In general, there are three major classes of neurons: sensory neurons, motor neurons, and inter-neurons. Sensory neurons, which are also called afferent neurons, carry messages from the sensory receptors toward the central nervous system to be finally interpreted by the brain. The receptor cells are specialized sensory neurons that are sensitive to the external stimuli such as light and sound.
The motor neurons, also called efferent neurons carry the messages away from the central nervous system toward the muscles and glands. They are responsible for all our actions, movements, and responses. Usually the motor neurons are long.
The sensory neurons rarely come in contact with the motor neurons. They rely on the network of inter-neurons, which are mainly located in the central nervous system with 85 per cent being in the brain. They receive messages from the sensory neurons, and relay the same to other inter-neurons or to motor neurons. It is estimated that for every motor neuron, there are as many as 5000 inter-neurons, which do the intermediate task.
Supporting Cells. Neurons constitute only about half the volumes of the central nervous system. The rest consists of a variety of supporting cells. Because neurons have a high rate of metabolism but have no means of storing nutrients, they must constantly be supplied with nutrients and oxygen or they will quickly die. Thus, the cells that support and protect neurons are very important.
The most important supporting cells of the central nervous system are the glia cells. These cells surround neurons and hold them in place. They also supply some chemicals to the neurons, which they need, to exchange messages with other neurons. They also insulate neurons from one another, so that neural messages do not get scrambled.
Two of the most important glia cells are astrocytes and oligodendroglia. Astrocytes produce chemicals that neurons need to fulfill their functions. In addition, astrocytes help control the chemical composition of the fluid surrounding neurons. It is very important to note that the chemical composition around the neuron must be kept within a critical level for the smooth transmission of neural impulses. On the other hand, the principal function of oligodendrogfia is to provide support to axons to produce myelin sheath.