The cells whose job it is to keep our body informed of conditions in the outer world are the nerve cells. In lower forms of life, nerve cells are located in the skin and they directly transmit messages to the deeper parts of the body. But in human beings and other complex organisms, most of the nerve cells are actually the body, though they may pick up their “messages” in the skin by means of delicate “antennae”.
The purpose of the nerve cells is to transmit messages throughout the body, each message to the proper place. The nerve fibres along which these messages go are constructed like a cable, and are amazingly efficient.
Actually, there are four chief types of’ nerve cells, or “nerves’, or nerve units. These are the completely independent units of nerve cells in the body, each organized to do its special job. One type receives messages such as heat, cold, light, and pain from the outer world, and conducts them to the interior of the body. These might be called “the sensory units”.
Another type might call “the motor unit”. It receives impulses from the sensory units and responds to them by sending a nerve current to various structures in the body, such as the muscles and the glands. The reaction that results is called a “reflex”. A “heat message”, for example, would make certain muscles react and pull a hand away from a hot surface.
A third type of nerve unit does a connecting job. It transmits mes- sages over longer distances in the body. It connects motor cells in one part of the body with Sensory cells in another part.
The fourth type of nerve unit has the job of carrying messages from the outer world, such as cold, heat, and pain, to the brain where we “translate” the message into feeling.
What Are Nerves Made Of?
Nerves are composed of myelinated and unmyelinated axons, Schwann cells surrounded by connective tissue.