Ultrastructural specificity of synaptic sites in nerve terminals mediating both presynaptic and postsynaptic inhibition.

The ultrastructural basis of presynaptic inhibition of the excitatory efferent axon innervating the opener muscle of the crayfish walking leg was studied using the electron microscope. The inhibitory efferent axon terminals form axoaxonal synapses onto excitor terminals in very close proximity to sites of excitatory transmitter release. There is no evidence that these synapses are located at regions of low safety factor of action potential conduction in the excitor. In the area of axoaxonal synaptic connections, the inhibitory axon also forms inhibitory neuromuscular junctions on the opener muscle. Synaptic vesicles are clustered in active zones bordered postsynaptically by extensive invaginations, or junctional folds, in the sarcolemma. These results are consistent with the hypothesis that the primary effect of presynaptic inhibition is to shunt depolarization-secretion coupling at specific sites of transmitter release in the target cell, rather than to block impulse propagation into a more generalized population of nerve terminals distal to the axoaxonal synapse. Further physiological implications of this structural arrangement are considered.