presynaptic terminal

A presynaptic terminal is the swollen tip at the very end of a sending neuron, the spot where one nerve cell hands its message off to the next. Think of a neuron as a long wire that carries an electrical signal; the presynaptic terminal is the little bulb at the wire's end, sitting just across a tiny gap from the cell it wants to talk to. Packed inside this bulb are dozens of tiny bubbles called vesicles, each one a sealed pouch filled with chemical messengers known as neurotransmitters — the actual words of the message.

When an electrical signal races down the neuron and reaches this terminal, it triggers a quick chain of events: channels in the terminal's wall open, calcium rushes in, and that calcium acts like a starting gun, telling the waiting vesicles to fuse with the outer membrane and dump their neurotransmitter into the gap. The chemicals drift across and land on the next cell, passing the message along. Because the terminal both stores the messengers ahead of time and releases them only on cue, it is the launch pad of nearly every signal your brain sends — and the precise place where many drugs, toxins, and medicines do their work.