Blocking of impulses in specialized regions of crustacean motor axons

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Abstract
Patterned stimulation blocks impulse propagation in some fine branches of both excitor axons to lobster [Homarus americanus] claw closer muscle. The site of the block was determined by monitoring spikes in the main closer nerve and in one of its primary branches, and excitatory postsynaptic potentials (EPSP) in muscle fibers. With paired pulses (3- to 8-ms interspike interval) at 5 and 10Hz, the 2nd of a pair of EPSP disappears even though primary axon branches are conducting paired spikes. The abrupt disappearance of the 2nd EPSP suggests that it is not due to transmitter depletion but rather to conduction block in fine branches. Unpatterned stimulation at 10 and 20 Hz did not produce blocking. In the single excitor axon to the opener and stretcher limb muscles in lobster and crayfish, patterned or unpatterned stimulation failed to block conduction to the distal opener muscle, failing to confirm a finding in crayfish previously reported by others. Conduction block at specialized axonal regions may not always have adaptive value.

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