Synapses are the functional contacts between neurons. They can be divided into two general classes:
electrical and
chemical. Electrical synapses permit direct, passive flow of electrical current from one neuron to another. The current flows through
gap junctions, which are specialized membrane channels that connect the two
cells. Chemical synapses enable communication via the
secretion of
neurotransmitters; in this case, chemical agents released by the presynaptic neurons produce secondary
current flow in postsynaptic neurons by activating specific
receptor molecules. The secretion of
neurotransmitters is triggered by
voltage-gated Ca
2+ ion channels, which elevate Ca
2+ within the presynaptic terminal. The rise in Ca
2+ concentration causes synaptic vesicles (presynaptic
organelles that store
neurotransmitters) to fuse with the
plasma membrane and release their contents into the space between the presynaptic and postsynaptic cells.
Neuroscience, Sinaur Associates (QP355.2.N487 1997)