Potential Changes produced by Application of Current Steps in Motoneurones

Abstract

IN cat spinal motoneurones, when current steps are applied across the membrane through the intracellular micro-electrode, the membrane potential changes in an approximately exponential way, but with a certain deviation. Thus, the soma-dendritic membrane of the motoneurone has been assumed to behave as a resistance and a capacitance in parallel^1,2, and the deviation has been attributed to the cable-like property of dendrites, the dominance of which in forming the effective surface area of motoneurones has been emphasized recently³. As will be described, however, the potential produced by current steps in motoneurone membrane shows a considerable overshoot, which has so far been overlooked because the current steps used previously were generally too brief to reveal the later decline of the potential. This electrical behaviour of the membrane suggests that the equivalent electrical circuit of the membrane is not a simple resistance-capacitance network.