1264 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



Clark (72) reports that complicated patterns of move- 

 ment may be evoked by electrical stimulation. These 

 patterns are constant from day to day and differ from 

 point to point. The response pattern starts with the 

 production of a sustained abnormal posture during 

 the stimulation, forceful rel)ound immediately after 

 the stimulation and then continues in a long sequence 

 of bizarre postures lasting several minutes as they 

 progress in slow lesiurely fashion from one part of 

 the body to another. Similar '.seizures' are produced 

 by mechanical stimulation (71) and by stimulation 

 of certain interior portions of the cerebellum (65). 

 The 'seizure" is present after bilateral cereiiral de- 

 cortication and deafferentation (73, 74). 



If the cerebellum is split in the mid-line, the 'seizure' 

 is unilateral but is capable of crossing small ijridges 

 of intact cortex anteriorly and posteriorly. McDonald 

 (205) has repeated this type of experiment using im- 

 plants on the anterior lobe. During stimulation, slow 

 collapse of extensor tonus occurs in the ipsilateral 

 forclimb or hind limb, depending upon the location 

 of the electrode. This is at times accompanied by 

 contralateral extension and is followed by post- 

 stimulation rebound and seizures. Sprague & Cham- 

 bers (317) indicate that with threshold stimuli ap- 

 plied to the fastigial nucleus, the same responses are 

 seen in the unanesthetized as in the decerebrate 

 animal, and that if the voltage is raised seizures may 

 occur. 



Perspective 



The organization of this portion ol the chapter on 

 the cerebellum was constructed in an attempt to help 

 the reader discover the meaning and significance of 

 the varieties of experiments which have dealt with 

 cerebellar stimulation. Certainly this is a difhcult task 

 in the light of the amount of data which has been 

 accumulated. Even more difficult is the task of under- 

 standing the details of the mechanisms involved in 

 the production of responses to cerebellar stimulation. 

 It is obvious, particularly in this last respect, that we 

 do not yet know enough to arrive at any clarity of 

 understanding. However, certain superficial general- 

 ities may be worth emphasizing in the way of a sum- 

 mary, even if the remarks only repeat what has been 

 said many times before. 



In spite of the bias introduced through the use of 

 the decerebrate preparation in so much of this work, 

 it nevertheless remains clear that cerebellar activities 

 are principally related to the adjustment of tonus of 



striated muscle. This is simply a different way of say- 

 ing that the initiation or precipitation of motor neuron 

 discharge does not seem to be the principal task of the 

 cerebellum. Instead, its forte is in the adjustment and 

 regulation of the time-space pattern of motor neuron 

 discharge precipitated by activity in some other por- 

 tion of the ner\ous system. This appears evident from 

 the time course of the reactions to cereljellar stimula- 

 tion which are somewhat more leisurely and pro- 

 longed than those resulting from cerebral or reflex 

 stimulation. It also may be inferred from the manner 

 in which cerebellar influences add to and subtract 

 from the potency with which cerebral and reflex 

 mechanisms may govern motor neuron activity. 



While the function of the cerebellum as a whole 

 may i)e regarded in this general light, there are indi- 

 cations that not all portions of the cerebellum are 

 equipotential insofar as the mechanisms through 

 which these influences are brought about are con- 

 cerned. Since so little has been accomplished with 

 the vestibular portions of the cerebellum in experi- 

 ments involving stimulation, it is justifiable to dismiss 

 this portion from our present considerations. The most 

 important diff"erences relate to the mechanism of 

 function of the anterior and posterior lobes. The 

 differences are not mutually exclusive and sharply 

 defined, but in the present state of our knowledge it 

 seems justifiable to emphasize them. 



The anterior loije would appear to be concerned 

 primarily with the regulation of motor neuron re- 

 sponse to reflex control mechanisms. In the execution 

 of this function its influences are exerted upon de- 

 scending pathways involving portions of the extra- 

 pyramidal motor nuclei such as the red nucleus, the 

 reticulospinal systems descending from the brain- 

 stem tegmentum and the vestibulospinal systems. 

 There is ample evidence that both facilitatory and 

 inhibitory capabilities, necessary to proper operation 

 of any control system whether biological or physical, 

 are also possessed by this control system. While the 

 pathways over which these functions leave the cere- 

 bellum have been fairly well clarified, it remains un- 

 certain whether the final mechanism is uniform for all 

 aspects of function. For example, we still do not know 

 whether all inhibitory responses are brought about 

 by cerebellar activation of a brain-stem inhibitory 

 mechanism or whether some may be brought about 

 by cerebellar inhibition of a brain-stem facilitatory 

 mechanism. The fact that the cerebellum also par- 

 ticipates importantly in the control of the gamma 

 motor neurons as well as in the control of alpha motor 

 neurons is a new and promising oliscrvation which 



