An axon is a long, slender projection of a nerve cell, or neuron that conducts electrical impulses away from the neuron’s cell body or soma. An axon is one of two types of protoplasmic protrusions that extrude from the cell body of a neuron, the other type being dendrites.
Axons are distinguished from dendrites by several features, including shape (dendrites often taper while axons usually maintain a constant radius), length (dendrites are restricted to a small region around the cell body while axons can be much longer), and function (dendrites usually receive signals while axons usually transmit them). All of these rules have exceptions, however.
Some types of neurons have no axon and transmit signals from their dendrites. No neuron ever has more than one axon; however in invertebrates such as insects or leeches the axon sometimes consists of several regions that function more or less independently of each other. Most axons branch, in some cases very profusely.
Axons are in effect the primary transmission lines of the nervous system and as bundles they help make up nerves. Individual axons are microscopic in diameter (typically about 1μm across), but may be up to several feet in length.
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The longest axons in the human body, for example, are those of the sciatic nerve, which run from the base of the spine to the big toe of each foot. Peripheral nerve fibers can be classified based on axonal conduction velocity, mylenation, fiber size etc. Different sensory receptors are innervated by different types of nerve fibers.
Proprioceptors are innervated by type Ia, Ib and II sensory fibers, mechanoreceptors by type II and III sensory fibers and nociceptors and thermo receptors by type III and IV sensory fibers. Growing axons move through their environment via the growth cone, which is at the tip of the axon.
The growth cone has a broad sheet like extension called lamellipodia which contain protrusions called filopodia. The filopodia are the mechanism by which the entire process adheres to surfaces and explores the surrounding environment, nerve fibers are gray.