Axoaxonal synapse location and consequences for presynaptic inhibition in crustacean motor axon terminals

Serial sections were made of several excitatory nerve terminals in the stretcher muscle of the spider crab, Hyas areneus , to document locations of inhibitory axoaxonal synapses responsible for physiologically powerful presynaptic inhibition. The excitatory terminals are varicose, often with small side branches joined to the main terminal by thin bottlenecks . Axoaxonal synapses occur predominantly on the varicosities, both primary and secondary, with a smaller number on bottlenecks . The distribution is often clustered at specific locations of the excitatory terminal. An electrical model was employed to ascertain the effectiveness of axoaxonal synapses at different locations on the terminal. The model plotted the potential distribution along the terminal with or without a synaptic conductance equivalent to one quantal unit of inhibitory transmitter action. It was assumed from recent work that terminal varicosities are not completely invaded by an action potential. The model predicts that large drops in potentials originating in the main axon occur in the terminals during inhibitory transmitter action, with the largest total drop produced by axoaxonal synapses on the terminal varicosities. The effectiveness of inhibitory action is critically dependent on the dimensions and internal resistance of the bottlenecks . Thus, the geometrical features of the excitatory terminal appear to play a key role in effectiveness of presynaptic inhibition.