A number of neurons occur in different parts of the nervous system.
These are classified as follows:
a. Unipolar neurons:
In these neurons, a single axon passes from cyton. There are no dendrites. Unipolar neurons are rarely found in the central nervous system of the adults, but they are present during the early developmental stages.
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b. Pseudounipolar neurons:
These cells are first typically bipolar and spindle shaped but as the development proceeds the two processes converge until they meet at one side of the body. This then elongates so that a fine process is formed with a T shaped division at the end. Pseudounipolar neurons are formed in all spinal ganglia and in the ganglia of cranial nerves except the VIIJ nerve.
c. Bi-polar neurons:
These cells have a single dendrite and a single axon projecting from opposite ends of the cells. Such neurons are present in the retina of the eye, in the olfactory epithelium and in the ganglia of the VIII cranial nerve.
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d. Multipolar neurons:
These occur most frequently when compared to other neurons. As the very name indicates they have a number of processes. Based on the number, arrangement and length of the processes they are categorised into two types viz., Golgi type I and Golgi type II. In Golgi type I there is a long single axon and numerous short dendrites, while in Golgi type II the axon is very short. The body of the multipolar neurons is located in the grey matter of the brain and spinal cord. The Golgi II cells are confined to the central nervous system but the axons of the Golgi type I form the motor components of spinal nerves.