Cardiopulmonary sympathetic afferent input to lower thoracic spinal neurons.

Spinal neuronal responses to stimulation of cardiopulmonary sympathetic afferent (CPS) fibers were studied in 25 alpha-chloralose-anesthetized cats. Eighty-two neurons located in the T7-T9 segments were tested for responses to electrical stimulation of CPS fibers. Activity of 55 neurons was altered; 37 were excited, 10 were inhibited, and 8 were both excited and inhibited. All 55 cells with CPS input also responded to stimulation of somatic receptors and the left greater splanchnic nerve (SPL). Somatic receptive fields were primarily located on the upper portion of the abdomen and left lower rib cage. Short and long latency responses occurred following CPS and SPL stimulation. Latencies of responses to CPS stimulation were significantly longer than latencies of responses to SPL stimulation (P less than 0.05). Early responses to CPS stimulation were significantly less in magnitude compared to early responses to SPL stimulation (P less than 0.05). Cell responses to CPS stimulation were reduced in magnitude for as long as 300 ms when a conditioning stimulus was applied to SPL. Inhibitory responses of 10 cells to CPS fiber stimulation were best observed during repetitive stimulation. Eight of the cells were also inhibited by repetitive stimulation of SPL. Injection of bradykinin (4 micrograms/kg) into the left atrium increased activity of 16/30 cells from 8 +/- 2 to 22 +/- 5 spikes/s. The results demonstrate that CPS fiber stimulation alters activity of lower thoracic spinal neurons but not as intensely as SPL stimulation. These neurons may participate in cardiac-abdominal visceral reflexes or the pain of cardiac origin that is referred to the abdomen.