual molecules. Apparently, each quantum corresponds to the release of a transmitter from one synaptic vesicle. A single EPSP is usually not capable of causing depolarization to the threshold required for the occurrence of an action potential. But the depolarizing effects of several EPSPs add up, and this phenomenon is called summation. Two or more EPSPs occurring simultaneously at different synapses on the same neuron can collectively produce a depolarization sufficient to excite an action potential in the postsynaptic neuron. It's called spatial summation. Rapidly repeated release of a transmitter from the vesicles of the same synaptic plaque under the influence of an intense stimulus causes individual EPSPs, which follow each other so often in time that their effects are also summed up and cause an action potential in the postsynaptic neuron. It's called time summation. Thus, impulses can arise in a single postsynaptic neuron either as a result of weak stimulation of several associated presynaptic neurons, or as a result of repeated stimulation of one of its presynaptic neurons. IN brake at synapses, the release of the transmitter increases the permeability of the postsynaptic membrane due to the opening of specific channels for K + and Cl - ions. Moving along concentration gradients, these ions cause hyperpolarization of the membrane, called inhibitory postsynaptic potential(TPSP).

Mediators themselves do not have excitatory or inhibitory properties. For example, acetylcholine has an excitatory effect at most neuromuscular junctions and other synapses, but causes inhibition at the neuromuscular junctions of the heart and visceral muscles. These opposing effects are due to the events that unfold on the postsynaptic membrane. The molecular properties of the receptor determine which ions will enter the postsynaptic neuron, and these ions, in turn, determine the nature of the change in postsynaptic potentials, as described above.

Electrical synapses

In many animals, including coelenterates and vertebrates, the transmission of impulses through some synapses is carried out by passing electric current between pre- and postsynaptic neurons. The width of the gap between these neurons is only 2 nm, and the total resistance to current from the membranes and the fluid filling the gap is very small. Impulses pass through synapses without delay and have no effect on their transmission medicinal substances or other chemicals.

Neuromuscular junction

The neuromuscular junction is a specialized type of synapse between the endings of a motor neuron (motoneuron) and endomysium muscle fibers (section 17.4.2). Each muscle fiber has a specialized area - motor endplate, where the axon of a motor neuron (motoneuron) branches, forming unmyelinated branches about 100 nm thick, running in shallow grooves along the surface of the muscle membrane. The muscle cell membrane - the sarcolemma - forms many deep folds called postsynaptic folds (Fig. 16.11). The cytoplasm of motor neuron terminals is similar to the contents of the synaptic plaque and, during stimulation, releases acetylcholine using the same mechanism discussed above. Changes in the configuration of receptor molecules located on the surface of the sarcolemma lead to a change in its permeability to Na + and K +, and as a result, local depolarization occurs, called end plate potential(PKP). This depolarization is quite sufficient in magnitude to generate an action potential, which propagates along the sarcolemma deep into the fiber along a system of transverse tubules ( T-system) (section 17.4.7) and causes muscle contraction.

Functions of synapses and neuromuscular junctions

The main function of interneuron synapses and neuromuscular junctions is to transmit signals from receptors to effectors. In addition, the structure and organization of these sites of chemical secretion determine a number of important features of the conduction of nerve impulses, which can be summarized as follows:

1. Unidirectional transmission. The release of the transmitter from the presynaptic membrane and the localization of receptors on the postsynaptic membrane allow the transmission of nerve signals along this path in only one direction, which ensures the reliability of the nervous system.

2. Gain. Each nerve impulse causes a release at the neuromuscular junction sufficient quantity acetylcholine to cause a spreading response in the muscle fiber. Thanks to this, nerve impulses arriving at the neuromuscular junction, no matter how weak, can cause an effector response, and this increases the sensitivity of the system.

3. Adaptation or accommodation. With continuous stimulation, the amount of transmitter released at the synapse gradually decreases until the transmitter reserves are depleted; then they say that the synapse is tired, and further transmission of signals to it is inhibited. The adaptive value of fatigue is that it prevents damage to the effector due to overexcitation. Adaptation also takes place at the receptor level. (See description in section 16.4.2.)

