THE PATTERNING OF SKILLED MOVEMENTS 



1699 



and sensory messages back from the muscle to the 

 brain, the part played by sensory control in the 

 coordinating of movement has been clearly recog- 

 nized. Claude Bernard stressed the regulatory role of 

 sensibility in "giving the signal which moderates or 

 which accelerates" (11), and Exner (31) enunciated 

 his principle of 'sensomotility' which asserted the 

 unitary whole of sensory-motor functions. Further- 

 more, it seems unnecessary to recall the emphasis laid 

 on the afferent input in all reflexological theories of 

 motor coordination. 



Observation of motor defects following sensory 

 disturbances in mammals, as well as clinical observa- 

 tions in man, has confirmed the outstanding im- 

 portance of sensory information in regulating motor 

 performance. The crude performance of ataxic 

 patients, as well as the motor impairment of deaffer- 

 ented animals, emphasizes the extent to which the 

 normal functions are impaired when the centers are 

 prevented from receiving muscular sensory messages. 

 Precision is lacking and adjustment remains crude, 

 although in man functional capacity may be later 

 partially restored with the vicarious aid of vision. 



The importance of sensory messages coining back 

 from the periphery for the initiation and the control 

 of the movement is especially well illustrated by the 

 early observations of Mott & .Sherrington (89) in a 

 monkey having partially or completely deaffercnted 

 forelimbs. The monkey appears to be reluctant to 

 make voluntary use of its completely deaflcrented arm. 

 The whole motor mechanism is however intact and 

 can be activated quite normally in primitive de- 

 fensive or attacking acts under emotional stress; but 

 in the normal course of life, there is no attempt l>\ 

 the animal to use such capacities in an intentional 

 manner. Thus, the elimination of sensory innervation 

 produces a kind of localized apraxia. It is noteworthy 

 that the preservation of small parts of the cutaneous 

 innervation of the member appears to be sufficient to 

 preserve the effective use of the otherwise deaffer- 

 ented limb (89 1. 



furthermore, experimental analysis has given us 

 important information concerning the functional 

 organization of the complex spinal machinery. The 

 excitatory state of the spinal keyboard, which ul- 

 timately conditions its reactivity to cortical com- 

 mands, appears to be finely modulated by a great 

 variety of regulatory influences of reflex origin. We 

 know, for instance, the contribution of stretch re- 

 flexes to the smoothness of muscular contraction, and 

 the participation of proprioceptive messages in the 

 complex postural adjustments of the body musculature 



(see Chapter XLI by Eldred on posture in this Hand- 

 book). 



Although less complete than the preceding, our 

 present knowledge of the functioning of the cortical 

 motor areas also indicates clearly the modulating 

 action of sensors origin which conditions the ex- 

 citatory state of the upper motor neuron kevboard 

 (15). Of particular interest for our purpose is the 

 role of the neocerebellum which seems to be directly 

 involved in the mechanism of control of the discharge 

 of corticospinal impulses (2, 1271 



The neocerebellar cortex which in man and, to a 

 lesser degree, in the other higher primates surpass,^ 

 the more primitive cerebellar structures in size and 

 functional importance assumes an important part in 

 coordinating voluntary movements. Injury to this 

 mechanism causes, among other effects, very char- 

 acteristic disorders in the spatiotemporal patterning 

 of the voluntary command. The weakness and the 

 lack of precision of the movement, the poor timing of 

 its components (asynergy I, and the forced oscillations 

 of the limbs at the start and at the end of the move- 

 ment (intention and terminal tremor) are common 

 li'.iiures of neocerebellar disorders which are mani- 

 fested in a variety of ways (56). 



Even at the levels at which volitional impulses 

 originate, the modulating action of sensory messages 

 at every moment keeps the activity of central struc- 

 tures in harmony with the varying position of the 

 body parts in movemenl and with the state of the 

 ever-changing external field of action. 



Still more broadly, we may consider the total 

 afferent influx as producing, bv u.iv of both its 

 specific and unspecific channels of distribution 

 throughout the central structures, a continuously 

 shifting background of central excitability. We have 

 already been led to consider the part of such dyna- 

 mogenic influence in the arousal of motivational 

 forces which put to work and which sustain the 

 voluntary control of action. 



Therefore the spatial and temporal pattern of 

 impulses required for a purposeful movement cannot 

 be seen as automatically and blindly released into the 

 channel of the executive pathways by the originating 

 structure. It i^ progressively built up by the spread of 

 central commands through the lower structures. It is 

 remodeled at each way station of the executive 

 system in accordance with the modulating influences 

 which converge from the peripheral sensory mech- 

 anisms. 



All these facts emphasize clearly the outstanding 

 role played by the regulative action of sensorv origin 



