Nerve cells usually have only one axon which arises typically from the cell
body but may emerge from a proximal dendrite (the end of a dendrite closest to
the soma). In either case, the site of origin is termed the axon hillock. Axons
have diameters ranging from 0.2 to 20 μm in humans (though axons of invertebrates
can reach 1 mm) and vary in length from a few μm to over a meter. They
may be encapsulated in a myelin sheath. Axons usually branch, particularly at
their distal end (furthest from the soma). These branches are referred to as axon
collaterals. The ends of an axon are swollen terminals (or boutons) and usually
contain mitochondria and vesicles. Some axons have a tuft of branches (a
terminal arbor) at their tip, each with its terminal bouton, some have boutons
along their length where they are described as varicosities. Axon terminals form
the presynaptic component of chemical synapses.