Threshold of ventral root stimulation to produce the IS-spike without the M-spike in the same cat motoneuron

In cats, anesthetized with pentobarbital sodium and immobilized with d-tubocurarine, it was discovered by changing the strength of L 7 or S 1 ventral root (VR) stimulation that the stimulation could produce in the same motoneuron the IS-spike, i.e., the spike of the initial segment, either via the M-spike or without the M-spike, which is believed to be the spike of the node of Ranvier as recorded in the soma. It was shown by simultaneously recording the compound action potential of the L7 or S 1 ventral root that the threshold of VR stimulation to produce the IS-spike via the M-spike was higher than that to produce it without the M-spike in all 18 motoneurons in which the threshold was studied. With weaker stimulation the M-spike was not produced and the IS-spike arose from a potential smaller than the M-spike (the M-potential) or directly from the baseline. The M-potential could also trigger the M-spike. These results disclose the possibility that the M-spike originates not in the axon, but somewhere else such as the dendrite.