Splenic, renal, and cardiac nerves have unequal dependence upon tonic supraspinal inputs.

Stimulation of visceral receptors can lead to unequal reflex responses in splenic, renal and cardiac sympathetic nerves. Activity of splenic nerves is often more excited or less inhibited than that of cardiac or renal nerves. This study was undertaken to determine potential differences in resting discharge among these 3 nerves. Dependence upon supraspinal drive was evaluated by comparing the relative decrease in activity of these nerves in chloralose-anesthetized cats 30 min to 2 h following high cervical spinal cord transection. After this transection, discharge rates of cardiac and renal nerves were significantly depressed to less than 50% of initial values. In contrast, splenic nerve activity was not significantly affected. To determine if this sustained splenic nerve activity resulted from greater responsiveness to potential external sources of excitation, splenic, renal and cardiac neural responses to factors known to affect sympathetic discharge in spinal animals were compared. Neither increased arterial pressure, decreased arterial pressure, systemic hypercapnia and acidosis, nor thoracolumbar dorsal rhizotomy revealed specific inputs responsible for the preferential maintenance of splenic nerve activity in spinal cats. It was concluded that ongoing activity of splenic nerves is less dependent upon supraspinal sources of excitation than is activity of renal or cardiac nerves. The cause of this difference among these 3 components of sympathetic outflow remains to be determined.