Peripheral generation and modulation of crustacean motor axon activity at high temperatures

Summary

We have examined the effects of temperature changes on the stretcher muscle and its motor supply in a crab (Pachygrapsus crassipes). An increase in temperature caused a decrease in the amplitude of evoked excitatory junctional potentials (ejp's). Above a critical threshold a single action potential in the excitor (E) or specific inhibitor (SI) axon provoked multiple spikes in the appropriate axon and concomitant ejp's or inhibitory junctional potentials (ijp's) in the stretcher muscle fibers. The critical temperature for generation of peripheral spikes was dependent upon the crab's thermal history.

In preparations in which a shock to the E axon evoked repetitive firing, stimulation of the SI axon at about the same time as the E axon abolished or curtailed the peripherally generated E axon responses. No reciprocal modulation of SI activity by the E axon was observed. GABA abolished the peripheral generation of E spikes and picrotoxin prevented SI modulation of E activity. We suggest that the site of SI modulation is at the axo-axonal synapses, possibly at the fine E axon branches and the ‘bottlenecks’ along the E axon where inhibitory synapses have been observed.