4. Integration. A postsynaptic neuron can receive signals from a large number of excitatory and inhibitory presynaptic neurons (synaptic convergence); in this case, the postsynaptic neuron is able to summarize signals from all presynaptic neurons. Through spatial summation, a neuron integrates signals from many sources and produces a coordinated response. At some synapses there is a facilitation in which, after each stimulus, the synapse becomes more sensitive to the next stimulus. Therefore, successive weak stimuli can evoke a response, and this phenomenon is used to increase the sensitivity of certain synapses. Facilitation cannot be considered as a temporary summation: there is a chemical change in the postsynaptic membrane, and not an electrical summation of postsynaptic membrane potentials.

5. Discrimination. Temporal summation at the synapse allows weak background impulses to be filtered out before they reach the brain. For example, exteroceptors of the skin, eyes and ears constantly receive signals from the environment that are not particularly important for the nervous system: only important for it are changes stimulus intensities, leading to an increase in the frequency of impulses, which ensures their transmission across the synapse and the appropriate response.

6. Braking. Signal transmission across synapses and neuromuscular junctions can be inhibited by certain blocking agents acting on the postsynaptic membrane (see below). Presynaptic inhibition is also possible if at the end of an axon just above a given synapse another axon ends, forming an inhibitory synapse here. When such an inhibitory synapse is stimulated, the number of synaptic vesicles discharged in the first, excitatory synapse decreases. Such a device allows you to change the effect of a given presynaptic neuron using signals coming from another neuron.

Chemical effects on the synapse and neuromuscular junction

Chemicals perform many different functions in the nervous system. The effects of some substances are widespread and well studied (such as the stimulating effects of acetylcholine and adrenaline), while the effects of others are local and not yet well understood. Some substances and their functions are given in table. 16.2.

It is believed that some medicines, used for such mental disorders, like anxiety and depression, affect chemical transmission at synapses. Many tranquilizers and sedatives (tricyclic antidepressant imipramine, reserpine, monoamine oxidase inhibitors, etc.) exert their therapeutic effect by interacting with mediators, their receptors or individual enzymes. For example, monoamine oxidase inhibitors inhibit the enzyme involved in the breakdown of adrenaline and norepinephrine, and most likely exert their therapeutic effect on depression by increasing the duration of action of these mediators. Hallucinogens type Lysergic acid diethylamide And mescaline, reproduce the action of some natural brain mediators or suppress the action of other mediators.

Recent research into the effects of certain painkillers called opiates heroin And morphine- showed that the mammalian brain contains natural (endogenous) substances that cause a similar effect. All these substances that interact with opiate receptors are collectively called endorphins. To date, many such compounds have been discovered; Of these, the best studied group of relatively small peptides called enkephalins(met-enkephalin, β-endorphin, etc.). They are believed to suppress painful sensations, affect emotions and are related to some mental illnesses.

All this has opened new avenues for studying brain function and the biochemical mechanisms underlying the effects on pain and treatment with the help of such various methods, as suggestion, hypno? and acupuncture. Many other substances such as endorphins remain to be isolated and their structure and functions to be established. With their help, it will be possible to gain a more complete understanding of the functioning of the brain, and this is only a matter of time, since methods for isolating and analyzing substances present in such small quantities are constantly being improved.

transmit information by releasing chemicals - neurotransmitters and neuromodulators. They are released from the endings of nerve cells strictly into special places of contact with other cells called synapses. This is either a section of a neighboring neuron or a muscle cell. The number of synapses is extremely large, which provides a large area for information transfer. In addition, between two cells, synaptic contact in turn can correspond to thousands of connections.

There are several types of synapses: chemical, electric And neuromuscular which is often called neuromuscular junction.

Chemical synapse

Chemical synapse has next building. At the nerve ending there is a swelling like an onion, which is called synaptic plaque. The cytoplasm of plaques contains mitochondria, some other cell organelles, but mainly synaptic vesicles. They contain a neurotransmitter, the very substance with which the nerve signal is transmitted through the synapse. The membrane of the synaptic plaque at the site of the synapse thickens and becomes thick, forming presynaptic membrane. The dendrite membrane in the synapse area is also thickened and forms postsynaptic membrane(Fig. 34). Between the two membranes there is a gap of about 20 nm wide - synaptic cleft. Neurotransmitters accumulate in synaptic vesicles, in particular acetylcholine, which then exit into the synaptic cleft. The action potential causes the simultaneous release of a neurotransmitter from many vesicles. The postsynaptic membrane contains protein molecules that act as transmitter receptors, as well as channels through which ions can enter the postsynaptic neuron.

