Axon
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The other type of process in the idealized neuron is the axon. Each neuron has only one axon and it is usually straighter and smoother than the dendritic profiles. Axons also contain bundles of
microtubules and
neurofilaments and scattered
mitochondria. The most MAPs in an axon have a lower molecular weight than those in the dendrite. A predominant
MAP in axons is
tau.
Microfilaments within the axon are usually associated with an area adjacent to the plasmalemma and often are the most dense at the
nodes of Ranvier. Beyond the initial segments, the axoplasm lacks rough endoplasmic reticulum and free ribosomes. The branches of axons are known as
axon collaterales. The axon itself is often surrounded by a membranous material, called the myelin sheath, formed by glia cells. The myelin sheath acts to insulate the plasmalemma of the axon in a way that necessitates the more rapid spread of the depolarization of the
plasmalemma and increases the speed of conduction of the nerve impulse (see
Chapter 3).
