An electrophysiological study of some reticulo-ruminal and abomasal reflexes in sheep

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Abstract
In halothane-anaesthetized sheep in which reticulo-ruminal (gastric) contractions were present, nervous discharges were recorded electrophysiologically in 64 single efferent gastric units (grouped in 7 types) dissected from the mid-cervical region of the left vagus. An examination was made of the reflex effects on the efferent discharges. Under isotonic recording conditions the total discharge was reduced but the peak frequency of most types of unit was enhanced compared with the discharge recorded under control isometric conditions. Abolition of reticular contractions by the use of preganglionic and post-ganglionic blocking agents resulted in an efferent discharge pattern similar to that recorded under isotonic conditions. Sudden deflation or inflation of the reticulum at the start of a reticular contraction had an effect that depended on the initial volume of the reticulum. It could be either excitatory and/or inhibitory. Acidification of the abomasal contents reflexly enhanced the discharge in 3 of 10 efferent gastric vagal units. An abomasal pH less than 1.0 appears to be less important in providing an excitatory drive to the "gastric centers" in halothane-anaesthetized sheep than in decerebrate sheep. The total reflex time is >1.3 sec for reticulo-reticular reflexes and >2.1 sec for reticulo-ruminal reflexes. The central reflex time is >370 msec. A tonic afferent input from "in series" reticular tension receptors during the quiescent period of the primary (reticulo-ruminal) cycle provides a reflex drive to the "gastric centers" and largely determines the rate and amplitude of the reticular and ruminal contractions. The enhanced afferent input, during a reticular contraction under isometric recording conditions, modifies the size, form and duration of the later phases in the contraction sequence of the reticulum and rumen. The reflex effects are predominantly excitatory at low and moderate levels of reticular tension and inhibitory at high levels.