THE EXTRAPYRAMIDAL MOTOR SYSTEM 



903 



specific movements studied by Hess (100-105) were 

 described earlier in the section on statokinetic struc- 

 tures in tiie brain stem. Only their principal features 

 may be briefly reviewed here. Electric stimulation 

 with implanted electrodes demonstrated the existence 

 of a system of motor mechanisms in the mesodien- 

 cephalic regions of the extrapyramidal system con- 

 trolling the position of the eyes, the head and the 

 trunk (figs. 8, 11). Subsequent coagulation produced 

 a mirror image of the pattern previously produced by 

 stimulation of the same structures, as shown in 

 figure 10. These observations indicate that posture is 

 regulated by constant tonic activity of several 

 antagonistic mechanisms operating in all three 

 dimensions. Elimination of one of these mechanisms 

 uncovers the activity of the antagonistic mechanisin, 

 normally in balance with it. Thus normal posture is 

 the result of a dynamic equilibrium of central antag- 

 onistic forces continuously active in the waking state 

 and controlled by peripheral afTerents. 



Hess has emphasized that intentional movements 

 of the body are directed toward certain aims {Erfolgs- 

 bezogen). To reach these aims a characteristic posture 

 and starting position of the body must necessarily be 

 assumed. 



"In the course of voluntary movements in conse- 

 quence of muscular action there result certain reac- 

 tive forces which are compensated, the governing 

 impulses being supplied mainly by vestibular and pro- 

 prioceptive mechanisms. These reflex preparations 

 of start-positions provide what may be called the 

 'dynamic support' upon which the voluntary aimed 

 movements are superimposed. In this functional con- 

 ception no diff'erentiation is made between pyramidal 

 and extrapyramidal innervation, since these terms 

 refer to anatomic relations. In order to emphasize the 

 dynamic situation, the term teleokinetic {lelos = aim) 

 is used for the voluntary directed phase of the motion ; 

 and the term ereismatic {ereisma = support) is used 

 to designate the other phase, which provides the 

 basic conditions for every accurately aimed motion" 

 [Hess (107)]. 



Figure 13 illustrates the action of the teleokinetic 

 and ereismatic mechanisms in a model experiment in 

 which three persons represent action and reaction of 

 a motor performance. The intentional teleokinetic 

 mechanism is represented by the upper person leaping 

 to the point marked by the arrow. The supporting 

 ereismatic mechanisms are represented by the second 

 person carrying the leaper and providing postural 

 mechanisms of support in anticipatory readiness for 



action, and by the third person supporting the second 

 to compensate for the rebound. In the left row it is 

 shown that all goes well for the intended movement 

 when the ereismatic mechanisms give the right sup- 

 port and the supporting persons know when the 

 upper jumps; the leaper reaches exactly the intended 

 point. The right row shows the same leap made by 

 the upper person but without proper preparation and 

 supporting activity b\- the others. In this case the 

 carrier falls backwards by the recoil of the leap and is 

 caught only in the last moment by the third. The 

 leaper jumps too short and falls because appropriate 

 postural support is lacking and the proprioceptive 

 reflex regulation of the second person comes too late 

 to compensate for the recoil. 



This model demonstrates some general principles 

 applicable to ereismatic supporting actions in the 

 motor system. Because the second and third men must 

 'feel' the weight and pressure of the first, 'know' the 

 moment of the leap and continuously 'adapt' to 

 various alterations of posture, the following three 

 points are evident: a) proprioceptive reflexes of 

 muscle and labyrinthine origin may be important 

 parts of supporting regulation but act too late to 

 compensate for unforeseen reactions of recoil; b) antici- 

 patory activation of the proprioceptive control of 

 supporting action therefore is necessary for effective 

 motor performances and involves central activation 

 of proprioceptive reflexes; and c) continuous self- 

 regulating modification of postural support by higher 

 central mechanisms compensating each other is 

 needed for successful integration of voluntary move- 

 ments. Although the conception of ereismatic innerva- 

 tion may seem to be purely theoretical, it indicates 

 the existence of physiological mechanisms not yet 

 sufficiently investigated. Thus, muscle spindle activity 

 is regulated at the spinal level by the gamma moto- 

 neuron system which in turn is regulated by supra- 

 spinal extrapyramidal structures, including the 

 reticular formation, the niger and the pallidum. 



Continuous integration of peripheral and central 

 impulses in different structures occurs in this motor 

 regulating system according to Hess" conceptions of 

 proprioceptive steering, von Hoist's principles of 

 Reaferenz (280) and other cybernetic rules. Several 

 servomechanisms with positixe and negative feedback 

 are probably active at different levels, cerebral and 

 spinal. At the lower levels they work according to 

 von Hoist's reafferent principle, the gamma system 

 serving as an additional amplifying protective 

 mechanism. At higher levels the \-estibular regula- 



