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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



3 



FIG. 17. To illustrate Group I B disynaptic inhibition of synergists and facilitation of antagonists. 

 Lefl. Curve A, simple facilitation of plantaris monosynaptic reflexes by weak Group I afferent volleys 

 in the afferent fibers of its synergist, the flexor longus digitorum. Curve B, the conditioning volleys 

 having been strengthened, but still confined to Group I fibers, an inhibitory action supervenes with 

 an added latency of approximately 0.6 msec, indicating the presence of an interneuron in the inhib- 

 itory pathway. Curve C, further conditioning effect caused by adding Group II fibers to the afferent 

 conditioning input. Right. Curve A, simple monosynaptic reflex inhibition of plantaris monosynaptic 

 reflexes by weak Group I afferent volleys from its antagonist, the extensor longus digitorum. Curve C, 

 further conditioning effect caused by addition of Group II fiber activity to the conditioning volleys. 

 Ordinates: test reflex amplitude in per cent of control value set at 100. Abscissae: time in msec, zero 

 time indicating coincidence of Group I conditioning and testing afferent volleys. [Rearranged from 

 Laporte & Lloyd (50).] 



1 Z 



Time in Jinsec. 



FIG. 18. Disynaptic reflex inhibition between muscles that 

 are not partners in a myotatic unit. Inhibition of monosynaptic 

 reflexes of triceps surae by afferent volleys in the nerve of flexor 

 longus digitorum. The initial phase of depression is due to 

 Group I fibers acting through an interneuron, the second phase 

 to Group II fibers. [After Laporte & Lloyd (50).] 



the afferent fibers from muscle discussed in connection 

 with figure 2, it is known that the entire Group II 

 band arises in the secondary, or flower-spray, endings 

 of the muscle spindle. Thus there is every reason to 

 suppose that the action they engender represents the 

 stretch flexor reflex. 



In the experimental situation the flexor reflex action 

 of Group II muscle afferent fibers is not powerful and 

 frequently does not secure an overt reflex discharge 

 (cf. fig. 21F). It is easily disclosed, however, by utiliz- 

 ing suitable monosynaptic reflexes as tests for influ- 

 ence upon the motoneurons. To obtain the Group II 

 reflex effect uncontaminated by Group I reflexes, the 

 muscle of origin for the Group II afferent volleys and 

 the muscle of origin for the test monosynaptic reflex 

 must not be partners in a myotatic unit. In figure 19 

 are to be found examples of the Group II action, 

 excitatory in a flexor nucleus, inhibitory in an ex- 

 tensor nucleus. The latency for onset of effect, approx- 

 imately 2 msec, is in part concerned with differential 

 conduction time (the Group II conditioning volleys 

 travel at lower velocity than the Group I testing 



