Tetrodotoxin-resistant propagating action potentials in presynaptic axon of the lobster.

Abstract

The properties of tetrodotoxin (TTX)-resistant action potentials were studied in the presynaptic axon near the nerve terminal of lobster neuromuscular junctions. In TTX-treated preparations, a depolarizing current, in the inhibitory axon produced an inhibitory postsynaptic potential (IPSP) in the muscle, but no action potential could be recorded antidromically at > 6 mm from the current electrode. Experiments using 3 microelectrodes in the axon near the nerve terminal proved that TTX-resistant spikes could propagate orthodromically; they declined antidromically with increasing distance. The asymmetrical propagation of the action potentials was not ascribable to the membrane-potential level, but rather to the direction of spike conduction. Cobaltous ion converted the propagating TTX-resistant spike to graded responses. 3-Aminopyridine enhanced the Ca component of the TTX-resistant spikes; it did not alter the asymmetry of spike propagation. There is a gradient of Ca-channel density in the nerve terminal that brings about different modes of activation of TTX-resistant spikes according to the direction of spike conduction.