Even though neurons vary greatly in size, shape, and length, they share some structural characteristics. All have a region enclosing the nucleus called the cell body (= soma or perikaryon). Additionally, all have one or more thin cytoplasmic processes (= neurites) that attach to the cell body. One type of the cytoplasmic processes is called the axon. There is only one axon per neuron, and it usually attaches to the cell body at a cone-shaped expansion called the axon hillock. The cell membrane of the axon is structurally modified to conduct electro-chemical impulses ( = action potentials). Axons often give off one or more large branches called collaterals. Before ending, the axon and its collaterals usually divide several more times. The last of these divisions are known as terminal branches (= telodendria). The tips of the terminal branches expand into small bulbs called synaptic knobs. They are also commonly referred to as end bulbs, axon terminals or terminal boutons (= buttons). In the synaptic knobs are vesicles that store and release neurotransmitter molecules. Action potentials begin just after the axon hillock, in an especially sensative section of the axon known as the initial segment (= the trigger zone). They then travel the length the axon and its collaterals to the synaptic knobs. Upon their arrival, the action potentials cause the synaptic knobs to release their neurotransmitter molecules into the surrounding area. Also projecting from most cell bodies are one or more, branched cytoplasmic processes called dendrites. The dendrites and (and frequently the cell body) function as the receptive zone of a neuron. Their cell membranes can detect changes in the surrounding environment (= stimuli), including neurotransmitter molecules release by other neurons. The stimuli are converted into electro-chemical signals that move through the cell body and generate action potentials in the initial segment of the axon. Note: instead of attaching to the cell body, the dendrites in some neurons attach directly to the axon.