THE CEREBELLUM 



I 26 I 



Poinpeiano (243, 245, 246) have demonstrated a 

 functional subdivision in the fastigial nucleus. Small, 

 highly selective lesions in this nucleus have enabled 

 them to differentiate the pathways for \ermal ipsi- 

 lateral inhibition and vermal ipsilateral facilitation. 

 Inhibitory acti\ity courses through the rostrolateral 

 portion of the nucleus and faciiitatory activity is 

 mediated by the rostromedial portion of the nucleus. 

 Both paths then rejoin in the ipsilateral inferior 

 cereljellar peduncle. 



Pompeiano (267, 268) has demonstrated that the 

 contralateral augmentation of e.xten.sor tonus accom- 

 panying slightly suprathreshold stimulation of the 

 anterior lobe \ermis is dependent upon fastigiobulbar 

 fibers which cross after leaving the cerebellum. Evi- 

 dence is also offered suggesting that the total response 

 is organized by bulbar or spinal mechanisms. 



The ipsilateral faciiitatory response to stimulation 

 of the hemispheral portion of the anterior lobe may 

 be duplicated by stimulation of the nuclei interpositus, 

 superior cerebellar peduncle and lateral reticular 

 formation of the medulla (317). In cats with chronic 

 fastigial lesions, it has been found that responses of 

 the two hemispheral strips (245, 270) are mediated 

 through the anterior one third of the nucleus inter- 

 positus. It would appear that the ipsilateral flexor 

 response from the medial strip courses through the 

 medial side of the cephalic portion of the nucleus 

 while the ipsilateral facilitation oi extensor tonus 

 from the lateral strip courses through the lateral 

 side of the cephalic portion of the nucleus. 



The course of these pathways within the brain 

 stem has been the subject of some investigation. 

 Inhibitory responses to vermal stimulation are still 

 obtainable after postcollicular decerebration (219, 

 234, 310) and so must be mediated through pathways 

 confined to the lower pons and medulla. However, 

 it has l:)een demonstrated (23, 32) that rigidity in- 

 creases after postcollicular decerebration as though 

 some inhibitory influences were operating over the 

 superior peduncle and mesencephalic pathways. 

 Hare, Magoun & Ranson (153) suggested that such 

 activity migiit be conveyed over fibers of the brachium 

 conjunctivum descending from the level of the red 

 nucleus. Pompeiano's (270) observations indicate 

 that the ipsilateral flexor response from the medial 

 strip of the hemispheral portion of the anterior lobe 

 is conveyed over a doubly crossed pathway involving 

 the contralateral red nucleus. Thus, this response 

 would not be obtainable in postcollicular preparation. 

 The ipsilateral extensor facilitation, on the other 



hand, persists after postcollicular decerebration 

 and is organized at the medullary level. 



The results of Nulsen et al. (254a) which have 

 appeared only in preliminary form have not been 

 confirmed physiologically and cannot be reconciled 

 with present anatomical information. 



The spinal pathways of these cerebellar responses 

 ha\e not been clearly isolated as discete tracts. It 

 would appear that {163, 269) the descending im- 

 pulses are conveyed over diffusely scattered fibers in 

 the lateral and ventral funiculi which do not cross 

 at the spinal level. This description of transmission 

 pathways is the same as the description of the reticulo- 

 spinal tracts (254). 



Anterior Lobe: Locus of Action. The locus of these 

 faciiitatory and inhibitory effects has not been clearly 

 determined. The .suggestion has frequently been made 

 that the inhibitory action is located in the spinal cord 

 (96, 219, 330, 331). However, in view of the influence 

 of cerebellar discharge upon the reticular formation 

 (310, 349, 350) and on the vestibular nuclei (97), it 

 is possible that the initibitory effects may appear as 

 the result of cessation of facilitation from inhibited 

 brain-stem structures. Although no data have been 

 presented, it is also possible that the faciiitatory effects 

 may be equally indirect. Granit et al. (135) stress the 

 necessity for considering both the alpha and gamma 

 motor neurons as final common paths for cerebellar 

 influences. 



Posterior Lobe: Postural Tonus. The occurrence of 

 responses similar to those obtained from the anterior 

 lobe has also been reported as a result of posterior 

 lobe stimulation. Among the reactive foci are the 

 simplex (VI, H VI) (152, 334), pyramis (VIII A, 

 VIII B) (32, 152, 317, 334), uvula (IX) (317) and 

 paramedian (H VII B, H VIII A) (152, 308, 310, 

 317). One gains the impression that the.se responses 

 are not as reliably produced or as powerful as those 

 derived from stimulation of the anterior lobe. 



Posterior Lobe: Eye Movements. Conjugate deviation 

 of the eyes has been reported as a result of posterior 

 vermis stimulation by numerous investigators (36, 

 86, 104, 112, 152, 153, 161, 231, 233, 250). These 

 have been obtained from the folium and the tuber 

 vermis (VII A, VII B), simplex (\'I) and pyramis 

 (VIII A, VIII B). 



Posterior Lobe: Cerebral Cortex. It is notable that 

 movements or alterations of tonus as primary events 

 resulting from cerebellar stimulation have almost 

 exclusively been related to the stimulation of vermal 

 structures in the anterior and posterior lobes. Never- 

 theless, stimulation of the cerebellar hemispheres, 



