Synapses in the nervous system

The concept of "synapse" was introduced at the end of the XIX century. C. Sherrington, implying by this term a structure that mediates the transmission of a signal from the end of the axon to the effector - the neuron, the muscle fiber, the secretory cell. During the study of synapses by morphologists, physiologists, biochemists and pharmacologists, their considerable diversity was revealed, while common features in structure and function were revealed; as a result, the principles of classification of synapses were developed.

The morphological principle of the classification of synapses takes into account what parts of the two cells they are formed and how they are located on the surface of the perceiving neuron (on the body of the cell, on the trunk or spine of the dendrite, on the axon itself). Accordingly, there are synapses axo-axonal, axo-dendritic, axo-somatic synapses. However, this classification does not explain either the functional role or the mechanism of operation of the synapse.

Morphological structure of the synapse

Morphologically the synapse is a structure of two demyelinated formations - the thickened synaptic end (synaptic plaque) at the end of the axon and the membrane segment of the innervated cell, through the synaptic cleft contacting the presynaptic membrane. The main function of the synapse is signal transmission. Depending on the method of signal transmission, chemical, electrical and mixed synapses are isolated. They differ according to the principle of work.

The mechanism of excitation in the electrical synapse is similar to the mechanism of excitation in the nerve fiber - PD presynaptic endings provides depolarization of the postsynaptic membrane. Such excitation transfer is possible due to the peculiarities of the structure of the synapses of this type-a narrow (about 5 nm) synaptic cleft, a large area of membrane contact, the presence of transverse tubules connecting the presynaptic and postsynaptic membranes and reducing the electrical resistance in the contact region. The most common electrical synapses in invertebrates and inferior vertebrates. In mammals, they are found in the mesenchyphal nucleus of the trigeminal nerve between neuronal bodies, in the Deiters vestibular nucleus between the body cells and the axon endings and between the "spinules" of the dendrites in the lower olive. Electrical synapses are formed between nerve cells of the same type in structure and function.

For electric synaptic transmission, there is a lack of synaptic delay, signaling in both directions, independence of signal transmission from the potential of the presynaptic membrane, resistance to changes in Ca2 + concentration, low temperature, some pharmacological effects, and low fatigue, since signaling does not require significant metabolic costs. In most such synapses, a "rectification effect" is observed, when the signal in the synapse is transmitted in only one direction.

Unlike electric synapses with direct excitation transfer, in a much larger number in the nervous system of vertebrates are represented chemical synapses (synapses with indirect signal transmission). In the chemical synapse, a nerve impulse causes the release of a chemical mediator from the presynaptic endings, a neurotransmitter that diffuses through the synaptic cleft (10-50 nm in width) and interacts with receptor proteins of the postsynaptic membrane, resulting in the generation of a postsynaptic potential. Chemical transmission provides a one-way signal and the possibility of its modulation (signal amplification, as well as the convergence of many signals on a single postsynaptic cell). The possibility of modulation in the process of signal transmission in synapses of chemical type provides the formation on their basis of complex physiological functions (training, memory, etc.). A wide synaptic cleft, the presence of a vesicle filled with a mediator with which the signal is transmitted, and in the postsynaptic - numerous chemosensitive channels (in the exciting synapse - for Na +, in the inhibitory synapse for Cl), can be attributed to the peculiarities of the chemical synapse's ultrastructure. Such synapses are characterized by a delay in the signal and a large fatigue compared with the electrical synapse, since their functioning requires considerable metabolic costs.

There are two main subtypes of chemical synapses

The first (the so-called asymmetric) is characterized by a synaptic gap of about 30 nm in width, a relatively large contact zone (1-2 μm), a significant accumulation of a dense matrix under the postsynaptic membrane. In the presynaptic plaque, large vesicles accumulate (30-60 nm in diameter). Chemical synapses of the second subtype have a synaptic cleft of about 20 nm in width, a relatively small contact zone (less than 1 μm), moderately expressed and symmetric membrane seals. They are characterized by small vesicles (diameter 10-30 nm). The first subtype is represented mainly by axodendritic, stimulating (glutamatergic), the second - by axosomatic, inhibitory (GABA-ergic) synapses. However, this division is relatively arbitrary, since cholinergic synapses have light vesicles 20-40 nm in diameter on electron micrographs, and monoaminergic (especially with norepinephrine) - large dense vesicles with a diameter of 50-90 nm.

Another principle of classification of synapses is for a substance used as a mediator (cholinergic, adrenergic, purinergic, peptidergic, etc.). Despite the fact that in recent years it has been shown that mediators of different nature can function in one end, this classification of synapses is still widely used.