Nervous control of reticulo-ruminal movements in sheep
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(1) Unitary activity was recorded with tungsten micro- electrodes from the medullary 'gastric centres' of halothane- anaesthetised sheep, in which primary cycle movements of the reticulo-rumen were initiated by sustained distension of the reticulum. Secondary cycle movements occurred in few preparations.(2) Recording sites were marked by electro-coagulation and identified in histological sections. The regions of the medulla oblongata in which recorded unitary aotivity bears a reproducible temporal relation to forestomach motility (the 'gastric centres') are bilaterally paired, and include the dorsal vagal nuclei and the reticular formation up to i mm dorsal and lateral to these nuclei between 1-2 mm caudal, and 5-6 mm rostral, to the obex.(3) Vaal preganglionic motoneurones were identified by an antidromic collision technique. The approximate conduction velocities of their axones, between the cervical vague arid the medulla oblongata , ranged from 10-24 m/sec, and each axone was ipsilateral with its cell body. Moto- neuronal discharges were associated with either reticular or ruminai contractions. Reticular units discharged only during primary cycles, whereas ruminai unit also discharged during secondary cycles of forectomach motility.(4) Gastric centre interneurones were recognised by their inability to be excited antidromically by stimulation of the vague nerves. The activities of Type A interneurones resembled those of reticular and ruminal motoreuronee; units with 'early' activity discharged only during primary cycle movements of the forestomach, where- as several unite with 'late' activity discharged during both primary and secondary cycle movements. Types B and C inter neurones have re sting diecharges which, respectively, increase or decrease in association with reticular contractions. Three irterneurones had discharges which resembled those of gastric vagal afferent units.(5) The discharges of vagal gastric motoneurones, Type A interneurones and interneurones with afferent-like activity were modified by procedures which change the gastric vagal input, and which reflexly modify the form or amplitude of forestomach contractions. The discharges of Types B and C interneurones were largely unaffected by changing the vagal input. The evidence indicates that neural, mechanisms under- lying the control of the frequency of the movements are independent of those controlling the form and amplitude of individual contraction cycles.(6) Bilateral vagotomy abolished the activity of gastric vagal motoneurones, whereas Types A,B & C interneurones continued to discharge with a rhythmicity of approximately 1 cycle/min. The rhythmic activity of gastric centre inter- neurones in the vagotomised preparation was dependent upon an input from higher, unidentified, regions of the central nervous system. Gastric centre rhythmicity may occur in response to an adequate vagal input in the absence of a central input, but the periodicity is then related to the level of afferent excitation.(7) It is concluded that neural networks in the medulla obldngata may be activated by inputs from the vagi or from higher centres, and that they are responsible for periodically activating gastric centre interneurones which may, in the presence of a vagal input, activate vagal preganglionic motoneurones innervating the forestomach. It is proposed that Type B interneurones activate 'early' Type A units which activate 'late' units, and that these Type A units excite, respectively, reticular and ruminal vagal motoneurones. The instantaneous, integrated input to 'amplitude' circuits determines the duration, spike frequencies and number of spikes in the discharges of many Type A interneurones and gastric vagal motoneurones.