Inhibition and excitation of thoracic spinoreticular neurons by electrical stimulation of vagal afferent nerves.

Neurons in spinal segments T1-T3 of cats projecting to the region of the nucleus reticularis gigantocellularis were tested for their responses to electrical stimulation of the vagus nerve. The effects of vagal efferent activity were blocked with methylatropine, thereby limiting vagal input to the effects of afferent fibers. Seventeen spinoreticular (SR) neurons were inhibited by vagal stimulation; one was excited; four were both inhibited and excited, and nine were unaffected. Half of the SR neurons were not spontaneously active; they were excited by somatic stimuli for 20- to 30-s periods in order to test for vagal inhibition. Vagal stimuli sometimes were sufficiently potent to completely suppress neuronal discharge during and just after 10-s periods of stimulation. Parameters of stimulus duration and frequency suggest activation of both A delta and C fibers in the vagus nerves. Vagal stimulation could inhibit increased firing from cardiac stimuli, such as coronary artery occlusion and injection of bradykinin into the heart. Two-thirds of the SR neurons so tested responded to bilateral sympathetic stimuli, whereas about one-half had excitatory somatic receptive fields restricted to the left foreleg and upper left side. The other half had bilateral, widespread, and complex somatic fields, including inhibitory responses. Such multiple inputs suggest an integrative role for spinoreticular neurons in sensory processing. Vagal inhibition and excitation of SR neurons probably operates via well known descending inhibitory systems from the nucleus raphe magnus and other medullary centers. The vagi are the only peripheral afferent pathway into this descending system that has been described to date.