Central projections of gastric afferent vagal inputs.

1. The medulla oblongata of thirteen halothane-anaesthetized, decerebellate sheep was explored systematically with recording micro-electrodes.2. The nervous activity recorded was designated either ;cyclical', if it was related to the periodic vagal outflow responsible for primary cycle movements of the reticulo-rumen, or ;afferent-like', if it was directly related to mechanical stimulation of the reticulum or the abomasum.3. Forms of mechanical stimulation were used which were known (from earlier ;single afferent fibre' studies) to evoke characteristic and distinguishable responses from four receptor types, i.e. reticular tension receptors, reticular epithelial receptors, abomasal tension receptors and abomasal mucosal receptors.4. Micro-electrode penetrations were made within a zone 4 mm lateral to the mid line, 6 mm rostral to and 2.5 mm caudal to the obex. In certain regions, ;afferent-like activity' was recorded, which corresponded to the discharge properties of one or more of the above receptor types.5. Units were sometimes excited or inhibited by more than one kind of receptor excitation, thus demonstrating the possibilities of a convergence of afferent projections and either a direct or a reciprocal relationship between receptor activity and central nervous ;afferent-like activity'.6. It is concluded that most, if not all, of the ;afferent-like activity' was recorded from the vicinity of interneurones.7. There was a partially overlapping viscerotopic organization of ;afferent-like activity' derived from abomasal and from reticular receptors.8. Regions showing ;afferent-like activity' were located dorso-laterally to regions showing ;cyclical activity' although there was some overlap between the two regions.9. ;Afferent-like activity' was recorded from regions which included the dorso-lateral part of the dorsal vagal motor nucleus, the adjacent reticular formation and the nucleus of the solitary tract between transverse planes 3 mm rostral to the obex and 1 mm caudal to the obex.10. Some of the gastric afferent vagal inputs were projected to contralateral locations via unidentified commissural connexions.