ANODAL BREAK RESPONSE OF SINGLE MOTONEURON IN TOAD'S SPINAL CORD

Abstract

Using single intracellular micropipettes in a bridge circuit, anodal break response of motoneurons was explored in unanaesthetized toad''s spinal cord in situ. Some motoneurons showed anodal break response to relatively weak current, while in others it could hardly be elicited even with much stronger current unless the membrane had previously been depolarized by subthreshold stimulation. The firing level of anodal break response was in general either near the original membrane potential or on slightly depolarized side of it and remained constant irrespective of the varying intensities of current. The latency of the response was either independehdent of current intensity or it became longer with increase in the current strength. The relationship between current strength and its duration was found to follow approximately the "exponential law". Hill''s constant of accommodation was calculated from the strength-duration curve in three cases and found to be 17.3, 31.7, and 43.3 msec. In one and the same motoneuron, the break response was initiated from the same site, initial segment or soma, as the make response. In some motoneurons, the break response was originated from the first node of myelinated axon. The results obtained with exponentially decaying current indicated that IPSPs, when elicited in more or less depolarized motoneurons, would be capable of initiating impulses by removing the accommodative process developed in a motoneuron, and would thus maintain cellular activities.