THE PATTERNING OF SKILLED MOVEMENTS 



l68 9 



of a prepared background of tonic activity of cortical 

 origin which most probably engages the spinal 

 internuncial mechanism. 



These facts have built a firm foundation for con- 

 cepts, originally expressed by Tower (121), concern- 

 ing the presumed function of the pyramidal tract. 



In the light of the results obtained by cortical 

 stimulation and by localized lesions of the pyramidal 

 bundle in the cat, the monkey, the chimpanzee and 

 man, Tower considers the temporal organization of 

 the pyramidal function to be characterized by two 

 complementary components. One, of tonic nature, is 

 the expression of the excitatory state of the cortical 

 keyboards; it brings a graded contribution to excita- 

 tion of the spinal segmental mechanisms. The other 

 is a phasic or episodic function superimposed upon 

 the tonic one "which appears as a specific contribu- 

 tion to individual acts of performances and often as 

 the entire performance." This specific action, ac- 

 cording to Tower, can be held responsible for the 

 initiation, for the control and for the spatiotemporal 

 patterning of the act. Thus, the fineness of the 

 topographical organization underlies the unique 

 feature of corticospinal function: the ability to bring 

 into action any portion of the skeletal musculature 

 and in all combinations. 



Therefore it is tempting, as Bernhard and his 

 collaborators suggest (12), to attribute to the direct 

 corticomotoneuronal connection a privileged role in 

 the control of the fine movements of the extremities. 

 The considerable growth in the number of thick 

 myelinated fibers in the human pyramidal tract 

 makes us expect a si ill greater development of this 

 system in man. Thanks to this monosynaptic liaison, 

 the motor cortical keyboard therefore directly con- 

 trols the discharge of the spinal motoneurons. Through 

 the docility of the spinal keyboard, the corticomoto- 

 neuronal system which stems from the precentral 

 gyrus can be considered as the chief executor of the 

 skilled movements. 



The Pyramidal and Extrapyramidal Contribution 



The objective attained by skilled performance is, 

 however, not an achievement of the corticomotoneural 

 system alone. The ability to control skeletal muscle 

 engaged in discrete action directed toward a given 

 end is dependent upon the coordinated action of 

 cooperating muscles and upon complex postural 

 adjustments. The initial activation of prime movers 

 is always accompanied by excitation of synergic 



muscles and of more proximally lying muscles fixing 

 the joints or sustaining the leading extremity, as 

 well as by graded contraction or relaxation of antag- 

 onists. The whole organization of corticospinal 

 relations is then put into play. 



To the action of the special contingent of thick 

 myelinated fibers, we must first add the participation 

 of the other components of the pyramidal bundle, 

 including the ones which stem directly from the 

 precentral gyrus. In spite of a considerable number 

 of studies, our knowledge on this point remains 

 imprecise. 



The most significant result of localized ablations 

 of area 4 is the definitive loss of the skilled use of 

 the extremities (101, 120, 121 1. Pribram and his 

 associates speak of a 'scotoma of action' which is 

 produced by precentral resections and which inter- 

 feres with the skilled use of certain parts of the 

 musculature (101). Such a deficiency may be at- 

 tributed to the destruction of parts of the motor 

 cortical keyboard. Voluntarj control ol the muscula- 

 ture is, however, possible l>\ paths other than the 

 pyramidal tract. The consequences of capsular 

 hemiplegias (affecting the pyramidal bundle at the 

 level of the internal capsule) are, as a mailer of fact, 

 much more serious than those following lesions con- 

 fined to the motor cortex. The pan played by the 

 pyramidal libers which stem from other cortical 

 areas (principally the parietal 1 ma} .iccount, partlv 

 at least, for the severit) of such disorders. 



On the oilier hand, all experimental and clinical 

 facts agree in recognizing a route oilier than the 

 pyramidal path lor conical action upon the seg- 

 mental mechanism. As Meiiler pointed out, "'Com- 

 parison of the spontaneous and electrically induced 

 behavior of animals after pvraniidoloinv with the 

 intact animals or with those deprived of all or part 

 of the frontal cortex makes it obvious that the con- 

 ducive substrate of most of the functions of the 

 frontal cortex is extrapyramidal" (86). 



By electrical stimulation of area 4 after section of 

 the pyramids, Tower obtained well-integrated 

 'synergic movements' and 'significant acts' (120). 

 These movements are poorly localized, however 

 They occur mostly in the proximal part of the mem- 

 bers and involve synergistic actions of axial muscula- 

 ture. They are slow in starting and stopping. Hines 

 (51 I applied the term 'holokinesis' to this somato- 

 motor activity of a generalized infantile type con- 

 trolled by extrapyramidal mechanisms (53). This 



