Spinal pathways mediating respiratory influences on sympathetic nerves.

The location of spinal pathways mediating the respiratory modulation of sympathetic nerve activity was determined. Left inferior cardiac sympathetic, phrenic, and external intercostal (T1) nerve activities were recorded in 16 alpha-chloralose-anesthetized, vagotomized, paralyzed cats. Baroreceptor reflex activation of sympathetic activity was tested by bilateral carotid occlusion. Eight cats received C6-C7 level ventral spinal cord hemisections followed by cumulative lesions leading to total spinal cord transection. Eight other cats received C6-C7 level dorsolateral funiculus (DLF) lesions followed by dorsal spinal cord hemisection and subsequent spinal cord transection. The respiratory modulation of sympathetic activity was quantitatively assessed using respiration-triggered computer summation of sympathetic activity. Ventral hemisection had no significant effect on the respiratory modulation of sympathetic activity or bilateral carotid occlusion responses. In contrast, bilateral DLF lesions eliminated both the respiratory modulation and bilateral carotid occlusion responses. Unilateral disruption of DLF pathways ipsilateral to the recorded sympathetic nerve indicated spinal level decussations. Thus bilaterally descending DLF pathways with spinal level decussations mediate the respiratory modulation of sympathetic activity.