ACCOMMODATION, LOCAL RESPONSE AND MEMBRANE POTENTIAL IN SPINAL MOTONEURONS OF THE CAT

Abstract

Accommodation and local response in lumbar spinal motoneurons of the cat were studied by using intra-cellular potential recordings and direct stimulations. With testing the membrane potential change elicited by linearly increasing or rectangular depolarizing currents applied directly, it was noticeable that irregular small potentials became more distinguishable as the membrane potential was brought near the firing level. It was visualized that the height of monosynaptic EPSP''s [excitatory post-synaptic potential] in motoneurons, being subthreshold to set up an action potential at the resting level, increased in appearance when the motoneuronal membrane was depolarized near the firing level with rectangular currents applied directly. An explanation for these phenomena is that the motoneronal membrane becomes to be apt to induce local responses near the firing level and EPSP''s are seemingly increased in size by local responses summing on them. It was assumed that these local responses may reinforce the triggering action of irregular small synaptic potentials to set up an action potential and may take part in preventing the motoneurons from accommodation. With studies on influences of changing the membrane potential upon accommodation in motoneurons, it was observed that the accommodative attitude was partly changeable depending upon the preset level of their membrane potentials. Accommodation of motoneurons was discussed in concern with the changeability.