HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY It 



surface and depth of the cortex {28, 38-40, 42, 44, 

 294). The negative component of the response is 

 more labile, more subject to depressing influences, 

 may be augmented by local strychnine and shows 

 temporal summation as a result of properly-timed 

 paired volleys. From these considerations, and from 

 the inversions of potential sign which occur at a 

 penetrating electrode (44, 215, 294), it appears 

 justifiable to conclude that the positive phase of the 

 evoked response develops from postsynaptic activity 

 in the granular layer of the cortex whereas the nega- 

 tive phase is related to the efferent discharge of 

 Purkinje cells. 



SENSORY INPUT. The electrical activity of the cere- 

 bellum is capable of being altered by impulses origi- 

 nating in a wide variety of sensory nerves. Before 

 dealing with the problems of localization on the 

 cerebellar cortex, it seems pertinent to consider the 

 nature of the afferent activity initiated in these 

 studies and the pathways of transmission involved. 



In those studies in which natural sensory stimuli 

 were used to activate receptors, there can be little 

 doubt about the nature and distribution of the af- 

 ferent fibers. It has been found that afferent activity 

 initiated in muscles, tendons and joints is delivered to 

 the cerebellar cortex (2, no, 140, 177-179). This is 

 in keeping with the long-recognized importance of 

 the cerebellum in relation to postural control. The 

 discovery that the stimulation of cutaneous receptors 

 (2, 75, 1 10, 131, 303, 311, 312) and even auditory 

 and visual receptors {28, 41, 117, 128, 129, 131, 311, 

 312, 319) could also activate the cereliellar cortex 

 was unexpected since it had not been predicted on 

 either anatomical or physiological grounds. 



The technique of evocation of cerebellar responses 

 jjy electrical stimulation of sensory nerves introduces 

 complications which have been of concern to some 

 investigators. With artificial stimulation, it has been 

 noted that the evoked response is dependent to a 

 certain degree upon the nature of the stimulated 

 aflferents and the conduction pathway. It has been 

 reported that the surface response is divisible into 

 three distinct portions having different latencies (140, 

 179). It is generally agreed that the initial component 

 of shortest latency results from the stimulation of 

 Group I afferents and that the impulses course 

 through the dorsal spinocerebellar tract (61, 107, 140, 

 178, 216, 248). There is a lack of clarity in the rela- 

 tionships between the later components of the re- 

 sponse reported by various investigators, particularly 

 with respect to their latency and to tlieir utility for 



localizing purposes. The component of intermediate 

 latencv has been ascribed to a response of neurons 

 of the cerebellar cortex (140) and to more slowly 

 conducted afferent activity (178, 248). A still later 

 component has been considered to originate from 

 impulses in more slowly conducting cutaneous af- 

 ferents (140, 178) and in slowly conducting afferents 

 in muscle nerves (216, 217, 248). 



The problem of the conduction pathway utilized 

 within the central nervous system has been attacked 

 principally with the technique of partial cord tran- 

 section. Because of the possibility of sparing a few 

 effective fiijers, this approach presents problems 

 which are not completely avoided by the technique 

 of recording from the ascending fiber systems. In the 

 opinion of some investigators, both the dorsal and 

 ventral spinocerebellar tracts conduct impulses of 

 both cutaneous and proprioceptive origin, and con- 

 vergence from the two sources upon single elements 

 of the tract occurs (61, 140, 148, 217). At the le\el of 

 the superior cerebellar peduncle (61) ventral spino- 

 cereijellar tract impulses may arrive over both crossed 

 and uncrossed pathways, and evidence of additional 

 crossing within the brain stem or cereiiellum has 

 been obtained as predicted by anatomical studies 

 (48). On the other hand, Oscarsson (256) reports 

 that the ventral spinocerebellar tract, at spinal cord 

 and peduncular levels, is acti\ated solely from con- 

 tralateral muscle nerve stimulation in which the 

 volley includes Group II) afferents. Oscarsson's 

 studies also led him to the conclusion that the dorsal 

 spinocerebellar tract is largely uncrossed and may be 

 activated by both Group la and lb afferents. 



The dorsal column system has been shown to be 

 activated by impulses originating in skin, muscle and 

 joint nerves (64, 216, 218). The ventral quadrant of 

 the spinal cord also furnishes an important pathway 

 to the cereijellum in the form of the crossed spino- 

 olivary and uncrossed spinoreticular fibers (26, 148, 

 217) capable of being acti\ated by both cutaneous 

 and proprioceptive stimulation. In these systems, too, 

 further crossing has been shown functionally and 

 anatomically (48) to occur at tlie upper end of the 

 pathway. 



The degree of localization of cerebellar responses 

 evoked by natural or by artificial stimulation of 

 sensory nerves has not been uniform in all studies of 

 this subject. In Dow's early studies on decerebrate 

 cats (107), sciatic, saphenous, median and ulnar 

 stimulation gave rise to responses which appeared 

 bilaterally over the anterior lobe (I-\', H I-H \'), 

 simplex (\"1, H \'I), pyramis (Mil A, \'III B) and 