Electric si-naps

Neuromuscular synapse (connection)

A special type of synapse is neuromuscular junction. This is a specialized connection between the ending of the motor neuron and the muscle fiber (Fig. 36). The axons of the motor neuron branch on the muscle membrane. The last one, the so-called sarcolemma, forms numerous postsynaptic folds. The endings of the motor neuron secrete cytoplasm, similar to the contents of the synaptic plaque, and during stimulation the mediator acetylcholine is released from it. The permeability of the sarcolemma surface for sodium and potassium ions changes and, as a result, local depolarization occurs. It is sufficient for the occurrence of an action potential, which causes muscle contraction.

Synapse(Greek synapsis contact, connection) - a specialized zone of contact between the processes of nerve cells and other excitable and non-excitable cells, ensuring the transmission of an information signal. Morphologically, a synapse is formed by the contacting membranes of two cells. The membrane belonging to the processes of nerve cells is called presynaptic, the membrane of the cell to which the signal is transmitted is called postsynaptic. In accordance with the affiliation of the postsynaptic membrane of the synapse, they are divided into neurosecretory, neuromuscular and interneuronal. The term “synapse” was introduced in 1897 by the English physiologist Charles Sherrington.

A synapse is a special structure that ensures the transmission of a nerve impulse from a nerve fiber to some other nerve cell or nerve fiber, also from a receptor cell to a nerve fiber (the area of ​​contact of nerve cells with each other and with another nerve cell). To form a synapse, 2 cells are required.

Synapse structure

A typical synapse is axo-dendritic chemical. Such a synapse consists of two parts: presynaptic, formed by the club-shaped extension of the axon terminal of the transmitting cell, and postsynaptic, represented by the contacting area of ​​the cytolemma of the receiving cell (in this case, the area of ​​the dendrite). A synapse is a space separating the membranes of contacting cells to which nerve endings approach.

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 cleft, the edges of which are strengthened by intercellular contacts. The part of the axolemma of the clavate extension adjacent to the synaptic cleft is called presynaptic membrane. The area of ​​the cytolemma of the receiving cell that borders the synaptic cleft on the opposite side is called postsynaptic membrane, in chemical synapses it is prominent and contains numerous receptors. In synaptic expansion there are small vesicles, so-called synaptic vesicles, containing either a mediator (a substance that mediates the transmission of excitation) or an enzyme that destroys this mediator. On the postsynaptic and presynaptic membranes there are receptors for one or another mediator.

Classifications of synapses

Depending on the mechanism of nerve impulse transmission, there are

  • chemical;
  • electric- cells are connected by highly permeable contacts using special connexons (each connexon consists of six protein subunits). The distance between cell membranes in the electrical synapse is 3.5 nm (usual intercellular distance is 20 nm); Since the resistance of the extracellular fluid is low (in this case), impulses pass through the synapse without delay. Electrical synapses are usually excitatory.
  • mixed synapses: The presynaptic action potential produces a current that depolarizes the postsynaptic membrane of a typical chemical synapse where the pre- and postsynaptic membranes are not tightly adjacent to each other. Thus, at these synapses, chemical transmission serves as a necessary reinforcing mechanism. The first type is the most common.

Chemical synapses can be classified according to their location and belonging to the corresponding structures:

  • peripheral
    • neuromuscular
    • neurosecretory (axo-vasal)
    • receptor-neuronal
  • central
    • axo-dendritic - with dendrites, incl.
    • axo-spinous - with dendritic spines, outgrowths on dendrites;
    • axo-somatic - with the bodies of neurons;
    • axo-axonal - between axons;
    • dendro-dendritic - between dendrites;

Depending on the mediator, synapses are divided into

  • aminergic, containing biogenic amines (for example, serotonin, dopamine;) o including adrenergic, containing adrenaline or norepinephrine;
  • cholinergic, containing acetylcholine;
  • purinergic, containing purines;
  • peptidergic, containing peptides. At the same time, only one transmitter is not always produced at the synapse. Usually the main pick is released along with another one that plays the role of a modulator.

By action sign:

  • stimulating
  • brake

If the former contribute to the occurrence of excitation in the postsynaptic cell (in them, as a result of the arrival of an impulse, depolarization of the membrane occurs, which can cause an action potential under certain conditions), then the latter, on the contrary, stop or prevent its occurrence and prevent further propagation of the impulse. Typically inhibitory are glycinergic (mediator - glycine) and GABAergic synapses (mediator - gamma-aminobutyric acid).

