Synapses in the nervous system

The concept of "synapse" was introduced at the end of the 19th century by C. Sherrington, who meant by this term a structure that mediates the transmission of a signal from the end of an axon to an effector - a neuron, muscle fiber, secretory cell. In the course of studying synapses, morphologists, physiologists, biochemists and pharmacologists revealed their significant diversity, while common features in structure and functioning were discovered; as a result, principles for classifying synapses were developed.

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

Morphological structure of the synapse

Morphologically, a synapse is a structure of two demyelinated formations - a thickened synaptic ending (synaptic plaque) at the end of the acton and a section of the membrane of the innervated cell, through the synaptic cleft in contact with the presynaptic membrane. The main function of the synapse is to transmit a signal. Depending on the method of signal transmission, chemical, electrical and mixed synapses are distinguished. They differ in the principle of operation.

The mechanism of excitation conduction in an electrical synapse is similar to the mechanism of excitation conduction in a nerve fiber - AP of presynaptic endings ensures depolarization of the postsynaptic membrane. Such excitation transmission is possible due to the structural features of synapses of this type - a narrow (about 5 nm) synaptic cleft, a large area of membrane contact, the presence of transverse canals connecting the presynaptic and postsynaptic membranes and reducing electrical resistance in the contact area. Electrical synapses are most common in invertebrates and lower vertebrates. In mammals, they are found in the mesencephalic nucleus of the trigeminal nerve between the bodies of neurons, in the vestibular nucleus of Deiters between cell bodies and axon endings, and between the "spines" of dendrites in the inferior olive. Electrical synapses are formed between nerve cells of the same type in structure and function.

Electrical synaptic transmission is characterized by the absence of synaptic delay, signal transmission in both directions, independence of signal transmission from the presynaptic membrane potential, resistance to changes in Ca2+ concentration, low temperature, some pharmacological effects, and low fatigue, since signal transmission 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.

In contrast to electrical synapses with direct transmission of excitation, chemical synapses (synapses with indirect signal transmission) are present in much greater numbers in the nervous system of vertebrates. In a chemical synapse, a nerve impulse causes the release of a chemical messenger from the presynaptic endings - a neurotransmitter, which diffuses through the synaptic cleft (10-50 nm wide) and interacts with receptor proteins of the postsynaptic membrane, resulting in the generation of a postsynaptic potential. Chemical transmission ensures one-way signal transmission and the possibility of its modulation (signal amplification, as well as convergence of many signals on one postsynaptic cell). The possibility of modulation in the process of signal transmission in chemical synapses ensures the formation of complex physiological functions on their basis (learning, memory, etc.). The ultrastructure of a chemical synapse is characterized by a wide synaptic cleft, the presence of vesicles in the synaptic plaque filled with a mediator that transmits a signal, and in the postsynaptic plaque, numerous chemosensitive channels (in the excitatory synapse - for Na+, in the inhibitory synapse - for Cl). Such synapses are characterized by a delay in signal transmission and greater fatigue compared to an electrical synapse, since their functioning requires significant metabolic costs.

There are two main subtypes of chemical synapses.

The first (the so-called asymmetric) is characterized by a synaptic cleft about 30 nm wide, a relatively large contact zone (1-2 μm), and significant accumulation of dense matrix under the postsynaptic membrane. Large vesicles (30-60 nm in diameter) accumulate in the presynaptic plaque. Chemical synapses of the second subtype have a synaptic cleft about 20 nm wide, a relatively small contact zone (less than 1 μm), and moderately pronounced and symmetrical membrane compaction. They are characterized by small vesicles (10-30 nm in diameter). The first subtype is represented mainly by axodendritic, excitatory (glutamatergic), the second by axosomatic, inhibitory (GABAergic) synapses. However, this division is rather arbitrary, since cholinergic synapses are found in electron micrographs as light vesicles with a diameter of 20-40 nm, while monoaminergic synapses (especially with norepinephrine) are found as large dense vesicles with a diameter of 50-90 nm.

Another principle of synapse classification is by the 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 ending, this classification of synapses is still widely used.