The paramedian reticular nucleus: a site of inhibitory interaction between projections from fastigial nucleus and carotid sinus nerve acting on blood pressure

1. The interaction between the pressor response to electrical stimulation of the fastigial nucleus (FN), the fastigial pressor response (FPR), and the depressor response to electrical stimulation of the carotid sinus nerve (CSN) was examined in paralysed anaesthetized cats.2. Blood pressure responses evoked by electrical stimulation of the FN and the CSN were mutually inhibitory and summed algebraically.3. The FPR was augmented after denervation of buffer nerves. Lesions of the FN did not alter the depressor response to stimulation of the CSN.4. Bilateral electrolytic lesions of the paramedian reticular nucleus abolished both the FPR and the CSN depressor response without altering base line pressure.5. With micro-electrode recording neurones were discovered within the paramedian reticular nucleus which responded to electrical stimulation of the FN or the CSN. These neurones were polysynaptically excited by stimulation of either the FN or the CSN but rarely from both, and could be further subdivided into cells responding with either a single spike or a burst discharge.6. The interaction between the FN and the CSN projections to the paramedian reticular nucleus was examined by conditioning-test studies. Eleven per cent of FN- and CSN-units were inhibited by conditioning stimulation of the heteronymous input. The interaction was exclusively inhibitory and observed only in units with latencies > 4 msec and having burst responses. The latency for inhibition was > 20 msec, peaked around 100 msec and lasted up to 300 msec.7. We conclude that the FRP is buffered by baroreceptors and that there is a mutually inhibitory interaction between projections from the FN and the CSN acting on sympathetic vasomotor neurones. The paramedian reticular nucleus appears to be an important site for the interaction.8. The findings support the view that interneurones mediating pressor and depressor responses are intermixed within the medial reticular formation of the medulla.

Spontaneous activity of cerebellar Purkinje cells and their responses to impulses in climbing fibres

1. Spontaneous discharges and evoked responses of Purkinje cells have been studied in the anterior lobe vermis of the cerebellum in cats anaesthetized with thiopentone sodium.2. Spontaneous activity was of two kinds: (a) single spikes which occurred in long trains and were discharged at average frequencies of 50-125/sec and (b) burst responses due to climbing fibre (CF) activation of the cell. These occurred at an average frequency close to 1/sec.3. CF responses were evoked by either stimulation of the Abeta fibres of the superficial radial nerve (SRN) or by an electrode inserted into the deep white matter near the fastigial nucleus (JF electrode).4. A suppression of the discharge of single spikes was frequently observed to follow a CF response, whether it occurred naturally or was produced by a stimulus. These pauses in spontaneous discharge (post-CF pause) lasted for approximately 100 msec, but they did not have a one-to-one relationship with the CF responses. Occasionally a pause in the spontaneous activity was elicited by stimuli that failed to evoke the cell.5. For a period following a peripheral stimulus, a Purkinje cell could not be further excited by a second peripheral stimulus (interaction). JF stimulation could still excite the cell. Evidence was obtained that there was no significant inhibition during the period of depressed excitability to peripheral stimulation.6. The control over the input of activity to the cerebellum through the CF system appears to be imposed at an extra-cerebellar site. The olivary nuscles was suggested as a strong possibility.7. Some possible mechanisms responsible for the post-CF pause were discussed. Disfacilitation of Purkinje cells by suppression of granule cell discharges seems to give the best fit to the data.

Autonomic effector responses to stimulation of nucleus fastigius

1. In anaesthetized cats electrical stimulation of the nucleus fastigius caused cardiovascular responses, electrodermal responses of the paws and pupil dilation with retraction of the nictitating membranes.2. The cardiovascular responses included rise of arterial pressure and peripheral vasoconstriction. Increase of pulse pressure and tachycardia also occurred.3. The effects of sympathetic blocking drugs indicate that fastigial stimulation causes sympathetic discharge to the heart and to the peripheral vessels. There was no evidence of alteration of vagal activity.4. Fastigial stimulation inhibited reflex bradycardia due to carotid sinus distension, vagal afferent stimulation or phenyl diguanide.5. Fastigial and hypothalamic influences on cardiovascular responses could be evoked after severing the nervous pathways connecting the two.6. The pressor response to fastigial stimulation was still present after bilateral division of the carotid sinus and vagus nerves.