The reflex regulation of reticulo-ruminal movements was analysed in the
present experiments using an electrophysiological 'single fibre* recording
technique. The afferent input to and the efferent output from the 'gastric
(retioulo-ruminal) centres' was repeatedly sampled by recording from single
gastric units dissected from the cervical region of the left vagus in 8 sheep
anaesthetized with chloralose and 70 with halothane. Primary cycle movements
of the reticulo-rumen were usually evoked by distending a reticular balloon
with 400-600 ml air.
By recording from afferent gastric units, it was found that the majority
of gastric mechanoreceptors are slowly-adapting 'in series' tension receptors
situated in the muscle layers of, principally, the reticulum, the reticuloruminal fold and the cranial sac (dorsal rumen). The afferent discharge from
these receptors increases during passive distension and during isometricallyrecorded contractions. Receptors in the lips and the floor of the reticular
groove and oma sal canal respond both during a contraction and, particularly,
to pressure. The 'resting discharge' generated by tension receptors is largely
determined by the intrinsic motility of smooth muscle cells. The mean conduction velocity in afferent gastric fibres is 12.4 m/sec.
By recording from efferent gastric units, at least 7 distinctive types
were discernible and, by relating their discharges temporally to movements of
the reticulum and of the rumen, it wa3 concluded that Types I, II and III occur
in fibres innervating the reticulum or associated structures, Type IV the rumen
and Types V, VI and VII other gastric structures not yet identified. In only
Type VII units is there a 'resting discharge' during the quiescent part of the
gastric cycle. By cold blocking vagal nerves, it was demonstrated that
separate efferent fibres innervate the reticulum and the rumen and, that the
dorsal vagal trunk carries predominantly excitatory afferent fibres to the
gastric centres whereas the ventral vagal trunk carries predominantly either
excitatory or inhibitory fibres depending on the experimental conditions.
By recording from afferent and from efferent gastric units at the same
time as altering conditions in the reticulum either physically or with certain
drugs, the changes in the afferent input to and the efferent output from the
gastric centres evoked by these manoeuvres were recorded and estimates of the
total and the central reflex time for gastric reflexes were made.
It is concluded from the present investigation that:
(a) the co-ordination of the complex sequence of primary cycle movements
is a function of the 'gastric centres', through their ability to
determine the parameters and temporal interrelationships of efferent
nervous discharges in the various types of gastric units innervating
different regions of the forestomach.
(b) the tonic afferent input from 'in series' reticular tension receptors
during the quiescent period of the primary cycle provides a reflex
•drive* to the gastric centres and largely determines the rate, the
duration and the amplitude of reticular and ruminal contractions.
(c) the enhanced afferent input to the 'gastric centres', occurring during
a reticular contraction recorded under isometric conditions, modifies
the form, the amplitude, the duration and the delay in onset of the
later parts of the contraction sequence of the reticulum and the rumen.