,258 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



petal influence may consist of inhibition as well as of 

 excitation. 



The complete significance of the variety of sources 

 of cerebellopetal influences will become clearer as 

 the information concerning the results of stimulation 

 and extirpation is de\'eloped later in this chapter. 

 At this juncture it is sufficient to point out that in 

 order to bring about coordination of reflex and 

 voluntary movements, the cerebellar mechanisms 

 require a continuous flow of information from all 

 possii)le sources. Complex motor acts such as that 

 of deglutition, or of shifting visual attention from one 

 to another oi)ject of regard, could scarcely be prop- 

 erly coordinated without the assistance of sensory 

 impulses of interocepti\e origin or of visual origin 

 nor without the information carried by impulses 

 originating from the volitional mechanisms of the 

 cerebral cortex. 



The demonstration of tlie existence of localized 

 receiving zones for some forms of cerebellar input, 

 and the contrasting demonstrations of more or less 

 diffuse and overlapping recei\ing zones for cere- 

 bellopetal impulses, have left unsolved many old 

 problems of localization of function in the cerebellar 

 cortex. The overlap of spinocerebellar and cerebro- 

 cerebellar projections in the anterior lobe was pre- 

 dicted on the basis of histological studies and com- 

 pletely nullifies the concept that the paleoccrebellum 

 is the exclusive mediator of spinocerebellar rela- 

 tionships. On the other hand, if it is granted that the 

 paramedian loljules may ije activated by association 

 fibers (i6g), what has been called the neocerebellum 

 may indeed be regarded as the exclusive mediator of 

 cerebrocerebellar relationships. Within that portion 

 of the cerebral cortex which is shared by spinocere- 

 bellar and cerebrocerebellar input, and which is also 

 related through the fastigius and interpositus to 

 extracerebellar structures, the significance of the 

 localized areas for afferent projection remains un- 

 certain. As Combs himself points out, there are 

 serious anatomical objections to calling upon the 

 lateral reticular nucleus of the medulla to bear the 

 entire burden of .such localized projections as have 

 been demonstrated. When experimental conditions 

 are proper for the observation of unlocalized activity 

 within this portion of the cerebellum, it is impossible 

 to say at present whether this is the result of a truly- 

 diffuse projection, in the physiological sen.se, or 

 whether the projection is actually to specific cortical 

 neurons which are not segregated into special com- 

 partments on a somatotopic basis. 



The third concept of importance which derives 



from the electrophysiological studies consists in the 

 striking and dramatic evidence afforded by micro- 

 wire studies of unit discharges from brain-stem nuclei 

 of the manner in which cerebellar mechanisms 

 operate with respect to the primary transmission 

 pathways of the l;)rain. In tiie light of these findings, 

 it seems not unreasonable to regard the cerebello- 

 fugal impulses as playing their role i)y modulating 

 the excitability of key neurons in transmission path- 

 ways which receive their principal activation from 

 sensory sources or sources higher in the brain. By 

 converging upon neurons primarily activated from 

 other sources, cerebellar impulses might be able to 

 e.xert an important control over traffic on the path- 

 way even though the pathway does not, anatomi- 

 cally, course through any part of the cerebellum. 



FUNCTIONAL ALTERATIONS PRODUCED BY 

 CEREBELLAR STIMULATION 



The major portion of the work reported during 

 the nineteenth century must be regarded as almost 

 useless because of the failure of investigators to dif- 

 ferentiate carefully between responses produced by 

 cereisellar stimulation and those produced by stimu- 

 lation through spread of current to nearby brain- 

 stem structures. It must i)e remembered that the 

 characteristics of excitability of neuronal structures 

 were very poorly understood and that techniques for 

 the control of stimulatina; pulse parameters were 

 practically nonexistent. For more complete informa- 

 tion concerning these earlier papers the reader 

 should consult van Rijnberk (346-348), Brun (53), 

 Spiegel (315) or ten Gate (324). 



In 1897 Lowenthal & Horsley (187) and Sher- 

 rington (295) first described inhibitory reactions 

 which were certainly due to stimulation of the cere- 

 bellar cortex. Nevertheless, Horsley & Clarke (160), 

 describing experiments on anesthetized animals, 

 later concluded that the cerebellar cortex was not 

 excitable by direct electrical stimulation. This er- 

 roneous conclusion coupled with the fact that many 

 investigators were looking for phasic movements 

 such as those produced by stimulation of the cere- 

 bral cortex, served to impede progress in this line of 

 experimentation for many years. Since that time, the 

 realization that anesthesia severeh depresses re- 

 sponses to cerebellar stimulation has conditioned the 

 choice of experimental preparation with the result 

 that a great many investigations have been carried 

 out on unanesthetized decerei:)rate animals. This 