Thus, inhibitory synapses are of two types:

  1. a synapse in the presynaptic endings of which a transmitter is released, hyperpolarizing the postsynaptic membrane and causing the appearance of an inhibitory postsynaptic potential;
  2. axo-axonal synapse, providing presynaptic inhibition.

Cholinergic synapse (s. cholinergica) - a synapse in which acetylcholine is the mediator. Some synapses have a postsynaptic seal, an electron-dense zone made of proteins. Based on its presence or absence, synapses are distinguished as asymmetric and symmetric. It is known that all glutamatergic synapses are asymmetrical, while GABAergic synapses are symmetrical. In cases where several synaptic extensions come into contact with the postsynaptic membrane, multiple synapses are formed. Special forms of synapses include spiny apparatus, in which short single or multiple protrusions of the postsynaptic membrane of the dendrite contact the synaptic extension. Spine apparatuses significantly increase the number of synaptic contacts on a neuron and, consequently, the amount of information processed. Non-spine synapses are called sessile synapses. 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 fusion of synaptic vesicles with the membrane, as a result of which the transmitter enters the synaptic cleft and connects with receptor proteins of the postsynaptic membrane, which are divided into metabotropic and ionotropic. The former are associated with the G-protein and trigger a cascade of reactions of intracellular signal transmission, the latter are associated with ion channels that open when a neurotransmitter binds to them, which leads to a change in 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 transmitter in the synaptic cleft is acetylcholinesterase. At the same time, part of the transmitter can move through the postsynaptic membrane (direct uptake) and in the opposite direction through the presynaptic membrane (reverse uptake). In some cases, the mediator is also absorbed by neighboring neuroglial cells. Two release mechanisms have been discovered: with complete fusion of the vesicle with the plasmalemma and the so-called “kiss-and-run”, when the vesicle connects to the membrane, and small molecules exit from it into the synaptic cleft, while large ones remain in the vesicle . The second mechanism is presumably faster than the first, with the help of it synaptic transmission occurs when the content of calcium ions in the synaptic plaque is high. The consequence of this structure of the synapse is the one-sided conduction of the nerve impulse.

There is a so-called synaptic delay - the time required for the transmission of a nerve impulse. Its duration is 0.5 ms. The so-called “Dale principle” (one neuron - one transmitter) has been recognized as erroneous. Or, as is sometimes believed, it is more precise: not one, but several mediators can be released from one end of a cell, and their set is constant for a given cell.

Moscow PsychologicalSocial Institute (MSSI)

Abstract on the Anatomy of the Central Nervous System on the topic:

SYNAPSES(structure, structure, functions).

1st year student of the Faculty of Psychology,

group 21/1-01 Logachev A.Yu.

Teacher:

Kholodova Marina Vladimirovna.

2001


Work plan:

1.Prologue.

2. Physiology of the neuron and its structure.

3.Structure and functions of the synapse.

4.Chemical synapse.

5. Isolation of the mediator.

6. Chemical mediators and their types.

7.Epilogue.

8. List of references.


PROLOGUE:

Our body is one big clockwork mechanism. It consists of a huge number of tiny particles that are located in in strict order and each of them performs certain functions and has its own unique properties. This mechanism - the body, consists of cells, connecting their tissues and systems: all this as a whole represents a single chain, a supersystem of the body. The greatest variety of cellular elements could not work as a single whole if the body did not have a sophisticated regulatory mechanism. Plays a special role in regulation nervous system. The whole complex work of the nervous system is regulation of work internal organs, control of movements, whether simple and unconscious movements (for example, breathing) or complex movements of a person’s hands - all this, in essence, is based on the interaction of cells with each other. All this is essentially based on the transmission of a signal from one cell to another. Moreover, each cell does its own job, and sometimes has several functions. The variety of functions is provided by two factors: the way cells are connected to each other, and the way these connections are arranged.

PHYSIOLOGY OF THE NEURON AND ITS STRUCTURE:

The simplest reaction of the nervous system to an external stimulus is it's a reflex. First of all, let us consider the structure and physiology of the structural elementary unit of 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 excitation and bilateral conduction.

Nerve cells are capable of not only perceiving and processing external stimulation, but also spontaneously producing 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 impulses per second (for motor neurons) to 600-800 impulses per second (for interneurons of the brain). The axon ends in many thin branches called terminals. From the terminals, the impulse passes to other cells, directly to their bodies or, more often, to their dendritic processes. The number of terminals in an axon can reach up to one thousand, which end in different cells. On the other hand, a typical vertebrate neuron has between 1,000 and 10,000 terminals from other cells.

