The Journal of Neurophysiology Vol. 87 No. 3 March 2002, pp. 1290-1302
Copyright ©2002 by the American Physiological Society

Spinal Inhibitory Effects of Cardiopulmonary Afferent Inputs in Monkeys: Neuronal Processing in High Cervical Segments

 ¹Department of Physiology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma 73190; and  ²Amgen, Thousand Oaks, California 91320

Chandler, Margaret J., Jianhua Zhang, Chao Qin, and Robert D. Foreman. Spinal Inhibitory Effects of Cardiopulmonary Afferent Inputs in Monkeys: Neuronal Processing in High Cervical Segments. J. Neurophysiol. 87: 1290-1302, 2002. Noxious stimulation of spinal afferents inhibits primate spinothalamic tract (STT) neurons in segments distant from the region of afferent entry. Inhibitory effects of cardiopulmonary sympathetic afferent (CPSA) stimulation remain after C₁ transection but disappear with spinal transection between C₃ and C₇. We hypothesized that spinal inhibitory effects produced by CPSA stimulation are processed by neurons in C₁-C₃ segments. One purpose of this study in anesthetized monkeys was to determine whether chemical activation of high cervical neurons reduced sacral STT cell responses to colorectal distension (CRD) and urinary bladder distension (UBD). First, effects and interactions of pelvic and cardiopulmonary visceral afferent inputs were determined in 10 monkeys on extracellular activity of sacral STT neurons recorded in deep dorsal horn. CRD and UBD increased activity in 95 and 91% of sacral STT neurons, respectively. CPSA and cardiopulmonary vagal stimulation decreased activity in 84 and 56% of STT neurons, respectively. CPSA stimulation decreased CRD-evoked activity in six of eight sacral STT neurons and decreased UBD-evoked activity in five of eight STT neurons tested. Excitatory amino acid application at C2 segment decreased CRD-evoked responses in 7 of 10 sacral STT neurons and decreased UBD-evoked responses in 9 of 12 STT neurons. The second purpose of this study was to examine responses of C₁-C₃ descending propriospinal neurons to stimulation of cardiopulmonary afferent fibers. If C₁-C₃ neurons process CPSA input to suppress STT transmission, then CPSA stimulation should excite C₁-C₃ neurons with descending projections. Effects of thoracic vagus nerve stimulation also were examined. Vagal stimulation inhibits STT neurons in segments below C₃ but excites C₁-C₃ STT neurons; we theorized that vagal inhibition of sensory transmission might relay in high cervical segments and, therefore, excite C₁-C₃ descending propriospinal neurons. Extracellular discharge rate was recorded for C₁-C₃ neurons antidromically activated from thoracic or lumbar spinal cord in 24 monkeys. CPSA stimulation increased activity of 16 of 45 neurons and inhibited one cell. Thoracic vagus stimulation increased activity of 20 of 43 neurons and inhibited one cell; stimulation of abdominal vagus fibers did not affect activity of six of six cells that were excited by thoracic vagal input. Mechanical stimulation of somatic fields excited 30 of 41 neurons tested. All neurons activated by visceral input received convergent somatic input from noxious pinch of somatic receptive fields that generally included the neck and upper body; 11 C₁-C₃ propriospinal neurons did not respond to any afferent input examined. Results of these studies were consistent with the idea that modulation of spinal nociceptive transmission might involve neuronal connections in high cervical segments.