In unipolar neurons, in which the single process bifurcates into a peripheral and a central branch, both branches are structurally axon-like. In bipolar neurons, the effector and receptor portions of the neuron are found at the extreme ends of the two processes, and the entire intermediate portion is conductive. It is estimated that some neurons receive as many as 100,000 axon terminals on their dendritic expansion. The region of origin of the axon from a nerve cell is termed the axon hillock and is devoid of Nissl substance. It is the most excitable part of the neuron and the site at which the nerve impulse is initiated.
Axons invariably acquire sheaths along their course.
The axon and its sheath are referred to as a nerve fiber. Nerve fibers may be myelinated or unmyelinated. Myelin sheaths are elaborated and maintained by oligodendroglia in the central nervous system, and by Schwann cells in the peripheral nervous system. The relationship of such nerve fibers to oligodendroglia in the central nervous system is not quite so intimate. The nodes are the site of voltage-gated sodium channels and ionic movement of impulse conduction.