Dendrites - shorter and more numerous processesneurons. They perceive excitation from neighboring neurons and conduct it to the cell body. There are pulpy and non-pulpate nerve cells and fibers.

Pulp fibers are part of the sensitive andmotor nerves of skeletal muscles and sensory organsThey are covered with a lipid myelin sheath. Pulp fibers are more “fast-acting”: in such fibers with a diameter of 1-3.5 micromillimeters, excitation spreads at a speed of 3-18 m/s. This is explained by the fact that the conduction of impulses along the myelinated nerve occurs spasmodically. In this case, the action potential “jumps” through the area of ​​the nerve covered with myelin and at the node of Ranvier (the exposed area of ​​the nerve), it passes to the sheath of the axial cylinder of the nerve fiber. The myelin sheath is a good insulator and prevents the transmission of excitation to the connection of parallel nerve fibers.

Non-muscle fibers make up the bulk of the sympathetic nerves. They do not have a myelin sheath and are separated from each other by neuroglial cells.

In pulpless fibers, cells act as insulators. neuroglia(nervous supporting tissue). Schwann cells - one of the types of glial cells. In addition to internal neurons that perceive and transform impulses coming from other neurons, there are neurons that perceive influences directly from the environment - these are receptors, as well as neurons that directly affect the executive organs - effectors, for example, on muscles or glands. If a neuron acts on a muscle, it is called a motor neuron or motor neuron. Among neuroreceptors, there are 5 types of cells, depending on the type of pathogen:

- photoreceptors, which are excited under the influence of light and provide the functioning of the organs of vision,

- mechanoreceptors, those receptors that respond to mechanical influences. They are located in the organs of hearing and balance. Touch cells are also mechanoreceptors. Some mechanoreceptors are located in muscles and measure the degree of their stretch.

- chemoreceptors - selectively respond to the presence or change in concentration of various chemicals, the work of the organs of smell and taste is based on them,

- thermoreceptors, react to changes in temperature or its level - cold and heat receptors,

- electroreceptors react to current impulses, and are present in some fish, amphibians and mammals, for example, the platypus.

Based on the above, 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 flow 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 nearby cells turns silver, 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 showed that, despite the fact that the cells in the brain are located extremely close to each other, and their processes are confused, each cell is still clearly separated. That is, the brain, like other tissues, consists of individual cells that are not united into a common network. This conclusion was made by a Spanish histologist S. Ramon y Cahalem, who thereby extended the cell theory to the nervous system. The rejection of the concept of a connected network meant that in the nervous system pulse moves from cell to cell not through a straight line electrical contact, and through gap

When did the electron microscope, which was invented in 1931, begin to be used in biology? M. Knollem And E. Ruska, these ideas about the presence of a gap received direct confirmation.

STRUCTURE AND FUNCTION OF SYNAPSE:

Every multicellular organism, every tissue consisting of cells needs mechanisms that ensure intercellular interactions. Let's look at how they are carried out interneuronalinteractions. Information travels along a nerve cell 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 Greek "Synapsis"- connection, communication). The concept of synapse was introduced by the English physiologist C. Sherrington in 1897, to denote the functional contact between neurons. It should be noted that back in the 60s of the last century THEM. Sechenov emphasized that without intercellular communication it is impossible to explain the methods of origin of even the most elementary nervous process. The more complex the nervous system is, and the larger number components of the nerve brain elements, the more important the importance of synaptic contacts becomes.

Different synaptic contacts differ from each other. However, with all the diversity of synapses, there are certain common properties of their structure and function. Therefore, we first describe general principles their functioning.

Synapse - is a complex structural a formation consisting of a presynaptic membrane (most often this is the terminal branch 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 across synapses remained unclear for a long time, although it was obvious that signal transmission 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 either electric or chemically. The electrical theory of synaptic transmission in the central nervous system was recognized until the early 50s, but it lost ground significantly after the chemical synapse was demonstrated in a number of cases. peripheral synapses. So, 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 synaptic potentials


CNS neurons allowed us to draw a conclusion about the chemical nature of transmission in interneuronal synapses of the spinal cord.

Microelectrode studies recent years showed that at certain interneuron synapses there is an electrical transmission mechanism. It has now become obvious that there are synapses with both a chemical transmission mechanism and an electrical one. Moreover, in some synaptic structures both electrical and chemical transmission mechanisms function together - these are the so-called mixed synapses.

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