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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



necessarily normal concomitants. Moreover, certain 

 cortical influences participate in a temporal and 

 spatial pattern whicli is not reproducible with elec- 

 trical stimulation. 



For these reasons, it soon became apparent that 

 substitution of ablation for electrical stimulation 

 might overcome certain of the difficulties (see 320, 

 449 for early references). The results so obtained, in 

 addition to providing anatomical data of prime 

 importance, have opened new fields of knowledge 

 although not necessarily contributing to the solution 

 of the original problems. 



Clearly, many of the difficulties are inherent in the 

 nature of the brain itself where inxestigations are 

 limited by complex structural and functional arrange- 

 ments. It has become increasingly evident that areas 

 of the cerebral cortex other than the so-called 'motor 

 area' may be responsible for motor activities or partici- 

 pate in their regulation. It is very likely that integra- 

 tion of a normal behavioral pattern of inovement 

 results from the sum of several subliminal activities 

 in groups of neurons which reciprocally influence 

 each other at different levels in the central nervous 

 system. Stimulation techniques potently emphasize 

 the ease of disruption of the delicate balance between 

 inhibitory and excitatory processes through which 

 the firing of neurons is regulated. On the other hand, 

 ablation is of little assistance here since cortical re- 

 section results in functional changes which can rarely 

 be construed as the inverse of the effects of stimula- 

 tion. 



It might, therefore, be considered that the co- 

 ordinated action of structiu'es involved in sensorimotor 

 activities is very easily disrupted when submitted to 

 these experimental procedures and that the experi- 

 menter sees only a distorted view of the original proc- 

 esses. We might, thus, conclude with Sherrington 

 (394) that "our expectation must be modest, for 

 inodest assuredly must be the achiesement reached 

 by such means in a problem of such a nature." 



A further difPxulty in assessing cortical motor func- 

 tions arises from the current practice in clinical ter- 

 minology of identifying the upper motoneuron syn- 

 drome as a 'pyramidal' one in contrast to the 'extra- 

 pyramidal' syndromes. Historically, the development 

 of this terminology, and of the related concept of two 

 separate systems involved in motor functions, arose 

 in the observation that motor disorders of particular 

 types result from lesions elsewhere than in the pyraini- 

 dal tract. Reappraisal of this concept of a double 

 motor system has been necessary (cf 196, 303, 325- 

 328) with the acquisition of anatomical and physi- 



ological data to be presented below and by other 

 authors in this work. Briefly, it must be conceded 

 that when the term 'upper motoneuron syndrome' 

 is used and account taken of a simultaneous involve- 

 ment of extrapyramidal functions, a distinction can 

 be drawn between this syndrome and those which are 

 primarily extrapyramidal in origin. This concept ap- 

 pears to have a firm basis in clinical and experimental 

 observations. Skilled movements are abolished in 

 primates, including man, by lesions in the so-called 

 'motor" area as well as by lesions which involve, 

 partialK' or totally, fibers coming from this area and 

 tra\eling in the pyramidal tract. Two facts, however, 

 are clear, a) Both initiation of mo\ements, using the 

 term 'initiation" in a purely mechanistic sense, as 

 well as their control require adequate information 

 from proprioceptive and exteroceptive receptors. The 

 patterning of a behaviorally appropriate sequence of 

 mo\ement is possible only with a background of in- 

 formation provided from these inputs (45, 141, 336). 

 b) The coordination of tonic and phasic activities in 

 the participatins; muscles as well as all necessary ad- 

 justments in posture, including muscles other than 

 those primarily in\ol\ed, results from integrated 

 actixity in all participating structures (pyramidal, 

 extrapsramidal, cerebellar and spinal). Additionally, 

 there are probalilv concomitant chanoes in the auto- 

 nomic sphere. 



The cortex is obviously one of the places where 

 regulation and modification of an initiated move- 

 ment can occur through convergence here of all 

 relevant information in a structure which is capable 

 of great integrative functions, and which in turn can 

 influence the efi'ector apparatus via the pyramidal 

 tract and by acting upon subcortical structures which 

 mediate extrapyramidal motor effects. Stimulation 

 of the primary inotor cortex, after section of the 

 pyramidal tract (cf 421, 422), is followed by synergic 

 movements which were interpreted as 'significant' 

 acts, even if limited to the proximal segments of the 

 limb (422). 



For this reason, a distinction between pyramidal 

 and extrapyramidal actixities contributino to motor 

 functions at the cortical le\el becomes quite arbitrary. 

 In addition, experimental and clinical findings can 

 demonstrate only predominance of function or func- 

 tions, but they can rarely exclude the coexistence of 

 other functions. We shall, therefore, attempt a de- 

 scription of the role of the cortex in sensorimotor 

 activities without pretense to a rigid scheme of organi- 

 zation. 



