Sympathetic nervous system response to mechanical stress of the spinal column in rats

The effects of mechanical stimulation of the spine on blood pressure, heart rate and the activity of selected sympathetic nerves (renal and adrenal) were examined in alpha-chloralose/urethane anesthetized rats. Spinal segments from T10 to T13 or from L4 to L7 were isolated from surrounding muscle and the upper and lower segments of the four segment units were fixed by means of spinal clamps. Forces from 0.5 to 3.0 kg were applied to the lateral aspect of the two mobile segments. Stimulations of the thoracic or the lumbar region produced large decreases in blood pressure (-29.8 +/- 3.1 mmHg) along with small decreases in heart rate (6.1 +/- 1.6 beats/min). Additionally, large and immediate decreases were observed in renal nerve activity. While responses attenuated during the course of stimulation, they generally outlasted the stimulus. The adrenal nerve, after an initial decline in activity, showed subsequent increases which were attributed to baroreceptor effects, since bilateral lesion of the vagi and carotid sinus nerves abolished them. After baroreceptor denervations only initial decreases in activity were observed by mechanical stimulation of the spine. The observed responses were found not to be due to spinal cord compression, but rather were ascribed to afferent fiber mediated reflexes. Cutting dorsal (sensory) roots T10 to L2, bilaterally, abolished the response to lower lumbar stimulation. Additionally, in recording from the peripheral end of the severed dorsal roots, large increases in afferent activity were observed by spine stimulation. The present study has demonstrated potent somatovisceral reflexes from mechanical stimulation of the spinal column. These results are discussed in relation to other studies of somatovisceral responses and with regard to various clinical reports.