Inhibition of barosensitive neurones evoked by lobule IXb of the posterior cerebellar cortex in the decerebrate rabbit.

1. Electrical stimulation of lobule IXb of the posterior cerebellar vermis evoked an increase in heart rate and arterial pressure in the decerebrate rabbit. This pattern of response was followed by a rebound bradycardia which was maintained until arterial pressure had returned to control levels. Activation of the arterial baroreceptors during IXb stimulation failed to elicit reflex changes in heart rate and arterial blood pressure. 2. Extracellular recordings were made from eighty-one single neurones in the nucleus tractus solitarius (NTS), and adjacent structures, that received inputs from the carotid sinus nerve, aortic nerve and/or vagus. A proportion of these were characterized as baro- or chemosensitive by physiological stimuli. The nature of the input from lobule IXb onto these cells was studied subsequently. 3. In twenty-seven out of thirty-five barosensitive neurones, stimulation of lobule IXb either decreased or abolished on-going activity. The latency to onset of inhibition was 21 ms in fourteen cells. In a further twenty-one neurones the spikes evoked orthodromically by electrical stimulation of either the aortic or sinus nerves were either inhibited or severely reduced in number during short-train conditioning stimulation of lobule IXb. This effect was most potent when the cortical conditioning stimulus was timed to occur 25 ms before the aortic or carotid sinus nerves were stimulated. 4. It is suggested that stimulation of lobule IXb in the decerebrate rabbit expresses its influence on the cardiovascular system in part by an inhibitory action on neurones receiving baroreceptor inputs at the level of the NTS.