Neuron also known as nerve cell is the structural and functional unit of nervous system. They are the longest cell of human body. It is similar to all the other cell in the body in having nucleus and all the other cell organelles in cytoplasm but it differs from the others cell by two ways:- (i) it has branches of process is called axons and dendrites. (ii) it cannot undergo division as it does not have centrosome. A single nerve cell form synapse with other neuron cell, i.e., they are not arranged in layers and they conduct impulses end to end. A neuron is composed of three major parts:- Cell body or soma, dendrites, axons.
On the basis of presence or absence of the myelin sheath, axons are of two types:- (a) Myelinated axons:- In the human body, many neurons are involved with the lipid-rich sheath called myelin sheath. Myelinated axons are the axons which possesses the myelin sheath around them. They are found in spinal and cranial nerves. It is not a continuous sheath. Between the adjacent sheath of a single axon on certain gaps are present. Between the two adjacent myelin sheath adjacent mylan shee the gap which are present is known as Nodes of Ranvier. (ii) Non-myelinated axons/ Unmyleinated axons:- They are enclosed by the schwann cells which do not produce any myelin sheath but forms neurilemma. The velocity of impulse is decided by the myelin sheath and not by any other factor.