THE CEREBELLLVI 



1259 



means that the larger share of observations have 

 dealt with responses which are superimposed upon 

 the abnormal background of decerebrate rigidity. 

 This fact has certainly colored the general viewpoint 

 of cerebellar function as revealed by stimulation in 

 such a fashion as to emphasize the role of the cere- 

 bellum in relation to postural reactions. 



Due to technical difhculties, cramped spatial rela- 

 tions and a certain lack of interest, the literature on 

 the reactions of submammalian forms to cerebellar 

 stimulation is quite scanty. The major portion of 

 such investigations has been carried out on birds, 

 and the results serve to establish the general similari- 

 ties of avian responses to mammalian responses, 

 although some very interesting specific questions have 

 been raised (34, 43, 46, 70, 192, 193, 200). 



Stimulation of Cerebellar Cortex 



ELECTRicwL sTiMUL.ATioN. Any attempt at a brief ex- 

 position of the studies of responses to electrical 

 stimulation of the cerebellum is complicated by 

 several difficulties. Since the cerebellum is so sensi- 

 tive to general depressing influences, it is no surprise 

 to encounter variations in results from one study to 

 another. The significance of anatomical details has 

 not been appreciated by all investigators with the 

 result that foci of stimulation are not always clearly 

 described. The responses themselves are complex 

 and often appear in opposite sign in various limbs. 

 In the following, the attempt is made to divide the 

 material anatomically and to discuss further on the 

 basis of a functional suijdivision in terms of response 

 types. 



Anterior Lohe: I'ermian Portion. Cerebellar cortical 

 stimulation in the vermian portion (between the 

 paravermian veins) may give rise to a decrease, in- 

 hibition, or an increase, facilitation, of pre-existing 

 motor neuron discharge. Since the inhibitory re- 

 sponses have priority both by virtue of longevity and 

 by sheer weight of paper devoted to their descriptions, 

 they will be considered first. 



As indicated earlier, Sherrington (295) and Lowen- 

 thal & Horsley (187) first described inhibition of 

 decerebrate rigidity during cerebellar stimulation in 

 cats, dogs (187) and monkeys {295). The collapse of 

 rigidity was said to be prompt and complete on the 

 ipsilateral side but also involved contralateral limbs. 

 There followed a long period during which anterior 

 lobe stimulation was not effectively used as an inves- 

 tigative tool, a silence that was broken by Bremer's 

 classic papers describing the inhibitory response in 



greater detail and describing also the powerful re- 

 bound contraction at the end of stimulation (32). 



Since that time, the inhibition of tonus accompany- 

 ing cerebellar stimulation has been the subject of 

 numerous studies (23, 33, 35, 96, 210, 221, 232-238, 

 309, 310, 317). Many of these studies have made 

 contributions to the presently held conclusion that 

 inhibition of decerebrate rigidity on the ipsilateral 

 side is the result of stimulation of the truly vermian 

 portion of the anterior lobe with threshold shocks 

 above 40 per sec. It has been demonstrated (152) 

 and confirmed (236, 310) that there is a certain de- 

 gree of somatotopic localization within the area 

 under consideration in that the forelimb is most 

 easily affected from the culmen (I\', \'), the hind 

 limb from the centralis (III) and the tail from the 

 lingula (I). This somatotopic arrangement is easily 

 obliterated by slightly suprathreshold stimulation 

 (267, 268). The inhibition of forelimb extensor tonus 

 is not reciprocal inhibition of spinal origin since it is 

 not accompanied by contraction of the fle.xor muscles 

 and since it develops more slowly, lasts longer and 

 shows recruitment (32, 96, 237, 317). The extensor 

 inhibition is extremely powerful in the sense that it 

 cannot be overcome by maximal vestibular and pro- 

 prioceptive reinforcement of decerebrate tonus (222- 

 224). 



It is also important to note that inhibition of many 

 other activities of the brain and spinal cord may also 

 be observed during stimulation of the vermian por- 

 tion of the anterior lobe. Not only is decerebrate 

 tonus decreased by cerebellar stimulation, but the 

 crossed extensor reflex and its myotatic appendage 

 may also be obliterated (32, 219). The running move- 

 ments of high decerebrates may be halted (32) 

 and the somatic and autonomic components of sham 

 rage in the thalamic animal may be held in abeyance 

 (231 ). Vasomotor reflexes (225, 226) and galvanic skin 

 reflexes (355) are among some of the simpler auto- 

 nomic functions inhibited by cerebellar stimulation. 

 Movements induced by chemical (227-229) and elec- 

 trical (309, 310) stimulation of the cerebral cortex 

 may be blocked by cerebellar inhibition. The strych- 

 nine convulsion of the spinal cord (37) may be re- 

 duced in frequency but not in amplitude by cerebellar 

 inhibition (330, 331). And finally, of utmost impor- 

 tance, is the observation that gamma neuronal ac- 

 tivation of muscle spindles may be inhibited by 

 cerebellar stimulation (134, 137). 



In spite of these numerous observations of the 

 powerful and widespread inhibitory effects of stimu- 

 lation of the anterior lobe vermis, the same cerebellar 



