Slow membrane potential changes accompanying excitation and inhibition in spinal moto- and interneurons in the cat during natural activation.

Abstract

Summary.Slow membrane potential changes accompanying excitation and inhibition processes have been studied in spinal moto‐ and inter‐neurons of spinal cats, using intracellular recording technique.Section I. Long‐lasting shifts in membrane potential level have been recorded in both moto‐ and interneurons during sustained rhythmical activity.A closer study of the slow membrane potential changes and their relation to spike potentials has revealed fundamentally similar events in moto‐ and interneurons; the quantitative analyses have mainly been performed on niotoneurons whose steady membrane potentials allow of studies for sufficiently long periods. Special attention has been given to the slow membrane potential oscillations during rhythmical firing.The firing level for the onset of rhythmical spike discharges was found to be relatively constant in one and the same neuron; with increase in discharge frequency a definite lowering of the firing level occurs, an approximately linear relation being found to exist between firing level and impulse interval.During periods of regular rhythmical firing subthreshold membrane potential oscillations may occasionally he seen associated with spikes of reduced amplitude or not accompanied by any recordable spikes.The potential analysis has led to a tentative differentiation between an initial quick and a later slow phase of repolarization. The quick repolarization level, more or less marked in different neurons, is of approximately the same value during natural activation and single ortho‐ or antidromic stimulation; it varies within relatively narrow limits during the large natural variations in membrane potential observed in the experiments.The slow repolarization level, which may show great variations even at one and the same membrane potential level, undergoes a pronounced reduction with increasing frequency of the rhythmical activity. The relation hetween the slow repolarization level and the impulse interval is found to be approximately linear.The typical variations in quick and slow membrane processes during various activity levels of a motoneuron have been diagrammatically represented. The possibility is discussed that the quick spike processes and slow potential variations represent changes in different membrane potential fractions, analogous to the Q‐ and L‐fractions established in peripheral nerve.The evidences in favour of an inherent oscillation mechanism in the inembrane activated at a critical level of depolarization and primarily responsible for the discharge frequency are discussed.Section II. Membrane potential changes during inhibition have been studied in moto‐ and interneurons using exteroceptive inhibitory stimulation. In both types of neurons natural activation of inhihitory sources resulted in a slow repolarization not exceeding the highest membrane potential recorded in the absence of excitatory stimulation.The inhibitory effects of synchronized afferent volleys have been analysed in motoneurons in different states of rhythmical activity. The amplitude of the repolarization dip produced by each afferent volley was found to depend on the membrane potential level at the onset of inhibition. During repetitive stimulation a steady state was gradually attained, the level of which varied with the initial state of activity of the neuron.Section III. Both in moto‐ and interneurons the occurrence of random miniature potentials of varying amplitude has been demonstrated, those of larger amplitude (10 mV) apparently being built up out of the smaller “unitary potentials” of 1–3 mV.Increase as well as decrease of the miniature potential activity could be produced by natural activation of extero‐ or proprioceptive sources influencing the neuron. Correlations to the membrane potential level showed a reduced amplitude range of miniature potentials during increased membrane potential.In addition to the most common type of miniature potentials representing an inside positive change, potentials of the opposite sign were occasionally observed. The functional role of such randomly occurring repolarization potentials has been exemplified and a comparison made with similar phenomena observed in other types of nerve cells.