RESPONSE OF SINGLE MOTONEURONS TO DIRECT STIMULATION IN TOAD'S SPINAL CORD

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Abstract
Responses of motoneurons in toad''s (Bufo vulgar is formosus) spinal cord to stimulating currents directly applied by an intracellular electrode were recorded through the same electrode. The microelectrode and the spinal cord were put into one arm of the Wheatstone bridge, which was so balanced that only an exponential rise of membrane potential was detectable on the records prior to the soma spike. Rheobase of motoneuron soma was of the order of 10-9 A. The mean value of chronaxie was 4.6 msec, which was about 20 times as large as that of myelinated axon. The critical membrane potential for spike discharge was approximately constant regardless of the intensity of rectangular stimulating currents. The potential-time curves obtained by charging the soma membrane with a rectangular current pulse indicated that the mean value of the time constant is 4.3 msec. By measuring the current intensity flowing across the soma membrane, D.C. resistance of soma membrane in the resting state was calculated. The mean value of the specific membrane resistance was 269 [OMEGA] cm2 and that of the specific membrane capacity was 17.5[mu]F/cm2. Facilitatory effects of catelectrotonus and inhibitory effects of anelectrotonus were found on the axon-soma conduction and synaptic transmission.