1274 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



tonic cerebellar influences on the inhibitory portion 

 of the medullary reticular formation. The importance 

 of differentiating; between signs of spasticity dependent 

 upon enhanced gamma efferent firing (134-136, 285) 

 and similar signs dependent upon primary facilitation 

 of alpha efferent discharge has recently been force- 

 fully emphasized (155, 332, 333). 



ORIGINS OF HYPOTONIA AND ASTHENIA. It will be re- 

 called that hypotonia is one of the prominent signs of 

 cerebellar ablation in the primates and appears follow- 

 ing the abatement of the dynamic signs in the quad- 

 rupeds. This alteration in motor neuron behavior also 

 has a comple.x origin which is not clearly understood. 

 The silencing of the gamma efferent system (135) is 

 one of the final steps which is probably in\olved, but 

 the intermediate steps have not been revealed. The 

 additive effects of cerebral and cerebellar ablation in 

 producing spasticity (185, 313, 314) certainly point 

 to an inhibitory function on the part of the cerebral 

 cortex, but again the intermediate steps are unknown. 

 The spinal cord itself must certainly be considered as 

 a third probability as an origin of hypotonia. Ev'en in 

 the acute decerebrate preparation, section of the 

 spinal cord at the upper thoracic level will be followed 

 by an augmentation of decerebrate rigidity in the 

 forelimbs — the Schiflf-Sherrington phenomenon (318). 

 This inhibitory influence which is furnished by the 

 normal spinal cord may be related to the abatement 

 of the spastic signs and, through its exaggeration, to 

 the subsequent atonia. This probability is supported 

 by recent experiments (12, i6) which differentiate 

 labvrinthine and tonic neck reflex mechanisms in- 

 volved in the Schiff-Sherrington phenomenon. 



Although no clear-cut evidence is available, it has 

 been suggested (36) with reason (281, 282) that as- 

 thenia is a manifestation of the withdrawal of a tonic 

 facilitatory action exerted on the cerebral cortex by 

 the cerebellum. 



Mechanisms of Influence on Phasic Reflexes 



Although no refle.x pathways are considered to 

 course through the cerebellum, the possibilities for 

 indirect cerebellar influences on reflexes are numerous 

 because of the number of structures and paths through 

 which the cerebellum might indirectly exert a modu- 

 lating influence. Modifications of behavior in the 

 gamma efferent system is undoubtedly one of the 

 features of reflex control which must be considered 

 (134). Influences on the brain-stem reticular forma- 



tion (213) and vestibular nuclei (97) offer other routes 

 for the production of alterations of reflex excitability 

 (300, 301). Even reflexes which are mediated through 

 cerebral cortical function (66, 316) may be secondarily 

 altered by the influence which the cerebellum exerts 

 upon the cerebral cortex (60, 88, 281, 282). 



Mechanisms of Influence on I 'oluntary Movement 



It is obvious that every voluntary movement must 

 be initiated from and superimposed upon a back- 

 ground of posture. It follows then that inadequacies 

 of postural control and abnormalities of postural re- 

 flexes (300, 301) will be reflected in disturbances of 

 phasic movements induced by voluntary action. How- 

 ever, in addition to this general deficiency of functions 

 which form a foundation for voluntary movement, 

 other, more specific possibilities should be pointed out. 

 It has already been mentioned that these disturbances 

 appear as manifestations of cerebellar dysfunction 

 primarily in the form of tremor, dysmetria and dys- 

 coordination. 



Although the brain-stem structures the disturbance 

 of which underlies the genesis of tremor are still in 

 doubt (62, 206), the manifestation of tremor as a dis- 

 turbance related to cerebral motor function seems well 

 established. Reasoning that tremor was a charac- 

 teristic of poorly controlled v'oluntary movement, 

 Fulton et al. (127) were led to demonstrate its aboli- 

 tion in the decerebellate cat by decortication. In an 

 extension of this work to the baboon and macaque 

 (9), it was found essential to ablate the entire pre- 

 central motor cortex in order to abolish cerebellar 

 tremor. Thus, disruption of the voluntary control 

 system abolished the signs of its disorganized function. 



It has been suggested that dysmetria and dysco- 

 ordination may have their origin, in part, in the break- 

 down of functional relationships between gamma 

 and alpha efferent systems (318). This suggestion is 

 supported by the observation that gamma discharge 

 precedes alpha discharge in many forms of muscular 

 contraction (136) and that cerebellar inactivation is 

 followed by a depression of gamma efferent discharge 

 (135). The older idea that a disturbance of reciprocal 

 innervation lay at the heart of these two signs of de- 

 ficiency has been disproved (264, 337, 338). In the 

 light of evidence of cerebellar modifications of cerebral 

 functions, it is probable that, like tremor, dysmetria 

 and dyscoordination also depend upon inadequacies 

 of organization of the voluntary control mechanisms 

 within the cerefjral cortex (36). 



