THE CEREBELLUM 



126 = 



dictates the reanalysis of many aspects of responses to 

 cerebellar stimulation. 



The posterior lobe, particularly the hemispheral 

 portion, in keeping with its phylogenetic history, seems 

 to turn its attention in the other direction, toward the 

 modification of cerebral control of motor neuron 

 discharge. This is in part inferred from the paucity 

 of reports of movement initiated by stimulation of the 

 hemispheres. It is likewise inferred from the fact that 

 the thresholds to stimulation of the motor and elec- 

 trical activity of the motor cortex are altered during 

 hemispheral stimulation. The evidence is not yet 

 clear as to how much of this 'upstream' influence 

 from the cerebellum influences the cerebral cortical 

 functions themselves. Certainly the alterations in 

 electrical activity and probably the alterations of 

 excitability represent a response of the cerebral cortex 

 per se. On the other hand, the various alterations pro- 

 duced by cerebellar stiinulation during movement 

 evoked by stimulation of the cerebral cortex may, 

 and probably do, depend upon alterations in ex- 

 citability produced somewhere along the subcortical 

 pathway to the motor neuron, and perhaps even at 

 the motor neuron. 



Although segregations and sequestrations of func- 

 tion such as those just outlined are attractive in that 

 they seem to help create order in a confused situation, 

 they cannot be adhered to dogmatically. Surely, 

 there are evidences of overlap and the margins are 

 not clear. And this is as it should be, for in any well- 

 run organization it is essential for the "right hand" 

 to know what the 'left hand' is doing. I'hus, altera- 

 tions of cortically-induced function may be obtained 

 from the anterior lobe and alterations of tonic motor 

 neuron activity may be obtained from the posterior 

 lobe. The potentialities for communication within 

 the cerebellum are tremendous on the basis of solely 

 anatomical considerations. That there is functional 

 order within this rich network is strikingh' demon- 

 strated by the stability of the patterns of induced cere- 

 bellar seizures In unanesthetized animals. 



From the anatomical point of view, somatotopic 

 organization within the efferent pathways from the 

 cerebellar cortex is of a high order as regards cortico- 

 nuclear relations (169). However, ijeyond this point, 

 anatomical data are inadequate to support the func- 

 tional evidences of somatotopy which have been noted. 

 It is important to recognize that such functional evi- 

 dence of somatotopic relations is demonstrable only 

 with threshold stimuli and disappears under condi- 

 tions conducive to intracortical spread of activity. 



.■^LTER.^TIONS OF FUNCTION PRODUCED BY 

 CEREBELL.iiR DESTRUCTION 



Introduction 



Nothing could be more logical than to attempt to 

 define the function of an organ by noting the de- 

 ficiencies suffered by an organism after its destruction 

 in whole or part. Although this method of approach 

 to cerebellar physiology has the longest and the most 

 crowded history, many of the studies have not been 

 susceptible of accurate interpretation. Here again, the 

 drawbacks introduced through lack of anatomical 

 knowledge and lack of attention to histological con- 

 trols have resulted in a great deal of confusion, par- 

 ticularly in the older literature. Lesions made surg- 

 ically were not always confined to the areas intended; 

 secondary destruction resulting from disturbances of 

 blood supply occurred; and lesions resulting from 

 simple exposure and from inadequate closure tech- 

 niques complicated the picture (122). Further diffi- 

 culties arose through lack of understanding of terms 

 used to describe deficiencies, and some of the most 

 hotly contested points seem to have hinged on defini- 

 tions and methods of examination designed to display 

 deficiencies. On the other hand, these defects are by 

 no means universal, and much of the older work has 

 been repeatedly confirmed by later investigators. 

 The studies of the results of ablation of cerebellar 

 material bear out the overwhelming importance of the 

 cerebellum in relation to the control of motor neurons 

 which was revealed by the studies of responses to stim- 

 ulation. Before entering into a consideration of the 

 ablation experiments, it would be well to consider 

 the meanings of some necessary words and phrases 

 without which the description of cerebellar signs 

 would be chaotic. 



We owe to Sherrington (296) the clean definitions 

 of the two basic types of function subserved by striated 

 muscle. Striated muscle may serve in the 'main- 

 tenance of attitude' through tlie development of a 

 continuous isometric contraction. This type of con- 

 traction he called postural tonus. Sherrington empha- 

 sized the importance of postural tonus in antagonizing 

 gravity but also called attention to the role of postural 

 tonus in the maintenance of various portions of the 

 body in stable relationship to each other. Striated 

 muscles may also serve as 'organs of motion,' engaging 

 in brief periods of activity serving to move the whole 

 body or its parts. Such contractions are phasic in their 

 nature and may originate through reflex or voluntary 

 activation. This distinction between postural tonus, 



