io8o 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



in response to cues from the forclimbs (252). Or the 

 coordination may be impressed centrally; the por- 

 tion of an eel extending behind vertebral segments 

 which have been removed or splinted against move- 

 ment still contracts in rhythm with the anterior seg- 

 ment (99). 



Reflex influences between distant limits are 

 demonstrable in normal (130) or paraplegic man 

 (185, 214, 228), as well as laboratory animals (213, 

 252). Typically, the pattern is diagonal, and pressure 

 to one forepaw of the cat (decerebrate) with elicita- 

 tion of flexor withdrawal causes the opposite forelimb 

 and ipsilateral hind limb to extend while the other 

 hind leg flexes (237). If alternate flexion and ex- 

 tension are imposed on the forelimbs, stepping of the 

 hindquarters in appropriate diagonal sequence may 

 occur even in the bilaterally deafFerented hind limbs 

 (252). It is surprising that group I afferents from 

 muscles of the forearm are apparently not concerned 

 in causing extension of the ipsilateral hind limb and 

 that electrical stimulation only of group III or 

 smaller fibers causes such extension (178). This is a 

 poignant reminder that little is yet known of the 

 physiological role of the smaller afferents in muscle. 



Inconsistent with this general pattern of response 

 obtained upon electrical stimulation of forelimb 

 nerves is inhibition of the flexor longus digitorum 

 muscle of the hind limb during stimulation of group 

 II muscular or cutaneous afferents from extreme 

 distal portions of the forelimbs (178). This inhibition 

 of a single physiological extensor in a generally 

 extending limb has presumably the purpose of pre- 

 venting protrusion of the claws. It, interestingly, is 

 mediated by rather direct connections, the inhibition 

 of the monosynaptic response of the flexor digitorum 

 motoneurons appearing as briefly as 2.5 msec, after 

 stimulation of a brachial trunk (178). 



Reflexes of locomotor significance also exist be- 

 tween companion limbs. Passive or reflex flexion of 

 one hind leg, for example, causes crossed extension 

 of the opposite one in the cat and dog (212, 213) 

 and also in man (130, 228) and, conversely, passive 

 extension of the contralateral thigh leads to crossed 

 flexion of the hip (241). Even within a single limb 

 the afferent influx from individual contracting 

 muscles favors locomotor patterns. Contraction of 

 the cat hamstring muscles, for instance, induced by 

 stimulation of the appropriate ventral root, causes 

 prompt inhibition of decerebrate or crossed extensor 

 tonus in tlie vastocrureus, which has its motor out- 

 flow at a different spinal level (41). Reciprocal con- 

 traction of flexors and extensors is, of course, basic to 



locomotion (122, 240, 244), although this does not 

 mean that augmentation and diminution of contrac- 

 tion may not proceed concurrently in antagonistic 

 muscles during portions of the walking cycle (221). 

 Such graded restraint by the relaxing muscles makes 

 movements smoother (122). 



The importance of cutaneous receptors in induc- 

 tion of locomotion is probably secondary. The eel, 

 completely de\oid of skin, still can swim (ggj; pigeons 

 (34) or cats (30) in which nerves are severed at the 

 ankle display only a trace of abnormality in their 

 gaits; and, as is well known, the spinal dog steps 

 more readily when held aloft than when its feet are 

 in contact with the ground (30). The dog may even 

 cease stepping bilaterally upon touching one foot to 

 a supporting surface (240). On the other hand, the 

 monkey which is deprived of cutaneous sensation at 

 the apex of an extremity shows disproportionate lo.ss 

 of u.se of that extremity, while he suffers little dis- 

 ability if, by section of selected dorsal roots, cutaneous 

 sensation is spared but muscles deafterented (203, 

 26=,). 



Pcrlndicitv of Locnmntion 



The intriguing question has been raised as to 

 whether the periodic pattern characteristic of progres- 

 sion derives ultimately from an oscillation autoch- 

 thonous to the central nervous pathways (266, 274) 

 or requires for its generation reverberating circuits 

 through participating muscles or limbs (100, 172, 

 173). That supraspinal structures may not be req- 

 uisite for rhythmic patterns is evident from the 

 stepping of the decapitate cat (240) and the chronic 

 spinal dog (49, 193) or, more spectacularly, from the 

 independent and spontaneous rhythms of the two 

 cord segments of the dogfish in which the cord has 

 been transected at two levels. Various central phe- 

 nomena, such as rebound, accommodation and 

 reciprocal action between motor centers, no doubt 

 contribute to the production of rhythmicity but do 

 not of themselves indicate whether it is fundamentally 

 central or peripheral. C^ertain spontaneous rhythmic 

 variations in pyramidal tract discharges (277) or in 

 monosynaptic refle.x responses (183, 184) do not 

 seem to bear any simple relation to locomotion. 



LOCAL .\FFERE.\TS AND RHYTHMICITY. A peripheral 



basis for reciprocating contraction lies in the chang- 

 ing composition of afferent inflow arising in any 

 muscle which is allowed to contract against yielding 

 resistance. During flexion of the leg, for example. 



