Temporal Summation of After-Hyperpolarization following a Motoneurone Spike

Abstract

WHEN recorded intracellularly, the soma-dendritic spike potential of cat spinal motoneurones (SD spike¹) is followed by a brief phase of depolarization which reverses to a prolonged hyperpolarization (duration, 60–250 msec.)^1,2. Investigation of after-hyperpolarization has yielded three experimental results which conjointly led to the hypothesis that it is due to a prolonged increase in permeability to potassium ions³: there is an increased membrane conductance; when the membrane potential is changed by applying current intracellularly, the size of the after-hyperpolarization is increased by depolarization and decreased by hyperpolarization, being reduced to zero at about 20-mV. hyperpolarization; this equilibrium potential is shifted in the depolarizing direction by an electrophoretic injection of sodium ions into the motoneurone, which would of course result in a decrease of intracellular potassium ion concentration. In the present investigation on after-hyperpolarization, successive spike potentials of a cat motoneurone resulted in a considerable summation of after-hyperpolarizations and there was a corresponding increase in the membrane conductance.