[Neural control of the motility of the reticulo-rumen]

This paper reviews the nervous mechanisms involved in the control of motility of the forestomach compartments in ruminants. The first part of the review reports the efferent vagal discharge which consists of several distinct and independent types of urinary activity and passes from the gastric centres to the reticulo-rumen. The patterns of each of these activities are temporally related to the contractions of special parts of the forestomach and occur in a sequence which could produce the coordinated series of movements found in the reticulum and rumen. The orderly sequence of motor events that constitutes the gastric cycle is due to this coordination of efferent vagal outputs arising in the gastric centres. The focal point of the second part of the paper is sensory feedback from the complex stomach to the centres. Four types of receptor have been identified according to their location and stimulus; these are tension receptors and epithelial receptors in the reticulo-rumen and tension receptors and mucosal receptors in the abomasum. Mechanical or chemical stimulation of these distinct receptor types leads to either facilitation or inhibition of reticulo-ruminal motility. The third part of the paper deals with the organization of the medullary gastric centres. The gastric vagal motoneurons are controlled by interneurons organized in two functionally distinct networks. The "rate" network for which the periodicity of its activity depends on the cumulative integrated afferent inputs from central and peripheral sources, determines the rhythm of gastric cycles. It is postulated to drive the "amplitude and form" network which adjusts vagal output to instantaneous gastric afferents, enabling the amplitude of gastric contractions to be adapted to peripheral stimulations. The role of the sensory feedback from the complex stomach in the control of the "frequency" network is discussed, taking into account new experiments on vagal deafferentation and the concept of an oscillating generator that would be more or less permanently inhibited by vagal afferents is reviewed. The respective roles of local regulation mechanisms, mediated in the intramural plexus, and of central mechanisms, in the control of forestomach motility, are briefly discussed.