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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



ness and complexity of the processes involved here 

 are well illustrated by the clinical observation that 

 cerebellar ataxia in man can be o\ercome, to a great 

 extent, by visual control of the movement. Even if 

 it is impossible to determine where the integrative 

 processes involved in this control take place, it is 

 highly significant that no other inputs can reduce 

 the ataxia consequent to lesions of the dorsal roots. 

 While in the latter case the disorganization of motor 

 control can be largely explained on a reflex basis, due 

 to loss of proprioceptive afferent vollevs in spinal 

 centers, the role of visual inputs in overcoming 

 cerebellar ataxia implies the inter\'ention of more 

 complex central processes in which higher functions 

 participate. Attention by the subject to the motor 

 performance is certainly one of these processes, and 

 here neurophysiological techniques have recently 

 provided certain basic correlates. 



It has been postulated that modulation of sensory 

 inputs by central activity is a process strictly bound 

 to the mechanism of attention (cf 279). Anatomical 

 observations have shown that collaterals of pyramidal 

 fibers terminate in the cuneate, gracile and trigeminal 

 nuclei (85, 243). Physiological data on the role of 

 these connections are not yet available, but it would 

 seem reasonable to assume that somato.sensory inputs 

 may be modified, at the level of these relay nuclei, 

 by the activity of pyramidal neurons. The anatomical 

 observations quoted suggest mechanisms and path- 

 ways through which stimulation of the reticular 

 formation may induce changes in the output of these 

 nuclei (193). The possibility that the motor cortex, 

 itself, can modify .sensory inputs directly would appear 

 to parallel the actions tentatively attributed to 

 other systems of corticifugal fibers arising in sensory 

 areas. Suggestions by earlier investigators in this 

 area have already been mentioned. The question has 

 been reviewed by Galambos (164) in relation to 

 the proijlem of central control of acoustic inputs. 

 Various sensory inputs other than somatic have also 

 been shown to be susceptible to reticular stimulation. 

 These results are reviewed in C'hapter LI I by French, 

 in Chapter XXXI by Livingston and in the mono- 

 graph by Ro.ssi & Zanchetti (381). 



Reverting to more strictly cortical aspects of 

 sensorimotor integration, the overlap between the 

 second sen.sory area and the motor representation in 

 primates would appear to indicate a close relation- 

 ship between sensory and motor activities in this 

 cortical region, but this problem deserves further 

 study. Experimental evidence concerning the pjossi- 

 bility of evoking movements from both primary and 



second sensory areas has been reviewed by Hess et al. 

 (197) who reported contralateral movements evoked 

 in the cat by stimulation of the primary sensory area 

 and homolateral movements from the second sensory 

 area. The role of polysensory areas in relation to 

 motor activity is unknown. Points have been mapped 

 on the lateral surface of the cortex of the cat which 

 respond to a variety of sensory inputs (cf. 15J. A 

 region has been found overlapping the representation 

 of vestibular cortical afferents where interaction of 

 acoustic and somesthetic inputs occur (57, 312). 

 It has been suggested that this polysensory area may 

 participate in integration of postural and purposive 

 motor activity (57), but as yet no supporting evidence 

 is available. 



Vestibular inputs certainly participate in the 

 regulation of posture and in the patterning of ac- 

 tivity on which a motor act is leased. While these 

 actions arc mostly subcortical (cf. 228), involving 

 particularly cerebellar, brain-stem and spinal mech- 

 anisms, the vestibular projections to the cortex (see 

 Chapter XXII by Gernandt in this work), as well as 

 interaction of \estibular with other sensory inputs 

 in the basal ganglia (392), suggest a contribution of 

 these inputs to the cortical regulation of motor 

 functions. Disorders of equilibrium are commonly 

 seen clinically where lesions are localized in the frontal 

 and parietal lobes, but it would seem that these 

 functions have a greater and more direct influence on 

 parietal lolje activities (cf. 1 13), possibly in connection 

 to the formulation of the so-called "body scheme' 

 (cf. 113, 292). Ecjuililjratory sensations were also 

 produced by stiimihition of the temporal cortex (358). 



Role of Pxramidal System in Relation 

 to Willed Movement 



Voluntary movement is the subject of Chapter 

 LX\'II h\ Paillard in this work. The present dis- 

 cussion considers only a few aspects of this problem. 

 The absence of adequate knowledge, which we have 

 frequenth emphasized in the course of this re\iew, 

 heavily be.sets the possibility of evaluating cortical 

 components in the initiation of a voluntary motor 

 act. It would appear even more difiicult to attribute 

 specific patterns of neuronal activity to such an act 

 since learned movements are essentially adaptive and 

 do not depend on the use of a particular group 

 of muscles (218, 253, 254). This fact would .seem to 

 refute the hypothesis of a simple and unicjue pattern 

 of neuronal acti\il\ in a given motor performance 

 when this is considered as a means to achie\e a goal. 



