B F Leek's research while affiliated with The University of Edinburgh and other places

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.

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.

In the normal state one is largely unaware of prevailing conditions in the abdominal viscera apart from the rather vague sensations which signal the fullness of the stomach, of the distal colon and of the bladder. This is all the more surprising, because the visceral nerves contain a preponderance of afferent (sensory) fibres. In the vagus nerves (Fig. 1) the proportion of afferent axons is 80–90% (Daly and Evans, 1953; Agostini et al., 1957), in the splanchnic nerves it is >50% (Foley, 1948) and in the pelvic nerves it is 30% (Ranson, 1921). Even these figures are probably too low, as Daly and Evans (1953) and Agostini et al. (1957) have demonstrated that many visceral afferent fibres are nonmyelinated and exist in larger numbers than hitherto suspected. On the purely numerical basis of their afferent: efferent axons ratio, the visceral nerves should be regarded, therefore, principally as sensory nerves and only secondarily as motor nerves. This is in contrast to the classical attitude derived from studies on the autonomic nervous system, which has unduly emphasized visceral motor pathways.

1. Neuronal activity bearing a temporal relationship with spontaneous reticulo‐ruminal movements was recorded with micro‐electrodes from the medulla oblongata in halothane‐anaesthetized sheep. Recording sites were located histologically after causing electro‐coagulation at the micro‐electrode tip. 2. One hundred and forty‐four gastric units were recorded from the dorsal vagal nucleus and up to 1 mm dorsal and lateral to the nucleus between transverse planes 1 mm caudal, and 4 mm rostral, to the obex. It is considered that records were obtained from the regions of cell bodies. 3. The discharges of thirty‐two vagal preganglionic motoneurones were identified by an antidromic collision technique. Conduction velocities ranged from 10–26 m/sec. They were located in the dorsal vagal nucleus and up to 0·5 mm dorsal and lateral to the nucleus. The majority of motoneurones innervated either the reticulum or the rumen. One ruminal unit discharged during both primary and secondary cycle movements. 4. One hundred and twelve units which were not orthodromically or antidromically activated by stimulating the vagus nerves were considered to be interneurones. Four types were distinguishable on the basis of their patterns of discharge during primary cycle movements. 5. The discharges of Type A interneurones resembled those of gastric motoneurones, having no resting discharge between contraction cycles. Their discharges were temporally related to either reticular contractions or rumen contractions during primary and secondary cycle movements. 6. Types B and C interneurones have resting discharges which, respectively, increased and either decreased or stopped during each primary cycle movement. 7. Discharges of only three units identified as interneurones resembled the discharges of gastric vagal afferent units.

1. The nervous activity in single afferent gastric vagal units was recorded electrophysiologically from halothane‐anaesthetized sheep with spontaneous reticulo‐ruminal movements present. 2. Sixty‐six afferent units innervating gastric mechanoreceptors were isolated from fifteen sheep. The receptors were located mainly in the medial walls of the reticulum and the cranial sac of the dorsal rumen, and also in the reticular groove, the reticulo‐ruminal fold, the dorsal and ventral sacs of the rumen and the omasal canal. 3. The mean conduction velocity ( C.V. ) for twenty‐seven units was 12·4 ± 1·0 m/sec ( S.E. ). For units with a pathway in the dorsal vagal trunk, the mean C.V. was 14·5 ± 1·0 m/sec ( S.E. ) and for units with a pathway in the ventral vagal trunk the mean C.V. was 6·6 ± 0·5 m/sec ( S.E. ). 4. From the receptors a slowly adapting response was elicited by tangential lengthening. These were tension receptors in series with contractile elements, as they were excited by increased tensions developed both passively by inflation of the viscus and actively by muscular contractions. 5. Receptors in the reticulum and the rumen appeared to be situated deep in the muscle layers, whereas those in the reticular groove structures seemed to be more superficial and gave the in series tension receptor response as well as a response to light pressure. 6. A resting discharge in tension receptor units was usually absent at low levels of distension but appeared and increased as the level of distension was raised. Intermittency and fluctuations in the resting discharge were related to intrinsic local movement involving the receptive fields. Increasing distension enhanced the intrinsic movements. 7. Even after the removal of the abomasum, reticular and ruminal (primary cycle) movements were evoked by distending the reticulum. It is possible that this manoeuvre enhanced intrinsic movements, which, in turn, caused an increased excitatory afferent input to the ‘gastric centres’ from in series reticular tension receptors. 8. The enhanced afferent discharge from reticular tension receptors elicited by an isometrically recorded reticular contraction reflexly inhibited the subsequent (primary cycle) contraction of the rumen. 9. Very few receptors were located in the caudal regions of the rumen whereas the cranial sac is richly supplied with tension receptors. The idea that the cranial sac may serve as the reflexogenic zone for secondary cycle movements of the rumen is discussed.