SENSORIMOTOR CORTICAL ACTIVITIES 



82:: 



This point of view offers at least a less discouraging 

 approach to the findings that isolation of the motor 

 cortex, or the infliction of a series of perpendicular 

 incisions in it (251, 357, 409, 451), do not produce 

 significant impairment of function. Furthermore, it 

 has already been stressed that plasticity is one of the 

 major attributes of the central nervous system, as 

 indicated by functional recovery following extensive 

 lesions. This factor of plasticity, which may be re- 

 garded as simply another aspect of the multiplicity 

 of patterns mentioned above, may account for the 

 changes which must occur in central processes in 

 order to bypass both central and peripheral defects. 

 Rearrangement of central control of motor processes 

 would in fact seem po.ssible, at least partially, partic- 

 ularly in certain instances in higher primates, in cir- 

 cumstances where the peripheral effector apparatus 

 has been altered, as by a muscle transplant (cf. 408). 



In addition to the effect seen in the quoted cases of 

 experimental interference, it has been shown that 

 the electromyographic responses recorded from 

 muscles not directly involved in a given voluntary 

 movement (iii) are reduced with age {112), prob- 

 ably as a result of a more effecti\e and economical 

 pattern of central activity. 



No data seem to be available to support concepts 

 of the role of incli\idual neurons in relation to the 

 total motor output {202). In addition, the meager 

 results of cytoarchitectonic studies in the interpreta- 

 tion of functions of different cortical areas strongly 

 emphasize the difficulties of correlating structure and 

 function in relation to integrative processes. As Golgi 

 (179) so succincth- remarked many years ago, the 

 specificity of function of different cortical zones 

 depends not on the organization of these zones them- 

 selves, as revealed by cytoarchitectonic studies, but 

 on the specificity of fibers entering and leaving these 

 regions. 



Such factors as attention, learning, memory and 

 emotion are processes which, although poorly under- 

 stood, certainly play an important role in the per- 

 formance of both stereotyped and novel motor ac- 

 tivity. It is, therefore, not surprising that knowledge 

 concerning the problem of willed movement is still 

 very meager. It must be recognized that none of the 

 neurophysiological data presented here can account 

 for the initiation and arrest of movement nor for the 

 purposive changes made in the course of a movement 

 on the basis of previous experience. It has been sug- 

 gested that actions such as the sudden starting or 

 stopping of motor activities, which may be regarded 

 as at least one aspect of will, take place \'ia pyramidal 



fibers arising in the primary motor area (423). This 

 view agrees with the observation that a loss of many 

 aspects of skilled movements follows ablation of 

 cortical motor areas, implying the participation of 

 the cortex in at least the initiation of these finely 

 patterned aspects of motor activity. As Penfield 

 (355) has pointed out, it is clear that the nature of 

 voluntary action is determined according to sensory 

 information. When, for example, one considers the 

 extraordinary dexterity of hand movements, it is 

 obvious that, in the full utilization of the prccentral 

 cortex, the nerve impulses which reach it must come 

 in a pattern which is vastly varied in time of arrival, 

 in rhythm and in the combination of ganglion cells 

 selected for activation. Penfield concludes that the 

 nature of voluntary action is determined in accordance 

 with the guidance of memory and the conclusions of 

 reason, and that complex integration must occur 

 before the appropriate motor impulses arise in the 

 prccentral gyrus. 



In Penfield's view, this volitional stream of im- 

 pulses impinging on the prccentral gyrus does not 

 arise cortically since neither removal of the area 

 anterior to the prccentral gvrus nor ablation of the 

 postcentral gyrus can entirely abolish skilled move- 

 ments. Penfield looks to the centrencephalic system 

 of the brain stem as initialing a stream of willed im- 

 pulses capable of producing the action that is ap- 

 propriate to all previously received information. It 

 might i)e expected that this \-olitional stream of 

 impulses must originate in ganglionic nuclei, such 

 as the centrencephalic .system, which have functional 

 or preparatory connections with sensory and elabora- 

 tion areas of both hemispheres. Walshe (463, 464) has 

 also directed attention to the possible subcortical 

 origin of these streams of controlling impulses, stating 

 that "the human pyramidal system of itself initiates 

 nothing, and to speak of it as responsible for this or 

 that category of movements is to ignore the source 

 and motive power of its activities." 



Disorders of willed movements, such as occur in 

 parkinsonism, in association with lesions of the basal 

 ganglia and brain stem, have served to kindle new 

 interest in the role of the corpus striatum in these 

 functions. These studies have been reviewed by Clark 

 (95), who points out that there are extensive con- 

 nections between the intralaminar thalamic nuclei 

 and the corpus striatum, and that the latter is in fact 

 a highly important 'sensory ganglion', which receives 

 by way of the intralaminar nuclei, patterns of afferent 

 impulses mediated by the reticular system as a whole. 

 Moreover, in birds the higher functional levels of 



