CHAPTER XLI 



Posture and locomotion 



EARL ELDRED 



Department of Anatomy, School oj Medicine, University of California at Los Angeles, and the 

 Veterans Administration Hospital, Los Angeles, California 



CHAPTER CONTENTS 



POSTURE 



Posture 



AtTerents Concerned in Posture 

 Afferents from Muscle 



Muscle spindles 



Tendon organs 



Central effects of muscle afferents 



Other afferents in muscles 

 Afferents from Joints 



Pacinian bodies 



Ruffini endings 



Tendon organs 



Central effects of joint receptors 

 Cutaneous Receptors 

 Entcroreceptors 

 Final Comment 

 Efferent Side of Posture 



Patterns of Motor Nuclei Activation 

 Patterns of Motoneuron Activation 

 Postural Maintenance in Man 

 Central Aspects of Posture 



Central Facilitation of Postural Reflexes 



Spinal afferent influences 



Vestibular influences 



Reticular influences 

 Significance to Postural Tone of Motor Innervation 



Spindles 

 Postural Adjustment 



Basic mechanisms 



Afferents in postural adjustment 



Postural adjustments in man 



Central le\els of mechanisms for postural adjustment 

 Locomotion 



Relations of Locomotion to Posture at Reflex Level 

 Afferent Modalities in Progression 

 Periodicity of Locomotion 



Local afferents and rhythmicity 



Distant afferents and rhythmicity 



Background facilitation and rhythmicity 



Rhythmicity and neural balance 

 Additional Effects of Afferents upon Locomotion 



Deafferentation on precision of movements 



Effect of afferent inflow on mode of progression 

 Brain and Locomotion 



of 



SIR CHARLES SHERRINGTON, more than any other in- 

 vestigator, has contributed to our understanding 

 of the key neuromuscular relationships underlying 

 posture. In his words, ''Standing is a large and com- 

 posite postural reflex and in its execution a fundamen- 

 tal element is the contraction of the antigravity mus- 

 cles counteracting the superincumbent weight that 

 would otherwise flex the joints and cause the i)ody to 

 sink to the ground" (170). 



The experiments which led Sherrington and his 

 co-workers to this \iewpoint were principally their 

 demonstration of myotatic reflexes in decerebrate 

 animals (43). Resection of those portions of the brain 

 anterior to an intercollicular plane releases the facili- 

 tatory activity of the lower brain stem from de- 

 scending inhibitory influences and results in a state 

 of exaggerated contraction of antigravity muscles 

 (237), a 'caricature of normal posture.' Denervation 

 of skin, other muscles and finally appropriate dorsal 

 roots reveals that the tonic contraction is chiefly 

 dependent upon afferent messages from the muscle 

 itself (71, 170). Furthermore, the muscle exhibits 

 plasticity for, when forced to lengthen, it is increas- 

 ingly resistant (because of the stretch reflex) up to 

 some critical point, whereupon with the suddenness 

 of the closing of a clasp knife it gives way (by means 

 of the lengthening reaction). If, after a pause, fur- 

 ther lengthening is attempted, the same initial re- 

 sistance and e\entual yielding are felt, so that suc- 

 cessive lengthenings with renewed assumption of 

 tension at the new length may be repeated every few- 

 degrees over the total range of excursion of the joint. 

 Finally, to complete the picture of plasticity, when 

 the muscle is now allowed to shorten, each new 

 length is marked by a new and specific level of con- 

 tractural tone (through the shortening reaction). 



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