Structure and function of a chemical synapse.

(a) In response to an action potential, calcium enters the presynaptic neuron axon terminal. This results in vesicle fusion with the plasma membrane, which releases neurotransmitter molecules into the synapse. The neurotransmitter molecules then bind to receptors in the plasma membrane of the postsynaptic cell. This causes ion channels to open (in this example) or close, which in turn changes the membrane potential of the postsynaptic cell. (b) Transmission electron micrograph of a chemical synapse in a rat brain (magnification 170,000X ).