POSTURE AND LOCOMOTION 



1083 



purpose of modulating the excursion ol the muscle 

 and so protect it against strains. 



It is not readily apparent how rhythmic alterna- 

 tions are engendered under flexor and extensor 

 influences of unvarying potency as, for example, 

 under simultaneous and steady stimulation of skin 

 nerves of contrasting effect. Perhaps the alternation 

 arises through a difference in accommodation of the 

 two central pathways. Flexor motoneurons, at least 

 in the decerebrate preparation, fire easily to single- 

 shock stimulation of appropriate nerves, but their 

 discharge dies out quickly and cannot be driven at 

 rapid frequencies (3, 74). Extensor neurons, on the 

 other hand, are facilitated and sustained Ijy repeti- 

 tive afferent stimulation. Thvis, under constant 

 afferent inflow, flexor acti\'ity would be favored 

 initially, the balance of effect then turning to ex- 

 tensor activity. Final reversal of the cvcle might then 

 appear through long-term accommodation of the 

 extensor pathway, aided by shifts in composition of 

 the afferent inflow. 



Additional Effects of Afferents 

 upon Locomotion 



DE.\FFERENT.\TION ON PRECISION OF .MOVE.VIENTS. 



DeafTerentation produces defects in locomotion which 

 are .separate from those resulting from loss of sup- 

 porting tone or impaired rhythmicity. The cat with 

 a chronically deafferented hind limb can use the leg 

 effectively in walking althou<>h with some abbrevia- 

 tion of the weight bearing phase of the step (252). 

 When running, the alisence of myotatic reflexes in 

 that leg becomes less important relative to the deluge 

 of impulses from remote moving parts, and the limp 

 may be unnoticeable. However, the leg oversteps 

 and, in general, participates more vigorously than 

 on the intact side (252). This is to be expected as, 

 in the deafferented hind limb of a cat, segmental 

 reactions including stepping (243) are markedly 

 more abrupt in onset, decline and total excursion 

 than those of the intact limb (217, 223, 237, 241, 

 252). Among factors which may contribute to this 

 hyperreflexia are : a) the loss for the actively contract- 

 ing muscle of inhibitory effects from tendon organs, 

 whereas loss of facilitatory effect from these tension- 

 sensitive organs in the passively tensing antagonist 

 would be much less (6) ; b) the absence, on one hand, 

 of 'unloading' of spindles in actively shortening 

 muscle and, on the other hand, of augmenting dis- 

 charge from the passively extending antagonist, 

 both actions which, from a consideration of annulo- 



spiral effects, tend to restrain alpha activation; and 

 (■) the absence of dampening contractile tone. Modu- 

 lation by the cerebellum and other higher central 

 structures, which is dependent upon sensory informa- 

 tion, of course, is also lacking. Lastly, caution should 

 be exercised in ascribing hyperactivity in the chroni- 

 cally prepared limb directly to loss of neuronal 

 circuits, for neurons deprived of afferent inflow 

 become sensitized (257). 



Coupled with the hypermetria of the deafferented 

 hind leg of the cat is a lack of precision and correc- 

 tive ability in placing the foot. Stalking along the 

 rungs of a ladder becomes impossible, and in walking 

 the cat often steps on the dorsum of the paw (163, 

 252, 262). In part, this malpositioning of the foot 

 may be a consequence of the fact that muscles about 

 the ankle and foot participate less completely in 

 postural tone than more proximal mu.scles (19, 203, 

 217, 226, 237), a provision which permits the paw to 

 adjust to inequalities of the ground by more local 

 reflexes. Movements appropriate to such function, 

 for example, are demonstrable in the foot of the 

 paraplegic patient which, if stroked on the outer 

 side of the plantar surface, may evert; if on the inner 

 side, invert (228). In part, also, improper use of the 

 foot may result from loss of less localized "placing 

 reactions.' In either case deficits in placing the foot 

 result as well from local deafferentation below the 

 ankle as from cutting all dorsal roots to the limb, 

 as shown in the forepaws and hind paws of cats and 

 dogs by Sherrington (241). 



EFFECT OF .AFFERENT INFLOW ON MODE OF PROGRES- 

 SION. The mode of progression of an animal is some- 

 times dependent on the nature of the afferent inflow 

 entering at segmental levels. Toads, for instance, 

 when floating freely, exhibit .swimming movements 

 exclusively. When one foot encounters resistance, 

 however, the leg stiffens in a 'retractor extensor 

 response' and other limbs move into a pattern of 

 diagonal progression (102). The same toad placed 

 upon moist ground engages in burrowing, a reaction 

 which is lost following deafferentation of the legs 

 (274). Even visceroceptors may influence locomo- 

 tion, for in the carp discharges from the swiin bladder 

 affect the set and stroke of the pectoral fins (149). 



Brain and Locomotion 



Aniinals vary greatly in their ability to maintain 

 locomotory movements after the spinal cord is cut 

 (212). Chronic spinal tadpoles are incapable of 



