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HANDBOOK OF PHYSIOLOCJY 



NEUROPHYSIOLOGY 11 



spontaneous translatory movements even when the 

 cord is cut before hatching (270). Spinal dogfish, on 

 the other hand, exhibit tireless spontaneous swim- 

 ming, although similar preparations of eels or other 

 teleosts are quiet unless prodded (166, 172). Cats, 

 dogs and rabbits with cervical cord transections ex- 

 hibit spontaneous running, although no complete 

 progression (240). Effective stepping of the adult 

 cat hindquarters, however, may be obtained even in 

 the absence of cues from the forelimbs, providing 

 the cord has been transected at an early age and 

 physiotherapeutic care is given (246). Monkeys 

 demonstrate alternating reflexes only in the tail 

 (236); and in spinal man, stepping is not found in 

 the presumably favorable period of 'neural balance' 

 between early flexor and later extensor rigidity (158, 

 227). It is evident that in most vertebrates contribu- 

 tions from supraspinal levels are essential for effec- 

 tive progression and, in some, even for production 

 of the alternating rhythms basic to progression. 



The cat, in which the brain stem is cut so as to 

 leave only the medulla in continuity with the cord, 

 has flaccid muscles and little spontaneous activity 

 (258). Tone, on the other hand, is prominent but 

 movements are in abeyance when the section just 

 spares Deiter's nucleus and portions of the facilita- 

 tory reticular formation (164, 240, 258). When the 

 decerebration is made yet more rostralward, as by 

 rendering anemic parts of the brain anterior to a 

 plane passing behind the red nucleus extensor, 

 muscular tone although subject to exacerbations is 

 often more nearly normal and then may be inter- 

 rupted b\' fits of running (217). Lastly, when the 

 section passes rostral to the superior coUiculi and 

 down to the optic chiasma so as to spare the red 

 nuclei, subthalamus and portions of the hypothala- 

 mus, the cat can rise to its feet and walk normally 

 along a straight line with the body well-supported 

 and coordinated (164, 220). Particularly if the ani- 

 mal is young, these movements may not be distin- 

 guishable from those of an unoperated animal of the 

 same age (280). When, however, such a cat en- 

 counters an obstruction, it acts like an automaton, 

 the head remaining pressed against the obstacle while 

 the legs continue to walk (164). Thus, reflex inecha- 

 nisms of progression are essentially complete, but 

 l)ehavioral defects are prominent. 



The question arises whether the lack of locomotory 

 movements in the classic decerebrate or in the 

 medullospinal animal is a result of the supraspinal 

 influences throwing the 'neural lialance' between 

 flexor and extensor tendencies too far off to permit 



alternating activity in the motor pools to develop. 

 Certainly in adult cats progressive movements seem 

 to be lost when, upon successive removal of higher 

 brain structures, rigidity first appears (164, 196); 

 and in chronic decerebrate cats running reappears as 

 the hyperextension diminishes (170). Progressive 

 activity is plentiful in the decerebrated puppy, 

 kitten (102, 280) or rabbit (102, 159), preparations 

 in which rigidity is inconspicuous. Furthermore, the 

 opossum, which has perhaps less encephalization of 

 locomotory functions, is capable of well-coordinated 

 progression, despite the presence of concurrent 

 rigidity produced by a section just anterior to the 

 inferior coUiculi (273). These observations suggest 

 that the brain stem may itself produce rhythmic 

 activity as distinct from merely impressing extensor 

 and flexor influences of constant tenor upon intrinsic 

 rhythmic mechanisms of the cord. Related, but not 

 conclusive, evidence on this point is the finding that 

 single-shock stimulation of the medulla may arouse 

 rhythmic acti\ity in the deafferented lumbosacral 

 cord (13). 



Progressive activity of the hind limbs is more in- 

 dependent of cephalic control than is that of the 

 forelimbs (239, 240). The rabbit with a low brain- 

 stem transection, for example, will upon stimulation 

 of an intercostal nerve exhibit bilateral movements 

 of the hind legs in complete absence of effects on 

 the forelimbs (164). Similarly, no amount of goading 

 can make the forelimbs step in a pontine cat, al- 

 though the hindcjuarters do so readily (igo). 



Tied in closely with the question of reflex capacity 

 for effective locomotion are certain facilitatory rela- 

 tions of the cortex to movement. The thalamic cat, 

 although capable of essentially normal progression, 

 appears reluctant to move and remains in a normal 

 sitting posture for long periods. In monkeys, basic 

 motor performance is more dependent upon higher 

 centers and thalamic animals seem incapable of 

 progression; a mere loathing to move in the presence 

 of ability for locomotion appears, however, following 

 extirpation of certain cortical areas (211). Cortical 

 lesions impair the ability of rats (27), rabbits (28), 

 cats (8), dogs (283) and monkeys (211) to place the 

 extremities properly upon supporting surfaces in 

 response to tactile stimulation (placing reaction) or 

 to make corrective hops when body balance is im- 

 periled (hopping reaction). To some degree this 

 loss may result from absence of such facilitatory 

 influence rather than a loss of an essential site of 

 integration of these reactions. Placing of a sort, for 



