TYPES OF NEURONS -
The neurons are classified on the basis of their structure and function.
(A) On the basis of the structure, the neurons are of four types.
(i) Nonpolar or Unpolarized Neurons - Each neuron bears several branched processes. There is no functional difference between dendrites and axon. Each process can bring an impulse to the cyton, or can take it away from the cyton. e.g. cnidarians (= coelenterates) such as Hydra.
(ii) Unipolar Neurons. Each neuron has a single process. The process divides into two further processes. One is axon and the other is a dendrite. This neuron is described as unipolar, but from a functional point of view it is to be regarded as bipolar. e.g. Most sensory neurons are unipolar. They occur in the dorsal root ganglia of spinal nerves and granule cells of olfactory bulbs (part of olfactory lobes of the brain).
(iii) Bipolar Neurons. These neurons have only two processes, an axon at one end and a dendrite at another end.
Bipolar neurons are found in the retina of eyes, olfactory epithelium and cochlear and vestibular ganglia (cochlea and vestibule are the parts of membranous labyrinth of internal ear).
(iv) Multipolar Neurons. These neurons have several dendrites and an axon. Motor neurons and interneurons are multipolar. They are the most common type of neurons. Multipolar neurons occur in the grey matter of the brain and spinal cord.
(B) On the basis of function, neurons are of three types.
(i) Sensory (= Receptor or Afferent) Neurons. They connect sense organs with the central nervous system (brain and spinal cord). They bring sensory impulse from sense organs to the central nervous system.
(ii) Motor (= Effector or Efferent) Neurons. They connect the central nervous system to the effectors (muscles and glands). They carry motor impulses from the central nervous system to the effectors.
(iii) Interneurons (= Connector, Relaying Or Adjustor Neurons). They are present in the central nervous system and occur between the sensory and motor neurons for distant transmission of impulses. They are neither sensory nor motor.