Synapse is a certain zone of contact between 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 outgrowth of nerve cells 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 from the receptor cell (the area where nerve cells and another nerve fiber come into contact with each other), requires two nerve cells.
A synapse is a small section at the end of a neuron. It helps to transfer informationfrom the first neuron to the second. The synapse is located in three areas of nerve cells. Synapses are also located where a nerve cell connects with various glands or muscles in the body.
What is the 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 the transmission of a nerve impulse. Two main cells directly affect the process of information transfer: 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) - certain chemicals with the help of 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 end part of the synapse is the receptive part of the cell, which is the postsynaptic ending (the contacting cell fragment 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 process of nerve impulse transmission.
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 large quantities, 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.
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 performthe function of neurotransmitters 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, in the myocardial synapse - norepinephrine, in the subcutaneous nuclei of the brain - dopamine).
When the neurotransmitters of the synapse 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 electrical in nature, but chemical. 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 frog's heart vessels with saline and stimulated the vagus nerve, creating a slow heart rate. When the fluid was transferred from the inhibited pacing of the heart to the unstimulated heart, it beat more slowly. It is clear that stimulation of the vagus nerve causedrelease into the solution of the inhibitory substance. Acetylcholine fully reproduced the effect of this substance. In 1930, the role in the synaptic transmission of acetylcholine in the ganglion of the autonomic nervous system 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 releasing and synthesizing a pair of mediators (coexisting mediators). Neurons also have neurotransmitter plasticity - the ability to change the main neurotransmitter during development.
Neuromuscular junction
This synapse carries out the transmission of excitation, but this connection can be destroyed by various factors. The transmission ends during the blockade of the ejection of acetylcholine into the synaptic cleft, as well as during the 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 stop 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. Somepoisonous combat substances, pharmacological drugs (neostigmine and prozerin), as well as insecticides block the conduction of excitation to the neuromuscular synapse 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.
Nerve synapse: 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. The synapse is made up of three main components: the postsynaptic membrane, the synaptic cleft, and the 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 a mediator and mitochondria (they are needed for the synthesis of a 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. This spacefilled 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, increasing the contact area.
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 (very similar to 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, which is 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.
