Viscerosomatic convergence onto T2-T4 spinoreticular, spinoreticular-spinothalamic, and spinothalamic tract neurons in the cat.

Neurons of T2 to T4 spinal segments were antidromically activated from the medullary reticular formation and the contralateral region in or near the ventral posterior lateral nucleus of the thalamus. Sixty-seven percent of the spinoreticular neurons projected to the ipsilateral, contralateral, or both ipsilateral and contralateral medullary reticular formation. In addition, 21% of the cells projected to the reticular formation and thalamus and 12% projected only to the thalamus. These cells received both visceral and somatic inputs. Electrical stimulation of cardiopulmonary sympathetic afferent fibers elicited an early peak of cell activity in 30 cells, an early and late peak in 38 cells, and only a late peak in 1 cell. Seventy-one percent of the cells had simple somatic receptive fields; these fields were localized to the left forelimb and left upper thorax. The remaining 29% of the cells had complex receptive fields that often extended to the hind limb as well as bilaterally. Classification of all cells according to threshold for activation of the somatic receptive field showed that 86% of the cells were high-threshold, 10% were wide dynamic range, and 4% were high-threshold but, in addition, were inhibited during hair movement. Viscerosomatic convergence onto these cells support Ruch's convergence projection theory for referred pain. These projecting neurons may be involved with referred pain associated with angina pectoris. In addition, they may also be involved with cardiovascular adjustments.