The effects of peripheral and central nervous influences on gastric centre neuronal activity in sheep.

1. Responses of identified vagal reticulo-ruminal motoneurones and gastric centre interneurones to changes in vagal afferent activity were examined in anaesthetized, decerebellate sheep.2. Procedures which reflexly modified the form of forestomach movements caused corresponding changes in the activities of motoneurones and Type A interneurones, whereas the activity patterns of Type B, and many Type C, interneurones were not affected.3. Distension of the pyloric region of the abomasum reduced the number of spikes in the periodic discharges of gastric centre neurones (motoneurones and Type A interneurones) with reticular activity, although the frequency of periods of activity was often increased. The afferent pathway for both effects was probably vagal.4. Unilateral vagotomy usually had little effect on the frequency and amplitude of forestomach movements, and did not influence the temporal relation between ipsilateral gastric centre discharges and the movements.5. Median division of the medulla oblongata only in the region between the gastric centres caused a loss of synchronization in the activities of the two centres, indicating the existence of commissural connexions at this level.6. Bilateral vagotomy abolished forestomach movements and motoneuronal activity, but rhythmic activity in gastric centre interneurones continued with a periodicity of approximately 1 min. This persisting periodic activity was unaffected by spinal section, but was not present after transection of the brain stem rostral to the medulla.7. Cyclical gastric centre activity could be elicited by reticular distension in preparations in which the medulla oblongata was isolated from higher regions of the brain, but, in contrast to many sheep in which the brain stem was intact, the existence of the activity was totally dependent upon peripheral afferent activity.8. The evidence indicates that medullary neurones responsible for periodic activation of vagal preganglionic reticulo-ruminal motoneurones may be excited by either or by both vagal afferent fibres from the fore-stomach or by descending, as yet unidentified, influences from the central nervous system.9. Possible roles for gastric centre interneurones in neural networks which control the periodic activation of motoneurones and which control the form of individual activity cycles are discussed.