A synapse is a specific contact zone between the processes of nerve cells and other non-excitable and excitable cells, which ensure the transmission of an information signal. The synapse is morphologically formed by the contacting membranes of 2 cells. The membrane related to the process of nerve cells is called the presynaptic membrane of the cell into which the signal enters, its second name is postsynaptic. Together with 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 an nerve fiber to be exposed to from a receptor cell (the area of contact between nerve cells and another nerve fiber), two nerve cells are required .
The synapse is a small section at the end of a neuron. With his help, information is transferred from the first neuron to the second. The synapse is located in three sections of nerve cells. Also, synapses are located at the place where the nerve cell comes into contact with various glands or muscles of the body.
What synapse consists of
The synapse structure has a simple outline. It is formed of 3 parts, in each of which certain functions are carried out during the transmission of information. Thus, this synapse structure can be called suitable for transmission of a nerve impulse. Two main cells act directly on the process of information transfer : 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 triggering of neurotransmitters in the cell (this word has several meanings: mediators, mediators or neurotransmitters) - certain chemicals with the help of which an electric signal is transmitted between 2 neurons.
The synaptic cleft is the middle part of the synapse - this is the gap between the 2 interacting nerve cells. Through this gap, an electrical impulse comes from the transmitting cell. The final part of the synapse is the receptive 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 Latin Media - transmitter, intermediary or middle). Such synapse mediators are very important in the process of transmission of a nerve impulse.
The morphological difference between the inhibitory and excitatory synapse is that they do not have a release mechanism for the mediator. The mediator in the inhibitory synapse, motoneuron and other inhibitory synapse is considered the amino acid glycine. But the inhibitory or exciting nature of the synapse is determined not by their mediators, but by the property of the postsynaptic membrane. For example, acetylcholine gives an exciting effect in the neuromuscular synapse of the terminals (vagus nerves in the myocardium).
Acetylcholine serves as an exciting mediator in cholinergic synapses (the presynaptic membrane in it plays the end of the spinal cord of a motor neuron), in the synapse on Renshaw cells, in the presynaptic terminal of the sweat glands, the medulla of the adrenal glands, in the synapse of the intestine and in the ganglia of the sympathetic nerve. Acetylcholesterase and acetylcholine were also found in fractions of different parts of the brain, sometimes in large numbers, but apart from the cholinergic synapse on Renshaw cells, the rest of the cholinergic synapses have not yet been able to identify. According to scientists, the mediator excitatory function of acetylcholine in the central nervous system is very likely.
Katelchomins (dopamine, norepinephrine and adrenaline) are considered adrenergic mediators. Adrenaline and norepinephrine are synthesized at the end of the sympathetic nerve, in the cell of the brain 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. An intermediate substance, which includes norepinephrine and dopamine, also perform the function of mediators in the synapse created at the ends of the sympathetic nerves. This function can be either inhibitory (secretory glands of the intestine, 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 synapse mediators complete their function, catecholamine is absorbed by the presynaptic nerve ending, and transmembrane transport is activated. During mediator uptake, synapses are protected from premature depletion during long and rhythmic work.
Synapse: main types and functions
In 1892, Langley suggested that the synaptic transmission of mammalian vegetative ganglia is not of an electrical nature, but of a chemical nature. After 10 years, Eliott found that adrenaline from the adrenal glands comes from the same effects as stimulation of the sympathetic nerves.
After this, it was suggested that adrenaline is able to be secreted by neurons and, when excited, is secreted by the nerve ending. But in 1921, Levy 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 of the
frog’s heart with saline and stimulated the vagus nerve, slowing the heartbeat. When fluid was transferred from inhibited heart stimulation to an unstimulated heart, it beat more slowly. It is clear that vagus nerve stimulation caused the release of a inhibitory substance into the solution. Acetylcholine completely reproduced the effect of this substance. In 1930, Feldberg and his collaborator finally established the role in the synaptic transmission of acetylcholine in the ganglion of the
autonomic nervous system .
Chemical synapse
The chemical synapse is fundamentally different from the transmission of irritation with the help of a mediator from presynapse to postsynapse. Therefore, differences are formed in the morphology of the chemical synapse. Chemical synapse is more common in the vertebral central nervous system. It is now known that a neuron is able to isolate and synthesize a pair of mediators (coexisting mediators). Neurons also have neurotransmitter plasticity - the ability to change the main mediator during development.
Neuromuscular synapse
This synapse transmits excitation, however, various factors can destroy this connection. Transmission ends during the blockade of ejection of acetylcholine into the synaptic cleft, also during an excess of its content in the area of postsynaptic membranes. Many poisons and drugs affect the seizure, the output that 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 stopping the contraction of the respiratory muscles.
Botulinus is a microbial toxin in the synapse; it blocks the transmission of excitation, destroying the syntaxin protein in the presynaptic terminal, which is controlled by the release of acetylcholine into the synaptic cleft. Several toxic warfare agents, pharmacological agents (neostigmine and proserin), as well as insecticides block excitation in the neuromuscular synapse by inactivating acetylcholinesterase, an enzyme that destroys acetylcholine. Therefore, acetylcholine accumulates in the area of the postsynaptic membrane, sensitivity to the mediator decreases, the postsynaptic membranes are released and the receptor block is immersed in the cytosol. Acetylcholine will be ineffective and the synapse will be blocked.
Nerve synapse: features and components
A synapse is a junction between two cells. Moreover, each of them is enclosed in its own electrogenic membrane. The nerve synapse consists of three main components: the postsynaptic membrane, the synaptic cleft and the presynaptic membrane. The postsynaptic membrane is a nerve ending that extends to the muscle and descends into the muscle tissue. In the presynaptic region there are vesicles - these are closed cavities with a mediator. They are always in motion.
Approaching the membrane of nerve endings, the vesicles merge with it, and the mediator 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 is converted to acetylCoA under the influence of the enzyme acetylcholine transferase).
Synaptic cleft among post- and presynaptic membranes
In different synapses, the size of the gap is different. This space is filled with intercellular fluid, in which there is a mediator. The postsynaptic membrane covers the site of contact of the nerve ending with the innervated cell at the mioneural synapse. In certain synapses, the postsynaptic membrane creates a fold, and the contact area increases.
Additional substances that make up the postsynaptic membrane
The following substances are present in the area of the postsynaptic membrane:
- Receptor (cholinergic receptor in the mioneural synapse).
- Lipoprotein (has a great similarity with acetylcholine). This protein has an electrophilic end and an ion head. The head enters the synaptic cleft, interacting 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 is destroyed by 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 trophic 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 a mediator, 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.