Graded and Decremental Response in Heart Muscle Fibers

Abstract

Simultaneous records of the transmembrane potential from two points in a single cardiac fiber indicate that during the refractory period when repolarization is about half complete graded responses appear close to the site of stimulation. These responses to cathodal pulses vary in amplitude with stimulus intensity and may show spatial decrement even when an overshoot is present close to the stimulating electrode. The results obtained indicate that the transition from graded to all-or-none response does not depend solely on the restoration of membrane potential. In some experiments stimulation was accomplished by employing one lumen of a double microelectrode; in others single Purkinje fibers were stimulated by action potentials propagated from attached papillary muscle. Similar graded responses were elicited in both instances. The experiments employing propagating action potentials as stimuli suggest the possible occurrence of graded activity as a response mechanism in the intact heart.