EVIDENCE THAT C-1 AND C-2 PROPRIOSPINAL NEURONS MEDIATE THE INHIBITORY EFFECTS OF VISCEROSOMATIC SPINAL AFFERENT INPUT ON PRIMATE SPINOTHALAMIC TRACT NEURONS

1. Lumbosacral spinothalamic tract (STT) neurons can be inhibited by noxious pinch of the contralateral hindlimb or either forelimb and by electrical stimulation of cardiopulmonary sympathetic, splanchnic, and hypogastric afferents. A previous study found that spinal transections between C2 and C4 sometimes abolished the inhibitory effect of spinal afferent input and sometimes left it intact. This suggested that propriospinal neurons in the C1 and C2 segments might mediate this effect. To test whether neurons in the C1 and C2 segments were involved in producing this inhibitory effect, the magnitude of the reduction in neural activity was measured in the same STT neuron before and after spinal transection at C1 or between C3 and C7. 2. All neurons were antidromically activated from the contralateral thalamus and thoracic spinal cord. For us to accept a neuron for analysis, the characteristics of the somatic input and the latency and shape of the antidromic spike produced by spinal cord stimulation had to be the same before and after the spinal transection. Also, spinal transection often causes a marked increase in spontaneous cell activity, which may affect the magnitude of an inhibitory response. To avoid this confounding problem, a cell was accepted for analysis only if it showed marked inhibition of high cell activity evoked by somatic pinch before spinal transection. For analysis 13 STT neurons met these criteria: 6 neurons were in monkeys with C1 transections, and 7 neurons were in animals with transections between C3 and C7. 3. After transections at C1, electrical stimulation of cardiopulmonary sympathetic or splanchnic afferents or pinch of the ipsilateral or contralateral forelimb or of the contralateral hindlimb still significantly reduced STT neural activity. In contrast, the inhibitory effect to the somatic stimuli and electrical stimulation of visceral afferents were abolished after transections at or below C3. 4. These data show that propriospinal neurons in the C1 and C2 spinal segments can mediate much of the inhibitory effect on STT neurons produced by somatic and visceral spinal afferent input.