The structural and functional unit of the nervous system is the nerve cell, or neuron, or neurocyte. In other words, each neuron is the elementary structure that is capable of perceiving stimulation and excitation, as well as transmitting excitation in the form of a nerve impulse to neighboring neurons or innervated organs and muscles.
The entire nervous system can be represented as an interconnected and interacting network of several trillion nerve cells. Despite their enormous diversity, we can talk about a number of common structural and functional signs inherent in all nerve cells.
The following main parts are distinguished in the neuron: the body, the processes, and their endings.
The body of a neuron, whose dimensions range from 4 to 130 microns, is a cluster of cell plasma in which the nucleus is the carrier of genetic information, mitochondria ( universal “generators” of energy necessary to support cell activity), and a large number of structures performing various specific functions. The surface of a neuron, its membrane, often referred to simply as a membrane, not only provides an exchange with the environment but also has the properties of a semi-permeable membrane. It has the structure where complex bio-electrogenesis processes underlying the main functions of the nerve cell development.
The processes of the nerve cells are the outgrowths of the cytoplasm. There are two types of processes, dendrites, and axon. Dendrites are short branching, gradually thinning and ending in surrounding tissues. The number of them reaches ten, and they increase the surface of the cell. In addition to dendrites, the nerve cell always has one axon (or neurite). This process is always larger, and less branched. Axon ends in a synapse, by means of which it interacts functionally with innervated structures. By its functional significance, the composition of the reflex arc distinguishes three types of neurons:
- Receptor (sensory or afferent), having sensitive nerve endings that are capable of perceiving irritations from the external or internal environment.
- Effector (efferent), the end of the axons which transmit the nerve signal to the working organ.
- Associative (intercalary or central), which acts as the intermediate in the reflex arc and transmit information from the sensory neuron to the effector neuron.
It should be borne in mind that on the body and sprouts of most nerve cells there is a very large number of synapses through which information from other neurons comes in. Despite the huge morphological and functional diversity of neurons, it is possible to identify a number of key properties and functions.
Among the most important properties are:
- The presence of a transmembrane potential difference, that is, between the outer and inner surfaces of the neuron envelope, a potential difference of the order of 90 mV is recorded at rest, the outer surface is electropositive with respect to the internal.The magnitude and direction of the transmembrane current vary with the state of the neuron.
- Very high sensitivity to certain chemicals (mediators) and electric current.
- The ability to neurosecretion, i.e., to the synthesis and release of biologically active substances into the environment or into the synaptic cleft.
- High level of energy processes, which necessitates a constant inflow of the main energy source – glucose and oxygen, necessary for oxidation.