THE CEREBELLUM 



'257 



proper (III, IV, V) and to a much smaller extent 

 from the intermediate portion of the anterior lobe 

 (H III, H IV, H V). 



Convergence of cerebellar and vestibular influences 

 on the same cells has also been demonstrated for the 

 vestibular nuclei by DeVito et al. (97). The most 

 active projection appears to originate from the 

 spinocerebellar areas. Units of the vestibular nuclei 

 could be stimulated or inhibited, or remained un- 

 modified during cerebellar polarization. 



Using single evoked volleys, Whiteside & Snider 

 (359) 3nd Henneman el al. (156) have examined the 

 evoked responses of the midbrain and diencephalon 

 (359) and cerebral corte.x (156) following single 

 shocks to the cerebellar cortex. Responses considered 

 to be asynaptically and others considered to be 

 transynaptically mediated were identified in many 

 portions of the midbrain and diencephalic tegmentum 

 and in the sensory relay nuclei of the thalamus. In 

 experiments involving stimulation of the cortex and 

 nuclei (156), the efferent systems were found to be 

 extremely sensitive to barbiturates. Shocks to the 

 anterior lobe and the simplex were followed by con- 

 tralateral responses in the sensory and motor areas 

 of the cerebrum. Paramedian stimulation evoked re- 

 sponses from the same areas bilateralh'. It proved 

 possible to activate the auditory area and surround- 

 ing zones bv stimulation of the auditory projection 

 zone of the cerebellum, but responses from cerebral 

 visual areas were not recorded. .Stimulation of the 

 ansiform lobule produced irregular and inconsistent 

 cerebral responses. From any point in the cerebellar 

 nuclei, the cerebral sensorimotor cortices were acti- 

 vated bilaterally. 



Perspective 



Out of the details of the many studies of cerebellar 

 anatomy and physiology utilizing electrophysiologi- 

 cal techniques there emerge several concepts of major 

 importance which will aid in an understanding of 

 cerebellar function. 



The first of these is the idea that even in the 

 absence of controlled and purposeful stimuli, the 

 neurons of the cerebellar cortex and nuclei are in a 

 state of activity at a relatively high level. In the usual 

 preparations studied it is impossible to refer to this 

 as spontaneous or resting activity since there is un- 

 doubtedly a wealth of tonic afferent barrage arriv- 

 ing at the cerebellum from many intero- and extero- 

 ceptors and from other portions of the brain. On the 

 other hand, it remains possible that also in the 



absence of the usual drive from extracerebellar 

 sources, the cerebellar neurons may discharge in a 

 truly spontaneous fashion. Whether this neuronal 

 activity is truly autochthonous or whether it is driven, 

 the level of activity is high and shows none of the 

 tendency to synchronization which is so characteris- 

 tic of the cerebral cortex. It seems reasonable to 

 consider, therefore, that cerebellar neurons are, for 

 the most part, very close to their critical level of 

 excitability and that presynaptic influences exert 

 their effects by altering the rate of discharge rather 

 than by initiating a burst of activity from otherwise 

 quiescent cells. In the same line of thought, the 

 high level of activity in intrinsic cortical neurons 

 must be reflected in a tonic discharge from the 

 neurons of the cerebellar nuclei, and indeed this has 

 been observed. Here again, it seems reasonable to 

 consider that variations in cerebellar influence over 

 extracerebellar neurons are brought about by varia- 

 tion of the frequency of cereijellofugal impulses, 

 serving to produce modulation in level of excitability 

 in the recipient neurons rather than by forcefully 

 initiating or terminating the postsynaptic responses. 

 That such a modulating influence may either be 

 excitatory or inhibitory has been amply demonstrated 

 by the studies of unitary discharges of brain-stem 

 structures during induced variation in cerebellar 

 activity. 



The second conclusion of major importance is that 

 cerebellar activity may be modified by cerebellopetal 

 activity and that the potential sources for such modi- 

 fying influences are extremely widespread. It is no 

 longer adequate to consider cerebellar functions in 

 relation to the proprioceptive system alone. While 

 it is undoubtedly true that impulses from propriocep- 

 tive receptors constitute one of the major sources of 

 cerebellar input, it is equally true that other varieties 

 of sensory input and input from other portions of 

 the brain cannot be neglected in any attempt to 

 understand cerebellar function. Many of the sources 

 of cerebellopetal impulses have been defined ana- 

 tomically for years, and the electroanatomical studies 

 in such cases have served largely to confirm and to 

 refine the findings of the older investigations. The 

 new information derived from electroanatomical 

 investigations emphasizes the effectiveness and the 

 distribution of impulses from auditory, visual, cu- 

 taneous and interoceptive sources as additions to the 

 proprioceptive input. It is important to understand 

 also that, even though the surface sign of the evoked 

 potential is essentially the same for all cerebellopetal 

 pathways, the physiological effects of the cerebello- 



