Abstract
1. The ventral root potentials following an electrical stimulation of afferent nerve were recorded in toads and bull-frogs during the application of polarizing currents along spinal cord and ventral root.
2. The motoneurone ceases to elicit an efferent impulse when its soma is polarized anelectrotonically as well as when polarized catelectrotonically to a considerable extent (cathodal depression). In the former case the slow potential shows retarded temporal decay, in contrast with its rapid decay observable in the latter case. The effects of the electrotonus on the motoneurone soma (including dendrites) and on the axon-hillock were considered separately and discussed for respective cases.
3. Repetitive stimulation seems, when it gives rise to fatigue, to shift the resting status of polarization at synaptic membrane towards increasingly depolarized state, until finally it reaches what is similar to the state of cathodal depression. Currents applied in a direction to polarize motoneurone soma catelectrotonically accelerate and emphasize the above mentioned effect of repetitive stimulation, and hinder the motoneurone to recover during a pause of stimulation. Polarizing currents in reverse direction retard or prevent the occurrence of “fatigue” in the motoneurone.
4. It is suggested that the resting status of polarization at synaptic membrane would be labile and alterable within a wide range, and that the failure of synaptic transmission under a state similar to that of cathodal depression may account for at least a certain kind of central fatigue in normal organisms.
