POSTURE AND LOCOMOTION 



1079 



movements of tail, rump and hind legs useful in 

 righting, although these efforts may not succeed 

 because of the absence of postural tone (190, 236). 

 The central organization of other reflexes which 

 have their afferent inflow entirely at spinal levels 

 involves, in addition, portions of the brain stem. 

 Neck reflexes are fully elicitable only if those portions 

 of the brain posterior to a transection through the 

 inferior coUiculus and rostral portion of the pons are 

 retained (201, 221). The 'center' for the body-on- 

 head reflex is said to be in the midbrain (188, 220). 

 Labyrinthine righting reflexes require, of course, the 

 presence of the brain stem at the level of the eighth 

 nerve and. additionally, the midbrain tegmentum 

 (220). Even the lowly frog fails to develop capacity 

 for vestibular righting when, in the tadpole, the brain 

 stem is sectioned at such a level as to exclude the 

 mesencephalon but leave intact the labyrinthine 

 inflow (270). Extreme among adjustment mecha- 

 nisms in the extent of neuraxis involved is visual 

 righting, for here the cortex must be preserved (220). 

 It is evident that complete postural performance, 

 both for sufficiency of contractile tone and of ad- 

 justment mechanisms, requires the presence of supra- 

 spinal levels. 



LOCOMOTION 



Relalinns of Locomnlion to Posture 

 at Reflex Level 



A clinician has said that "postural tunc follows 

 movement like a shadow"; and, indeed, from ob- 

 servation of the 'associated movements' of paralyzed 

 limbs which accompany the use of sound limbs in 

 hemiplegic patients, it is easy to come to the concept 

 that all voluntary movements are accompanied by 

 appropriate postural adjustments of the rest of the 

 musculature (269). Locomotion and posture are 

 even more closely related, and reflexes basic to stand- 

 ing contrii)ute inseparably to progression also. The 

 conversion of the dog leg, for example, to a rigid 

 support by stimulation of the foot pads (positive 

 supporting reaction) is an integral part of the pro- 

 traction phase of 'marking time", or if extension- 

 retraction is vigorous, of a 'step.' The basic reflex of 

 posture, the stretch reflex, adds to the perfection of 

 the locomotor pattern in limiting the ballistic ex- 

 cursion of limbs, preventing lurching of the bod\ 

 to the side of a lifted extremity, restraining antago- 

 nists in alternating contractions, etc. 



Reflexes of postural adjustment also fit well into 

 progression. For example, in man the rotation of the 

 right side of the pelvis forward facilitates flexion of 

 the right leg, extension of the left, flexion of the left 

 arm and extension of the right, actions all appro- 

 priate to walking (261). To cite another example, 

 the dog standing on one hind limb has maximum 

 extensor tone in that leg when it is in the position 

 most favorable for weight bearing while, conversely, 

 extensor tone in the free limb becomes greater as the 

 hindquarters of the dog are shoved from this stabile 

 position, the 'Stemmbein' efTect (220). This reaction 

 fits equally well in corrective hopping or progressive 

 stepping. 



Locomotion employs, in addition, reflexes not re- 

 lated to posture. The 'extensor thrust', for example, 

 although of the same sign as the supporting reaction 

 and elicited from the same general area, differs in 

 that it is excited best by brusque tensing or relaxing 

 of the skin about the toe pads (238), primarily affects 

 hip muscles rather than ankle extensors and is phasic 

 but powerful in efTect (239). Thus, the reflex is better 

 adapted to galloping or springing (springing reflex) 

 than standing. Opposite in sign is the negative sup- 

 porting reaction which permits the leg to be lifted 

 and, assisted by passive changes of this compound 

 pendulum, to be carried forward in a step (19, in, 

 233). Also strictly kinetic in nature are the progres- 

 sion reflexes which require accelerating forces acting 

 on the semicircular canals (and in minor degree, the 

 vestibular organs) for elicitation (44, 220); the rabbit, 

 for example, when lowered briskly downward and 

 forward extends its forelegs, a reaction of obvious 

 value in bounding progression (44J. 



Afferent Modalities in Progression 



The role of specific afferent endings in locomotion 

 is incompletely known (79, 178, 201, 213), although 

 the older work, as presented in the classical mono- 

 graphs of Magnus (187), Rademaker (220) and 

 Creed et al. (43), generally indicated the importance 

 of proprioceptors and described progression in terms 

 of interlocking proprioceptive reflexes. Exteroceptive 

 stimuli or volitional excitation might initially cause 

 the head to move relative to the shoulder; then 

 receptors in joints and muscles of the neck could 

 initiate movements of the forelimbs; and these in 

 turn call forth adjustments of the trunk and hind 

 limbs. Transmission of such .sequences can be purely 

 mechanical; the hindquarters of the thoracic spinal 

 cat may still move in appropriate diagonal sequence 



