The axon is one of two types of protoplasmic protrusions of the neuronal soma. Moreover, they have demonstrated significant changes in the diameter along the single axon. The variation of the diameter has important functional implications since the speed of propagation of the impulse (i.e., action potential), besides being dependent on the presence of the myelin sheath, is directly proportional to the diameter of the axon. The diameter is variable as it ranges between 1 and 25 micrometers. Histological observation of axon shows a cylindrical structure, but recent 3D electron microscopy studies demonstrated that probably axon has not the shape of a perfect cylinder. Furthermore, compared to projecting neurons, interneurons usually have smaller soma, fewer organelles, and a reduced amount of cytoplasm (axoplasm). The longest axons of the human body are those that make up the sciatic nerve where the length can exceed one meter. In contrast, interneurons, that work within local circuits, have a short axonal terminal (up to several mm). Considering the functional distinction between projection neurons and interneurons, cortical projection neurons (CPNs), also termed as pyramidal neurons and spinal cord projection neurons (dorsal horn neurons), usually have long axons (from several mm and up to 1 m). Functionally, it carries electrical impulses and projects to synapses with dendrites or cell bodies of other neurons or with non-neuronal targets such as muscle fibers.Ĭoncerning length, the length of axons varies according to the function of the neuron. In size, the axon may represent over 95% of the total volume of the neuron.
Axons are the elongated portion of the neuron located in the center of the cell between the soma and axon terminals. In 1860, the German anatomist Otto Friedrich Karl Deiters (1834-1863) described the basic structure of the nerve cell and identified two different protoplasmatic protrusions of the cell body that he termed as "axis cylinder," and "protoplasmatic processes," respectively axons and dendrites.
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