SUMMARY1. Functional interrelationships between the hippocampus and the cerebellum have been investigated in the anaesthetized cat. Plots of the stimulating and recording sites as well as the latency range of the responses indicated the extent of ascending and descending lines of operation between these two structures.2. Stimulation of the fastigial nucleus evoked the discharge of single hippocampal units on both sides of the brain. Early responses had a mean latency of 12 msec and late responses had a mean latency of 21 msec. Increasing the intensity of the stimulus had little effect on the patterns of discharge.3. There was no topographical organization within the hippocampus. On the other hand, the activity evoked by a cutaneous stimulus was shown to be greatly depressed by a preceding cerebellar stimulus, particularly at intervals 30-40 msec between the two stimuli.4. Cerebellar responses evoked by stimulating the hippocampus were found mainly in lobule VI of the posterior lobe. Early and late responses were frequently recorded in the same trace, ipsilateral stimulation yielding the shortest latencies. Increasing the intensity of the stimulus increased the likelihood of there being a response and increased the number of spikes in each discharge. Hippocampal stimulation also had a profound influence on resting cerebellar discharges.5. Symmetrical points in the two hippocampi were chosen for conditioning and testing sequences. The conditioning stimulus had a long-lasting inhibitory effect on the test response followed by a slow recovery.6. The location and extent of hippocampal influences on the cerebellum were determined by plotting the presence or absence of a response at each stimulated site. The results indicated the existence of bilateral descending projections containing fast and slow components in conformity with the known conduction properties of mossy fibre and climbing fibre inputs.7. The physiological significance of interrelationship between the hippocampus and the cerebellum is discussed. It seems that there are many similarities as well as fundamental differences in the cerebellar control of movement under normal circumstances and in conditions of stress.

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SUaMMRY1. The projections of the ipsilateral and contralateral splanchnic nerves on the cerebellum were studied in cats under sodium thiopentone anaesthesia.2. The splanchnic nerves were electrically stimulated. 4. At the level of the upper cervical spinal cord, fibres from the ipsilateral splanchnic nerve were conveyed in both the dorsal and lateral funiculi; fibres from the contralateral nerve were restricted to the ipsilateral lateral funiculus.5. On the basis of the characteristic potential fields generated in the cerebellar cortex, it is concluded that the majority of the splanchnic afferents studied terminate in the cerebellum as climbing fibres. A few responses of short latency may have been due to a mossy fibre input.6. In interaction experiments, a conditioning stimulus to one splanchnic nerve depressed the response to stimulation of the other nerve for up to 80-100 msec, the effect being most pronounced at 40-50 msec. During this period, spontaneous activity of the cerebellar unit was also suppressed.7. The organization of the splanchnic pathways to the cerebellum and their terminal connexions are discussed.

SUMMARY1. Evoked single unit discharges have been recorded from the right side of the anterior lobe of the cerebellum following distension of the gallbladder, mechanical stimulation of abdominal viscera and electrical stimulation of the right splanchnic nerve.2. The majority of the evoked units were located in the posterior part of the culmen, lateral to the paravermian vein. Unit depths ranged from 0-2 to 5 0 mm below the pial surface.3. Units responding to distension of the gall-bladder or mechanical stimulation of the viscera were not affected by other peripheral stimuli.4. Responses to gall-bladder stimulation were either excitatory or inhibitory. Responses to mechanical stimulation of the viscera were mainly inhibitory.5. Units evoked by electrical stimulation of the right splanchnic nerve fell into two groups: (1) high threshold. The majority of units responded to activation of the Ad group of splanchnic afferents with a latency of 20-25 msec, and a high probability that the unit would respond to successive stimuli over long periods of time. (2) Low threshold. Some units were evoked by excitation of the A,f group of splanchnic afferents only. They had a shorter latency and a lower probability of response, which was not increased by exciting Ad fibres.6. It is suggested that the two groups of splanchnic afferent fibres project to the anterior lobe of the cerebellum as two separate functional systems.

SUMMARY1. Electrical stimulation of the right splanchnic nerve evoked the discharge of single units in the right cerebellar cortex of the anaesthetized cat. The majority of the units also responded to electrical stimulation of an area of skin on the right side of the anterior abdominal wall. The units were all located in lobules Vb-f (culmen) of the anterior lobe of the cerebellum, lateral to the paravermian vein.2. The response evoked by a conditioning stimulus (C) to one afferent pathway was followed by a period during which the response of a testing stimulus (T) to another pathway was diminished or absent. Diminished responsiveness was indicated by a reduction in the probability of the unit firing and a prolongation of the mean latency of the test response.3. The effects of interaction depended on the interval between conditioning and testing stimuli and could be demonstrated with intervals as long as 150 msec. Complete blocking of the test response occurred with C-T intervals between 30 and 40 msec; at intervals shorter than 25 msec the effects were less intense.4. If the C-T interval was constant, increasing the strength of the conditioning stimulus or decreasing the strength of the testing stimulus produced in each case a more powerful and prolonged inhibitory effect on the test response.5. When two successive stimuli were applied to the splanchnic nerve, the discharge evoked by the first stimulus was invariably followed by complete block of the response to the second stimulus over a wide range of intervals.