POSTURE AND LOCOMOTION 



I081 



the discharge from annulospiral cnduigs in the pas- 

 sively extending tliigh muscles is increased, with 

 resultant growing facilitation of the same and syn- 

 ergistic muscles and suppression of flexors. Extensor 

 contractions finally appear. Then, as thigh extensors 

 actively shorten, endings in the spindles Ijecome un- 

 loaded while the discharge from tendon organs 

 increases, thus weakening the extensor contraction 

 and setting the stage for the next cycle. Hence, in 

 theory (and neglecting an active role of gamma 

 efferents) might be explained the reciprocating 

 contractions of a pair of opposing muscles in an 

 otherwise denervated leg (30, 32, 243). Not only 

 would such swings in composition of afferent inflow 

 favor alternation at the spinal level, but cerebral 

 {83, 271) and cerebellar (147) cortices would be 

 predisposed toward movement of the opposite sign, 

 a net result which has been referred to as a 'reversal 

 effect' (43). 



DIST.\NT .AFFERENTS AND RHYTHMICITY. Local cirCuitS 



are not essential for repetitive movements as these 

 can occur in deafferented whole limbs or single 

 muscles (13, 31, 170). In elicitation of the scratch 

 reflex, for example, irritation of a small skin focus 

 over the shoulder of the dog may call into rhythmic 

 play some 19 muscles of the hind limb despite the 

 absence of all dorsal roots to the extremity (245). 

 Also, the deafferented hind leg of the cat may par- 

 ticipate fairly effectively in progression under the 

 changing excitement of influences from other moving 

 parts (252). 



In the toad a diagonal pattern of leg placement 

 characteristic of normal walking is retained if one, 

 two or even all four extremities are deafferented 

 (274). In fact, vestiges of diagonal progression re- 

 iTiain so long as the dorsal and ventral roots of any 

 one segment are intact, even though this innervates 

 anal regions (loi). Deafferentation of this last seg- 

 ment abolishes the diagonally coordinated movement. 

 From this and experiments on other forms. Gray & 

 Lissman have come to the view that an external 

 circuit must be intact for the appearance of periodic 

 movements of progression. Others using tadpoles 

 (274) or teleosts (266) deny such dependence. Cer- 

 tainly in mammals comprehensive deafferentation 

 prohibits rhythmic inovements except under extreme 

 conditions, such as cutting or electrically stimulating 

 the spinal cord (31, 32). Pollock & Davis (219), for 

 instance, prepared chronic cats in which the first 18 

 to 23 pairs of dorsal roots were severed and which 

 terminally were subjected to decerebration and to 



severance of the cord beneath the last deafferented 

 segments. No movements were seen in the upper 

 part of the body, whereas the hind limbs promptly 

 exhibited stepping. In. mammals even more re- 

 stricted deafferentation can impair participation iiy 

 an extremity in locomotion. If the cat foreleg is 

 deafferented, its efforts in progression are feeble and 

 quick to fatigue (163); and any extremity of the 

 monkey is sexerely incapacitated by deafferentation, 

 although 'associated movements" may persist (162, 

 163, 203). That the residual progressive movements 

 remaining in deafferented limbs result from sensory 

 barrages elsewhere is indicated by the fact that, if 

 the sound limbs are prevented from moving, the 

 operated leg is incapable of movement even if the 

 animal is enraged, as Sprong (252) demonstrated 

 for the cat. This would, incidentally, point to joint 

 receptors as the sensory terminals concerned, for 

 muscle and tendon receptors would be markedly 

 affected in the restrained leg. Patients, in contrast, 

 may exhibit associated movements with contractions 

 of sound extremities in which there is no actual ex- 

 cursion, as in the handclasp (269). 



B.-\CKGROUND FACILITATION .AND RHYTHMICITY. Al- 

 though the puppy which has undergone .section of 

 the thoracic spinal cord in the early days following 

 birth is said to be capable of walking, turning, climb- 

 ing and even jumping (246), the hindquarters of the 

 puppy (263) or monkey (264) which are isolated from 

 suprasegmental influences and additionally deaf- 

 ferented evince no overt spontaneous activity. The 

 suspended spinal dogfish, a preparation which ex- 

 hibits tireless swimming activity, is still spontaneously 

 active if one half its dorsal roots are cut, capable of 

 short-lived rhythmic movement upon stimulation if 

 onlv one pair of sensory roots remains, but utterly 

 quiescent if all 65 roots are cut (104, 173). These 

 results and the arguments of the last section appear 

 to refute the idea that the rhythmic activity charac- 

 teristic of progression arises in the cord. However, 

 such experiments are not conclusive, as the back- 

 ground of facilitatory nervous activity is also re- 

 duced. The progre.s.sion of chronically deafferented 

 toads, for example, is more sluggish and easily fa- 

 tigued the greater the extent of deafferentation (100, 

 ■274). Furthermore, the relationship is not linear as 

 the effect of cutting the dorsal roots to all limbs is 

 markedly greater than one would expect by simply 

 adding the effects of separate fore- and hind-limb 

 deafferentation (100, 274). Under these circum- 

 stances it is conceivable that a locomotor pattern of 



