io82 



HANDBOOK OF PHVSIOLOGV 



NEUROPHYSIOLOGY II 



activity is still present centrally but is incapable of 

 overt expression. Such hidden events in mammalian 

 gamma efferent circuits during complete absence of 

 alpha activity are commonplace (57). Rhythmic 

 alternation in ventral roots of positive and negative 

 slow potentials, which are usually associated with 

 extensor and flexor responses, respectively, have 

 been observed in deafferented lumbosacral segments 

 in the absence of contractions (13, 14). 



Gray & Lissman (100) would seem to have over- 

 come the above objection. As mentioned already, 

 they found that the toad left with intact dorsal roots 

 to only one of its limbs was capable of feeble but 

 coordinate ambulation in all limbs. If then further 

 roots were sectioned, this time the ventral roots 

 to the limb with intact aflferents, no progressional 

 movements were elicitable. Conversely, if three 

 limbs were de-efferented without disturbing the 

 afferents, while in the fourth limb dorsal roots alone 

 were cut, only monophasic movements of retraction 

 were obtainable (loi). These observations seem to 

 prove for this animal the experimenters' thesis that 

 the reciprocating movements of diagonal progression 

 are initiated and paced by a reverberating circuit 

 between the extremity and tiic cord. 



RHYTHMICITY AND NEURAL BALANCE. Not ail loco- 

 motor movements are abolished in the toad in which 

 all dorsal roots are cut, for the hind legs may still 

 engage in symmetrical swimming movements (100) 

 which disappear only following destruction of the 

 labyrinths (103). Both diagonal and symmetrical 

 progression patterns are apparently intrinsic to the 

 mammalian cord also, the symmetrical pattern being 

 more resistant as it alone may remain during early 

 recovery from cord transection (32, 193), asphyxia 

 (32), alcohol intoxication or aeroneurosis in man 

 (130). The rabbit is instructive in this respect for 

 the intact or thalamic animal has hopping progres- 

 sion but, after spinalization, stepping, marking time 

 and crossed extension reflexes are prominent (164). 

 This and the deaflferentation experiments upon 

 toads suggest that strong and symmetrical supraspinal 

 influences mask inequities in the effects of afferents 

 entering from the two sides at the spinal level. In 

 accord with this, elimination of the powerful laby- 

 rinthine influence increases markedly the incidence 

 of running in the decerebrate cat (217). 



Probably of decisive importance in determining 

 whether symmetrical or diagonal coordination will 

 prevail is the degree of balance that is struck be- 

 tween influences impinging upon the two sides. The 



decerebrate cat, in which stimulation of sensors- 

 nerves (30, 243) or cord surfaces on the two sides 

 (245) is carefully equated, may gallop, although 

 usually an unbalanced state of 'double reciprocal 

 innervation' oljtains and stepping emerges (242). 

 Such nicety of balance may be unobtainable in the 

 whole animal; thus, it was found impossible to ad- 

 just the toad hind limbs on a drum .so that rotation 

 of the drum caused other than alternate replacement 

 of the limbs (102). 



Many observations indicate the necessity of a near 

 balance between flexor and extensor influences for 

 the appearance of alternating movements (43). In- 

 duction of stepping in a leg or pair of antagonistic 

 muscles by stimulation of two sensory nerves of op- 

 posing effect (31, 66), or by matching a flexor reflex 

 against a background of extensor rigidity, requires a 

 certain optimal intensity of stimulation (14, 31, 66). 

 The decerebrate cat, while recovering from the ether, 

 shows running actions which, with the onset of ex- 

 tensor rigidity, are occluded. Similarly, the decere- 

 brate alligator when provoked to movement has 

 initiallv sufficient balance between flexor and ex- 

 tensor groups to permit progression, but the stimula- 

 tion of walking itself disturbs this balance and after 

 a few steps, the animal halts in extreme extension, 

 unable to progress further (5). The worsening of 

 progression in the spastic patient when very active 

 is perhaps a comparable phenomenon. 



In a similar vein, but of greater functional interest, 

 is the observation that, while in the labyrinthec- 

 tomized and anemically decerebrate cat ventroflex- 

 ion of the head elicits strong forelimb flexion and 

 dorsiflexion of the head brings on extension, an 

 intermediate position sponsors rhythmic movements 

 of the forearms (217). This intermediate po.se is, of 

 course, that in wiiich the walking animal naturally 

 carries his head. 



It is in regard to tiie lialance of flexor and extensor 

 influences that a guess may be made as to one func- 

 tion of flower-spray endings. At low tensions of 

 muscle, annulospiral endings are proportionately 

 more active than flower-spray afferents, so that 

 relatively pure facilitation occurs and some degree 

 of antigravity tone is produced unopposed. At higher 

 stretches flower-spray discharges enter and a flexor 

 influence arises. Thus, a neural balance in the af- 

 ferent discharge impinging on the homon\ mous 

 motoneurons is set up which other incident influences 

 may easily send into rhythmic activity. Tendon 

 organs, the discharge of whicii appears at even 

 hiarher tensions, mav serve the somewhat different 



