Descending inhibition of spinal neurons in the cardiopulmonary region by electrical stimulation of vagal afferent nerves.

Neurons in the left upper thoracic spinal cord of cats anesthetized with chloralose and relaxed with succinylcholine were activated by electrical stimulation of sympathetic chains or by pinching of the skin. Most neurons were spontaneously active, and 22 of the 51 neurons studied were antidromically activated from the spinomedullary junction, demonstrating their projection to higher centers. Stimulation of the central cut ends of either cervical vagus resulted in inhibition of firing in 26 neurons, acceleration in 6 neurons, both effects in 2 neurons, and no effect in the remaining 17 neurons. The parameters of stimulation determined whether inhibition of facilitation was produced in the two neurons that showed either effect. Two spinal neurons were fired by vagal stimulation with latencies of 16 and 20 msec. Activity in vagal afferent fibers is believed to impinge upon brain stem structures that have been shown by others to cause descending inhibition (and facilitation) of spinothalamic and spinoreticular neurons. This would provide a mechanism for modulation of sensory information from the cardiopulmonary region.