THE PATTERNING OF SKILLED MOVEMENTS 



l68 5 



arise: what are the factors which determine the 

 choice of the muscles to be engaged in a given move- 

 ment and the variable spatiotemporal ordering of 

 their activation, and how does this selective opera- 

 tion operate in the nervous system. We shall look 

 first for the elements required to answer the second 

 question. 



EXECUTIVE PATHWAYS OF SKILLED MOVEMENTS 



Pathological data, as well as the more precise 

 findings of animal experimentation, agree in em- 

 phasizing the role of cortical mechanisms in sub- 

 serving this kind of finely discriminative muscular 

 activity. 



The careful studies of Tower (1201, I lines (51) 

 and others on monkeys with lesions of the pyramidal 

 system, which were reviewed by Tower (121) in 

 1949, have shown the essential part played by the 

 corticomotoneural tracts in conveying the nervous 

 messages that appear to be strictly necessar) for the 

 evocation of skilled movements. Although impaired 

 in various degrees by disturbances in other parts of 

 the nervous system, the ability to perform skillful 

 acts appears to be definitely lost after interruption of 

 this direct pathway between the cortex and the 

 peripheral effectors. 



Since the work of HughlingS Jackson (lib), the 

 concept of a hierarchical functional organization of 

 the nervous structures has become familiar. As a 

 matter of fact, when we try to identify the nervous 

 structures implicated in the expression of skilled 

 movements, two levels immediately command our 

 attention. At the lower level we find in the medulla 

 oblongata and spinal cord groups of motor neurons 

 which hold the peripheral organs under their direct 

 control (Jackson's first level). At the cortical level 

 are located the structures in which arc assembled 

 the neurons of origin of the pyramidal paths (Jack- 

 son's second level ). 



Both of these levels present a remarkable somato- 

 logy of organization. The existence in these two 

 regions of topographically ordered keyboards chal- 

 lenges us to search for the meaning of such a structural 

 arrangement in relation to the spatial patterning of 

 effector commands. 



The Lower Motonei 



Keyboard 



The relative accessibility of the spinal structures 

 early made these regions favored grounds for neuro- 



physiological experimentation. Following the path 

 opened by Sherrington, our knowledge of the ana- 

 tomical and functional organization of these struc- 

 tures has progressed considerably. The study of 

 reflex activity of the spinal cord has contributed very 

 largely to the establishment of the fundamental basis 

 upon which our conceptions of nervous mechanisms 

 are constructed. Each muscle is found to be asso- 

 ciated with a contingent of motoneurons grouped in 

 the ventral part of the spinal grey matter according 

 to a rather strict topographical distribution. The 

 motoneuron, with the muscle fibers that receive its 

 axonic terminations, forms the functional unit of the 

 spinal keyboard. It is the "motor unit' as defined by 

 Sherrington (108). In normal functional conditions 

 each impulse of a motoneuron evokes in the corre- 

 sponding muscle bundle a synchronous contraction 

 of its components (65). This apparent rigidity of 

 organization ol the motor unit does not exclude a 

 great suppleness in the function of the basic mech- 

 anism. 



At the level ol the motor units, the intensity of the 

 niecli.mie.il effect resulting from neuronal discharge 

 evidently depends not onl\ on the number of muscle 

 libers simultaneously brought into play (Sherrington's 

 ratio of innervation), but also on the frequency of the 

 impulses. Due to the viscoelastic properties of muscu- 

 lar tissue, the resulting mechanical effect will increase 

 with the frequency of the impulses. The optimum 

 effective rhythm is of the order of 50 per see. 1 tetanic 

 fusion). The unit develops under these conditions a 

 tension which is five to six times that developed in a 

 single twitch. At the level of the muscle, the coopera- 

 tive action of the various motor units oilers a comple- 

 mentary means of gradation of contraction through 

 recruitment of motor units in an ever increasing 

 number. The most favorable conditions for the co- 

 operative activity which assures subtle and nuanced 

 mobilization of the effector 0114.111 results, as has 

 been shown, from the asynchronism in the activa- 

 tion of the motoneurons. 



In other words, the time and intensity characteris- 

 tics of muscle contraction depend on the timing of 

 the activation of the several units of the motoneuronal 

 keyboard (temporal dispersion), on the proportion of 

 active to inactive units and on the frequency of their 

 discharges. 



It is noteworthy that the anatomical arrangement 

 of the segments most usually engaged in the execu- 

 tion of skilled movements, the distal part of the 

 members, undergoes an increase in the number of 

 the muscles moving the mobile parts accompanied 



