10/8 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



neck and body to be realigned, and in which the 

 neck receptors cause the repositioning of extremities 

 are all compensatory. 



AFFERENTs IN POSTURAL ADJUSTMENT. Receptors Con- 

 cerned with the correction of more profound aberra- 

 tions of body position are characterized by their 

 specialized nature, such as the maculae and cristae, 

 or advantageous location as is the case with the 

 vestibular organs and upper cervical intervertebral 

 joints. Receptors about intervertebral joints, particu- 

 larly those where major segments of the body move 

 with respect to one another, have important postural 

 effects. Thus, jarring of spinous processes (51) or 

 flexion of the pelvis on the trunk eliciting the tonic 

 lumbar reflex in man (64, 261) and monkey (16) 

 cause contraction of the erector musculature; flexing 

 of the neck on the thorax diminishes extensor tonus 

 through the vertebra prominens reflex (188); and 

 strains imposed upon the upper three (194) or four 

 intervertebral (217) joints lead to pronounced pos- 

 tural effects by means of the attitudinal reflexes and 

 neck-upon-body righting reflexes. 



It is difficult to formulate a general rule covering 

 the effects of movements at intervertebral joints upon 

 postural patterns. Behavior among species differs; 

 extension of the neck in the cat cau.ses extension of 

 forelegs and flexion of hind legs (188), whereas in 

 man and rabbits (187, 188) all extremities extend. 

 The effects of movements at various le\els differ; 

 bending of the neck to one side causes extension of 

 both contralateral extremities, w^hereas bending of 

 the lumbar spinal column is said to cause protrusion 

 of the leg and flexion of the arm (129). Additionally, 

 some intervertebral reflexes reverse at a certain 

 extreme of articular excursion. Moderate flexion of 

 the neck of the labyrinthectomized decerebrate cat, 

 for example, typically elicits powerful flexion of the 

 neck and forelimbs, but if pressure on the head is 

 exaggerated the neck may spring into extension 

 (217). Similarly, but conversely, moderate active 

 flexion of the thorax upon the lumbar region in 

 man arouses activity in the sacrospinal muscle which 

 at full voluntary flexion is completely inhibited (64). 

 These effects seem generally to be bivalent in the 

 sense, for example, that retraction of the head 

 causes contraction of the extensor muscles of the 

 foreparts, while bowing of the head activates the 

 flexors (217). Also, they are reciprocal in that en- 

 hanced facilitation of extensor activity is associated 

 with active inhibition of flexors and vice versa (85). 



Neck reflexes are mediated by receptors of axial 



joints (194) and this is probably true for other trunk 

 reflexes. That more peripherally located receptors 

 also may participate in postural adjustment is sug- 

 gested by the fact that the labyrinthectomized and 

 blindfolded thalamic cat can right itself when laid 

 upon its side or e\en when foot pads alone are 

 stimulated (188). Whether this body-on-body reflex 

 is requisitely cutaneous is uncertain as muscle re- 

 ceptors are not indifferent to lateral pressure upon 

 the gross muscle (146). Indeed, spinal man shows 

 traces of body-on-body reflexes when suspended in 

 water (216), in which situation gravitational forces 

 acting upon proprioceptors presumably initiate the 

 reflex. Cues from proprioceptors, incidentally, are 

 capable of mediating imperfect but unassisted stand- 

 ing in the blindfolded patient with chronic bilateral 

 \estibular nerve section (68). 



It should be mentioned in passing that it is not 

 entirely reasonable to assume that utricular receptors, 

 in evoking the static labyrinthine reflex, serve only 

 the function of maintaining antigravity tone, for the 

 position of the head which most favors extensor con- 

 tractions (vertex downward) is opposite to that 

 assumed in standing. If teleological explanations must 

 be applied, the vestibular eflfects are more in the 

 nature of a righting reflex, in which, by extension of 

 the limbs, the body's center of gravity is displaced 

 ventralward and righting assisted. 



POSTURAL ADJUSTMENTS IN .M.\N. Ncck, trunk and 

 labsrinthine reflexes are recognizable in man, espe- 

 cially in early developmental stages (86, 247, 275), 

 or in patients with brain damage (20, 113). In the 

 intact adult their presence is demonstrable through 

 use of refined methods of examination, invoKdng 

 either a) facilitation by hypertensing (276) or severely 

 exercising the part of the body under observation 

 (114) or /)) detection of subtle changes in reflexes 

 (35), muscular tension (182) or electromyographic 

 activity (129, 261). The assistance that these reflexes 

 may gi\e in rational physiotherapy is becoming 

 recognized (168, 284J. 



CENTRAL LEVELS OF MECHANISMS FOR POSTURAL AD- 



]USTMENT. Most rcflcxes of postural adjustment in- 

 vohe wide extents of the neuraxis. Labyrinthine 

 influences, for example, are manifest in hind limbs 

 as well as forelimbs, and stimulation of proprioceptors 

 in the neck of the duck may affect the tail feathers 

 (148). Some of these reflexes are complete in basic 

 pattern at cord levels. The spinal cat or dog, for 

 instance, when laid upon its side, exhibits those 



