Vagal gastric efferent activity and its reflex changes in conscious sheep

Electromyographical recordings from neck muscles reinnervated by the vagus nerve allowed monitoring of vagal output to the reticulo-rumen in conscious sheep. The vagus nerve was sectioned below the nodose ganglion and its central end anastomosed with the peripheral end of the spinal accessory nerve, which innervates the trapezius and mastoido-humeral muscles. Fifty-three vagal efferent units recorded were identified and classified into seven distinguishable types on the basis of their temporal relationship with gastric movements. With respect to their patterns of discharge during the primary cycles type I to type V units were similar to units of the corresponding types described by Iggo & Leek (1967a) in anaesthetized sheep. Evidence for the hypothesis that each type of gastric unit innervates a distinct region of the reticulo-rumen is strongest for types I and IV. Type I units which exhibited a bimodal pattern of discharge closely associated with reticular motility, are presumed to innervate the reticulum. Some of them, so-called Ib because of their second burst with a long tail and their activity during the secondary cycles, are considered to activate the zone of attachment of the reticulum to the rumen. Type IV units which were active during the ruminal contractions of both primary and secondary cycles are considered to innervate the dorsal rumen. Discharges in these gastric units were observed when gastric motility was partially or totally abolished after administering autonomic blocking drugs for each unit tested (n = 25). They consisted of (i) a shortening of intervals between the onset of two successive bursts, and (ii) an increase in the duration of discharge and the number of potentials in the efferent units. As the abolition of gastric motility led to a decrease of gastric afferent input, these results suggest that the gastric centres are under a permanent inhibitory influence from the afferent drive.