POSTURE AND LOCOMOTION 



IO71 



foot pads where the afferents are presumably cutane- 

 ous (19) and of large caliber (iii). Slowly-adapting 

 mechanoreceptors in the distribution of the sural 

 nerve have been described in the frog (180) and rab- 

 bit (6g). This is especially interesting as stretch of the 

 skin over extensor surfaces causes contractions of the 

 underlying muscle in cats (58, 109), dogs (iii), 

 bullfrogs (ill) and sometimes in man (228), a pattern 

 which fits in well with postural activity. This relation 

 between extensor skin and muscle is not universal, 

 as stimulation of the skin of the back of the labyrin- 

 thectomized decerebrate dog causes a collapse of 

 extensor tone (239). 



Other cutaneous receptors may possibly contribute 

 to posture. Unmyelinated 'C fibers, for example, 

 may weakly facilitate extensor muscles (160) and 

 under certain types of pressure (pinpoint) have un- 

 ceasing discharges (285). Cold receptors satisfy the 

 criterion of having a slow rate of adaptation (117, 

 286) and stimulation of cutaneous surfaces by cold 

 may lead to extensor response (214, 238). Cooling 

 of the sole causes, incidentally, a positive Romberg 

 response in man [Heyd quoted by Harris (iii)]. 



Enteroreceptors 



When man changes from the horizontal to the 

 erect position, widespread displacement of mesen- 

 teric and even retroperitoneal structures, including 

 the kidneys (199), occurs and hollow viscera such 

 as the bladder may experience considerable alter- 

 ations in pressure (92). Pacinian bodies and other 

 receptors, which presumably are affected by these 

 changes, are abundant in the posterior body wall, 

 while the bladder wall and other viscera contain 

 slowly-adapting endings which are sensitive to pres- 

 sure (128). Stimulation of visceral receptors and 

 nerves, although usually favoring contraction of 

 abdominal flexor and psoas muscles, may .sometimes 

 cause contraction of the vastocrureus or lead to 

 progressive movements (198). In intact man the knee 

 jerk is enhanced during strong visceral contractions 

 (132), and in spastic man the close relation of visceral 

 receptors to flexor and extensor spasms is notorious 

 (215). The observation that chronic midbrain ani- 

 mals are sometimes incapable of standing except while 

 defecating may have a similar explanation (143). 



Final Comment 



The criteria, cited earlier, which sensory endings 

 should satisfy if they are to figure prominently in re- 



flex posture, are met, so far as present knowledge goes, 

 by the annulospiral endings of muscle spindles and 

 perhaps by the Ruffini endings of joints. The over- 

 whelming importance of muscular afferents, that is 

 endings in the spindles, in contractions of tonic nature 

 was demonstrated conclusively by Sherrington. 

 However, as shall be seen, joint receptors have also 

 been suspected of playing a prominent role in posture. 



EFFERENT SIDE OF POSTURE 



The final expression of postural integration is 

 determined by the combinations of motor nuclei 

 which are activated and by the pattern of moto- 

 neuron activation within the nuclei. 



Pa I terns of Motor Nuclei Activation 



The word 'posture', although defined variously 

 by the orthopedic surgeon (202), kinesiologist (253), 

 behavioral psychologist (87, 229) or physiologist 

 (170), connotes in each usage active muscular re- 

 sistance to the displacement of body parts by gravity. 

 For the quadruped this means contraction by those 

 derivatives of the primitive dorsal musculature 

 which extend the limbs, arch the back and raise the 

 head and tail (197, 239). However, abdominal mus- 

 cles (244) and the anal sphincter (65, 139, 197), 

 which are fundamentally flexors, also participate, 

 as do the special muscles of mastication. In man even 

 the levator palpebrae and facial muscles may be sus- 

 pected of some form of antigravity tone. Contraction 

 of flexors would appear to be preponderant in the 

 arms of the monkey (217), wings of the pigeon (244) 

 or hind legs of the frog (244), while in the arboreal 

 sloth, the ijalance of postural integration is completely 

 flexor so that the caricature of normal posture seen 

 in the decerebrate sloth is one in which the animal is 

 curled into a tight ball (24, 226). The one feature 

 which consistently characterizes motor nuclei active 

 in postural tone is their inner\-ation of antigravity 

 muscles. 



The question ari.ses whether in the intact animal 

 the activity of antigravity motor nuclei is coupled 

 with reciprocal inhibition of antagonist nuclei. 

 Certainly in tlie anesthetized or decerebrate animal 

 (19), flexor participation in the positive supporting 

 reaction may be strong, but the fact that contractions 

 often radiate to all muscle groups under extreme ex- 

 citation (167, 228) makes inference from these exag- 

 gerated states to the normal one uncertain. Electro- 



