946 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



lOO/s.c 1 I 1 M n 



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FIG. 24. Crossed conditioning of flexor reflexes by Group II 

 and Group III aff'erent libers. Test reflex recorded from the 

 semitendinosus and elicited by stimulation of the peroneal 

 nerve. Contralateral conditioning by volleys in the tibial nerve 

 at two strengths. These conditioning volleys produced ipsilater- 

 ally the reflexes shown in the iiisel. Hollow circles, crossed condi- 

 tioning of flexor reflex by the weaker volleys; solid circles, by 

 the stronger volleys. The essential distinction is the powerful 

 crossed inhibition of the flexor motoneurons by Group III 

 afferent fiber action. [From Perl (78).] 



ceptive for an ipsilateral extensor reflex rather than a 

 flexor reflex. 



Crossed Reflexes of Cutaneous Origin 



The classical crossed reaction of the decerebrate 

 preparation is the so-called crossed extensor reflex. 

 Immediately after spinal section the overt contra- 

 lateral response is flexion which is after a period of 

 time replaced by extension (76). There are, therefore, 

 crossed excitatory connections to both flexor and ex- 

 tensor motor nuclei, although with change of state 

 one end result may predominate. The nature of these 

 connections, at least as they are thrown into action 

 from cutaneous sources, has been clarified to some 

 extent by Perl (78). 



When Group II afferent fibers are stimulated, the 

 action of crossed knee and ankle flexor motoneurons 

 is facilitated (fig. 24, open circles) and discharge 

 occurs occasionally. If the Group III fibers be active 

 as well, there is no significant change in flexor facilita- 

 tion; but a prolonged period of inhibition follows 

 (flg. 24, filled circles). 



In an extensor nucleus little effect, and that in the 

 direction of inhibition, is encountered as a conse- 

 quence of stimulating contralateral Group II fibers 

 (fig. 25, open circles). Once Group III fibers are stim- 

 ulated there is an enduring facilitation (fig. 25, filled 

 circles) of the crossed extensor motoneurons which is 

 fully comparable to the inhibition of crossed flexors 

 caused by similar stimulations. 



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FIG. 25. Crossed conditioning of quadriceps monosynaptic 

 reflexes by afferent \olleys of two strengths applied to the 

 saphenous nerve. Inset shows monitor records of the conditioning 

 afferent activity. Hollow circles, effect of the weaker crossed 

 saphenous afferent \olleys. Solid circles, strong facilitation of the 

 extensor motoneurons by the action of crossed saphenous 

 Group III fibers. [From Perl (78).] 



If now one considers these contralateral effects to- 

 gether with the ipsilateral eff'ects discussed earlier, it 

 is seen that Group II aflferent fibers of cutaneous 

 origin in action set in motion a bilateral flexor reflex. 

 What may be its functional role is not clear, largely 

 it may be said because the receptor origin is not clear. 

 Nevertheless, the machinery for this bilateral flexor 

 response is present and the response itself probably is 

 not a nociceptive reaction. On the contrary, origin 

 and nature of the Group III response, flexion ipsi- 

 laterally, extension contralaterally, is sufficiently 

 elaborated as to leave little doubt that Group III 

 aff'erent fibers are responsible for the classical reflex 

 couplet of the nociceptive flexor reflex and the reflex 

 of crossed extension (30) . 



Reflex Effects of l^nmyelinated Afferent Fibers 



Reflex function of the unmyelinated, or C fibers, 

 was first demonstrated unequivocally by Clark et al. 

 (14) who described respiratory and arterial pressure 

 changes consequent to stimulation of them. 



In the cat, Laporte cS: Boer have encountered in 

 ventral roots of the lumbar enlargement a prolonged 

 reflex discharge due to the stimulation of C fibers 

 (49). In a more complete study of "C fiber reflexes' in 

 batrachians, Laporte (46) and Laporte & Boer (48) 

 have shown that the reflex discharge is directed into 

 flexor muscles and that ipsilateral extensors are in- 



