Medullo‐Spinal Modulation of Sympathetic Output and Spinal Afferent Input

Associated Inhibition of Sympathetic and Afferent Activities. In intact animals, it is important that most “tonic” sympathetic activity be regulated by brainstem systems that have access to “central commands” from higher centers and to highly specific cranial nerve afferents. The importance of this supraspinal regulation is manifested by severe derangements of sympathetic regulation after spinal cord injury. A major component of these derangements is an exaggerated responsiveness of sympathetic neurons to visceral and somatic afferent activity. Although much is known about medullo‐spinal systems that excite sympathetic preganglionic neurons, little is known about those systems that, in intact animals, isolate preganglionic neurons from spinal afferent input. This brief review will consider the evidence that either identical or cospatial medullary systems regulate the output of sympathetic preganglionic neurons and afferent input to the spinal cord. Further, evidence from this laboratory is presented, which indicates that a recently described system in the rostral cervical spinal cord, perhaps in concert with medullary systems, may play an important role in regulating the excitation of sympathetic preganglionic neurons by spinal afferents.