Changes in sensory-evoked synaptic activation of motoneurons after spinal cord injury in man." Brain 131(Pt 6

Following spinal cord injury (SCI), prolonged muscle spasms are readily triggered by brief sensory stimuli. Animal and indirect human studies have shown that a substantial portion of the depolarization of motoneurons during a muscle spasm comes from the activation of persistent inward currents (PICs).The brief (single pulse) sensory stimuli that trigger the PICs andmuscle spasms in chronically spinalized animals evoke excitatory post-synaptic potentials (EPSPs) that are broadened tomore than 500ms, the duration of depolarization required to activate a PIC in the motoneuron. Thus, in humans, we investigated if post-synaptic potentials (PSPs) evoked from brief (_20ms) sensory stimulation are changed after SCI and if they are broadened to T500ms to more readily activate motoneuron PICs and muscle spasms. To estimate both the shape and duration of PSPs in human subjects we used peristimulus frequencygrams (PSFs), which are plots of the instantaneous firing fre-quency of tonically active single motor units that are time-locked to the occurrence of the sensory stimulus. PSFs in response to cutaneomuscular stimulation of the medial arch or toe of the foot, a sensory stimulus that readily triggers muscle spasms, were compared between non-injured control subjects and in spastic sub-jects with chronic ( 1 ̀ year), incomplete SCI. In non-injured controls, a single shock or brief (_20ms) train of cutaneomuscular stimulation produced PSFs consisting of a 300ms increase in firing rate above baseline with an interposed period of reduced firing. Parallel intracellular experiments in motoneurons of adult rats reveale