Responses and afferent pathways of C₁-C₂ spinal neurons to gastric distension in rats

Some evidence shows that the upper cervical spinal cord might play an important role in propriospinal processing as a sensory filter and modulator for visceral afferents. The aims of this study were to determine (1) the responses of C₁-C₂ spinal neurons to gastric distension and (2) the relative contribution of vagal and spinal visceral afferent pathways for transmission of gastric input to the upper cervical spinal cord. Extracellular potentials of single C₁-C₂ spinal neurons were recorded in pentobarbital anesthetized male rats. Graded gastric distension (20-80 mm Hg) was produced by air inflation of a latex balloon surgically placed in the stomach. Sixteen percent of the neurons (32/198) responded to gastric distension; 17 neurons were excited and 15 neurons were inhibited by gastric distension. Spontaneous activity of neurons with inhibitory responses was higher than those neurons with excitatory responses (18.1±2.7 vs. 3.8±1.7 impulses s^-1, p<0.001). Twenty-eight of thirty-two (87.5%) neurons responded to mechanical stimulation of somatic fields on head, neck, ears or shoulder. Most lesion sites of neurons with excitatory responses were found in laminae V, VII; however, neurons with inhibitory responses were in laminae III, IV. Bilateral cervical vagotomy abolished responses of 4/8 neurons tested. Spinal transection at C₆-C₇ abolished responses of the other four neurons that still responded to gastric distension after bilateral vagotomy. Results of these data supported the concept that a group of C₁-C₂ spinal neurons might play a role in processing sensory information from the stomach that travels in vagal and spinal visceral afferent fibers.