All neurons have a cytoplasmic process called an axon (= nerve fiber), which usually attaches to one side of the cell body via a cone-shaped axon hillock. Several proteins embedded in the cell membrane (= axolemma) allow the axon to conduct action potentials (= electro-chemical impulses). Action potentials begin just after the axon hillock, in a sensitive section of the axon known as the initial segment (= the trigger zone). Axons vary in length from a few millimeters to just over a meter. They are sparsely branched compared with most dendrites. However, they often gives off one or more large collateral branches that lead to other cells. Before ending, the axon and its collaterals split into clusters of small, thin terminal branches (= telodendria, terminal arbor, terminal arborization). The tips of the terminal branches expand into rounded synaptic knobs, (= end bulbs, axon terminals, synaptic terminals, terminal boutons (buttons)). In the synaptic knobs are vesicles that store and release neurotransmitter molecules. Action potentials are generated by threshold level signals from the dendrites and cells body. Once begun, they travel the length the axon and its collaterals to the synaptic knobs, where they cause neurotransmitter to be released.