The simplest components of the nervous system are cells called neurons. They are in many ways similar to other cells of the human body, such as blood cells or muscle cells, but they have a unique feature – they can communicate with each other. To understand the nature of the process of transmission of nerve impulses need to consider the unique structural properties of neurons. From Figure 3-1 it can be seen that there are cellular bodies in a neuron that are similar to the bodies of any other cells. Among them is the nucleus containing the genetic information for a given neuron and controlling the metabolic processes in the cell. Several related formations, called dendrites, and one long cylindrical formation, the axon, depart from the neuron cell body. Such formations have only a neuron, and their specific functions are associated with them.
Axons are of different lengths, but in any case they are longer than shown in the figure, sometimes several thousand times longer than the diameter of the cell body. The axon is covered with a white fat coat called myelin. (Not all axons are covered with such a shell, and the “open” ones are gray). Myelin can be compared to electrical wire insulation. This is a suitable comparison, since the main function of the axon is to transmit electrical current. Axon transmits information by transferring electrical charge from one end of the neuron to the other. The current is always transmitted from the cell body, which sends an electrical impulse to small branches at the end of the axon. The difference in potential is small (about 110 millivolts). When an axon conducts a current, it is called excited, when not – in a state of rest.