Excitation and Inhibition of Spinal Motoneurons

Spinal cord motoneurons of the cat were stimulated by a needle electrode and reflexly via dorsal root impulses. Responses to direct stimulation originated in dendrites, initial axon segment, or myelinated axon, but apparently not in the cell body. The sites were differentiated by latency changes of the response as needle position and stimulus strength are varied, by ability to follow repetitive stimulation, and by responses to asphyxia duration and anesthesia depth. Facilitatory afferent impulses increased the excitability of the motoneurons, whether the direct test stimulus was activating either dendrites or initial axon segment, but inhibitory afferent impulses decreased the direct response only when dendrites were being activated. Facilitation is therefore nonlocalized and can be accounted for by the usual eddy currents. Inhibition is interpreted as a partial depolarization and impedance decrease of the cell body, induced by afferents reaching it directly and resulting in a short circuiting of eddy currents from excited dendrites to initial axon segment. The central delay of the spinal cord monosynaptic response (about 0.8 msec.) is accounted for by conduction time in the afferent fibers (0.5 msec.), determined by antidromic stimulation of these and by the time lag of afferent conditioning effects, and in the motoneurons, 0.2–0.3 msec. It is thus doubtful if any true ‘synaptic delay’ exists. The long intramedullary afferent conduction time also fully accounts for the lag seen in afferent inhibition, in harmony with the existence of a direct inhibitory pathway.