Durations of After-hyperpolarization of Motoneurones supplying Fast and Slow Muscles

Abstract

WHEN an impulse invades the soma and dendrites of a mammalian motoneurone, the spike potential is always followed by a phase of increased membrane potential, that is, by a hyperpolarization, which has hitherto been referred to as the positive after-potential¹. Since these responses are now investigated by intracellular recording, where hyperpolarization is signalled by an increased negativity, the term ‘after-hyperpolarization’ is to be preferred. Antidromic impulses have been fired into motoneurones from the homonymous muscle nerves. Under such conditions the after-hyperpolarization will often be superimposed on the inhibitory post-synaptic potential generated through the Renshaw cell mechanism². By stimulating the muscle nerve at just threshold strength for the axon of the motoneurone under observation, two series of traces are obtained, one showing the inhibitory potential alone, the other the inhibitory potential plus the after-hyperpolarization (cf. Fig. 1). In this way reliable measurements have been secured for the total duration of the after-hyperpolarization.