The process or projection of greater length than the dendron or dendrites coming out from the cell body of the neuron is known as the axon. It is usually single, long, slender process, sometimes branched.
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It is branched at its end and these axonal endings, now called telodendric, make connections with other neurons or with effector cells.
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They exist in a variety of types. Some endings are knob-like structures called synaptic knobs orboutons terminaux, others are filamentous, ribbon-like endings ; still others have a single specialized ending such as the end plate found at junctions with skeletal muscles. Many axons have simple, morphologically undifferentiated endings.
The protoplasm of the axon is called axoplasm and at axonal endings, may contain concentrations of mitochondria, numerous small membrane-bounded vesicles, and a collection of enzymes associated with the chemical transmitters.
In addition to the plasma membrane, the axon is covered over by a myelin or medullary sheath.
This sheath varies in thickness in different neurons and is composed of non-living lipid and protein formed by the Schwann cells.
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This sheath does not form a continuous cylindrical envelope but instead it is broken up at regular intervals by means of definite constrictions, the nodes of Ranvier.
The region between two nodes is known as the internode. Outside the myelin or medullary sheath there is one more very delicate covering known as the neurilemma or sheath of Schwann.
Presumably one Schwann cell provides the sheath for each internode region. This sheath contains nuclei at regular intervals.
Axons of the neurons, on the basis of the vertebrate motor neuron, were defined as processes that carry impulses away from the cell body but now this definition is true only for a few neuron types.