EFFECTS OF PRESYNAPTIC VOLLEYS ON SPREAD OF IMPULSES OVER THE SOMA OF THE MOTONEURON

Abstract

A centripetal volley of impulses which arrived at the spinal cord over ventral root fibers of cats and rabbits produced a series of potential changes in the ventral horn. A brief initial positive potential-change signaled the approach of the impulses in ventral root axons. A negative deflection of a duration of 1-2 msec. followed. It was due to the invasion of the motoneuron soma (cell body and dendrites) by the centripetal impulses. The negativity passed over into a much more prolonged and smaller positive phase. A rapid series of essentially identical centripetal volleys evoked negative deflections of diminishing size. The response of the motoneuron soma is evidently modified by frequencies of stimulation (e.g. 300 per sec.) which the motor axons follow readily. These observations on spinal motoneurons confirm Lorente de No''s findings for cranial motoneurons. Conditioning dorsal root volleys altered the motoneuron soma response to centripetal testing volleys in motor axons. The negative potential-change in the ventral horn due to the motoneuron soma was increased, or as the case might be, depressed. The result depended upon the conditioning dorsal root fibers, the tested motoneurons, and the interval between the arrival at the cord of the conditioning and the testing volleys. It is concluded that retrograde conduction in the motoneurons occurs with a decrement. The degree of penetration into the cell body and dendrites can be augmented or decreased by the consequences of activity in sensory and premotor neurons.