The peripheral generation of action potentials in excitatory motor neurons of a crab.

The effects of temperature on the firing patterns of the excitatory motor supply to the bender muscle were examined in autotomized walking limbs of the crab Pachygrapsus crassipes. A brief electrical shock applied to the limb nerve evoked an action potential in one or both of the excitor axons and an excitatory junctional potential (ejp) in the bender muscle fibers. However, at high temperatures a single shock to one of the excitor axons (E2) resulted in a short train of nerve spikes and a concomitant burst of ejps. Extracellular recordings made from two sites along the E2 axon demonstrated that the additional action potentials are generated peripherally. Furthermore, myogram recordings from bender muscles in untethered animals indicate that peripheral generation of spikes does take place in intact crabs exposed to high temperatures. The peripheral generation of action potentials exhibited a critical temperature threshold which could be altered by acclimation and by low levels of ethanol in the saline bathing the autotomized limb preparation. This phenomenon may, therefore, be closely associated with membrane fluidity.