Lability of conduction velocity during repetitive activation of an excitable epithelium

Conduction velocity lability was studied in the electrically excitable epithelium of Euphysa japonica by means of intracellular recordings. Three classes of response latency change were identified in response to bursts of stimuli: an initial jump, uniform drift and abrupt jumps in latency. In each case an increase in stimulus frequency produced an increase in latency. The initial jump in latency, which occurred between the first and second response of a series, was related to the afterpotential of the first response. The increased latency of the second response appears to result from the drop in membrane resistance during the hyperpolarizing afterpotential. The uniform drift in latency remains unexplained but may be the result of ion accumulation within the tissue, progressive inactivation of the ionic channels involved in producing the action potential, or junctional phenomena. The abrupt jumps in latency, which often preceded failure to respond, were found to be impulse initiation phenomena.