FIGURE 6-2 Steps involved in excitatory and inhibitory neurotransmission. 1. The nerve action potential (AP) consists of a transient self-propagated reversal of charge on the axonal membrane. (The internal potential E. goes from a negative value, through zero potential, to a slightly positive value primarily through increases in Na+ permeability and then returns to resting values by an increase in K+ permeability.) When the AP arrives at the presynaptic terminal, it initiates release of the excitatory or inhibitory transmitter. Depolarization at the nerve ending and entry of Ca2+ initiate docking and then fusion of the synaptic vesicle with the membrane of the nerve ending. Docked and fused vesicles are shown. 2. Combination of the excitatory transmitter with postsynaptic receptors produces a localized depolarization, the excitatory postsynaptic potential (EPSP), through an increase in permeability to cations, most notably Na+. The inhibitory transmitter causes a selective increase in permeability to K+ or Cl-, resulting in a localized hyperpolarization, the inhibitory postsynaptic potential (IPSP). 3. The EPSP initiates a conducted AP in the postsynaptic neuron; this can be prevented, however, by the hyperpolarization induced by a concurrent IPSP. Transmitter action is terminated by enzymatic destruction, by reuptake into the presynaptic terminal or adjacent glial cells, or by diffusion.

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