CHAPTER XCVIII 



POSTURAL COORDINATION 



The maintenance of posture is accomplished by the tonic contraction 

 of the skeletal muscles. Not only must the muscles assume a new tonic 

 state in each new position of the body, but the tone of the muscles must 

 alter rapidly and in harmony with voluntary and reflex contractions 

 of a tetanic nature. Each voluntary act must also be made with reference 

 to the preexisting posture of the parts involved. The smooth continuity 

 of the normal movements of the body is dependent on the mechanism 

 which correlates postural with voluntary and reflex tetanic contraction, 

 and disturbances in this mechanism give rise to incoordinated movement, 

 ataxia, and the abnormal states of contraction seen in the muscles of 

 sufferers from nervous disease. 



The Reflex Adjustment of Tone. The tonic contraction of skeletal 

 muscle is maintained only in the presence of a reflex arc. The afferent 

 neuron of this arc arises from the muscle itself. This is shown by the fact 

 that after cutting all the nerve trunks to neighboring muscles and the 

 skin the tone of the muscle persists, but it disappears at once on sever- 

 ing the dorsal root through which the afferent fibers from the muscle 

 pass. The receptors of the afferent neuron which lie in the muscle 

 substance are called proprioceptors, and the reflexes which they ini- 

 tiate proprioceptive reflexes. The efferent path of the arc is completed 

 by the motor neurons of the muscle. In the presence of this arc tone 

 is not only maintained, but plastic changes in tone may be induced by 

 impulses traveling over it. 



Two types of reaction are recognized which bring about changes in 

 the postural tone of skeletal muscle. If the knee of a spinal or decere- 

 brate mammal is flexed by pushing the foot with the hand, the resistance 

 which the tone of the extensors opposes to the movement may be felt to 

 give way, almost suddenly, as the joint moves into its new position. 

 On releasing the limb, it will now remain in the flexed condition as the 

 result of a reflex change in the tone of its muscles. The explanation 

 of this is that stretching the extensor muscle has stimulated its proprio- 

 ceptors and set up a reflex which causes a lengthening reaction of the mus- 

 cle. If the knee is now forced into an extended position a similar phenome- 

 non occurs and the tone of the muscles adjusts itself to the extended posi- 

 tion. The proprioceptors of the extensors have in this case been excited by a 

 decrease in the tension of these muscles and have given rise to a short- 



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