944 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



FIG. 21. Fle.Nor reflex discharges provoked by stimulation of Group II and Group III afferent 

 fibers in cutaneous and muscle nerves. A, B. Stimulation of the sural (cutaneous) nerve at Group II 

 strength. C, D. Stimulation of the sural nerve at Group III strength. E. Blank sweep. F. Stimulation 

 of the gastrocnemius ner\"e at Group II strength which, in this experiment, secured no reflex dis- 

 charge. G, H, 1. Stimulation of the gastrocnemius nerve at Group III strength. .\\\ recordings from 

 the peroneal nerse which supplies the pretibial flexors and dorsiflexors of the digits. [From Lloyd 

 (54)-] 



the source of Group II acti\ity aroused by natural 

 stimulation be a flexor or an extensor muscle. Further- 

 more, the degree of stretch necessary to provoke 

 action of this system is so small as not conceivably to 

 be painful. Hence the Group II flexor reflex or muscu- 

 lar origin seems indeed to be concerned with reflex 

 regulation of posture and movement rather than re- 

 action to painful stimulation. 



Group III Flexor Reflexes 



.Stimulation of Group III muscle aff'erent fibers 

 provokes a flexor reflex (54), an example of which is 

 seen in figure 21 G, H, I. The precise function of these 

 fibers has not been established (37), but it is a reason- 

 able assumption that they are nociceptive, in which 

 case the Group III flexor reflexes would be true noci- 

 ceptive withdrawal reflexes. 



REFLEX ACTION OF CUTANEOUS ORIGIN 



According to inspection (95) and myographic 

 analysis the dominant reflex pattern elicitable by 

 stimulation of the skin, or of cutaneous nerves, is one 

 of ipsilateral flexion and contralateral extension (30). 

 Exceptions to the rule exist (19, 26, 89) and there are 

 in observation of the dominant reflexes evidences of 

 the existence of concealed reflexes (15, 96). 



Low and High Threshold Flexor Reflexes 



Fibers in both the Group II band (i.e. alpha and 

 beta fibers) and the Group III band (i.e. delta fibers) 

 on stimulation give a flexor reflex result in the ipsi- 

 lateral limb (fig. 21, A-D). The minimum anatomical 

 pathway for these reflexes contains one internuncial 

 relav. The two sorts of flexor reflex differ somewhat in 

 character. Presence or absence of after-discharge, as 

 was noted briefly in connection with figure 8, was 

 found by Tureen (88) to depend upon whether or not 

 the delta (i.e. Group III) fibers were stimulated. In 

 a more recent study. Brooks & Fuortes (13) have 

 confirmed the difference between the response to 

 'weak' and "strong" stimulation with respect to after- 

 discharge. Although they did not identify the afferenl 

 fibers concerned, their weak stimulation may be pre- 

 sumed to have stimulated Group II and their strong 

 stimulation to have embraced Group III (fig. 22). 

 Their belief that the entire Group II reflex might be 

 monosynaptic was not .substantiated Ijy the later 

 study of Alvord & Fuortes (2), which latter is in agree- 

 ment with the generality that flexor reflexes are 

 polysynaptic. Brooks & Fuortes made the important 

 observation that the 'weak' stimulus in single shock 

 circumstances did not bring about an organized with- 

 drawal of the limb whereas the 'strong' stimulus did. 

 The former, yielding "an inconspicuous twitch" is 

 considered to express ""a very simple and unorganized 

 reflex property of the spinal cord . . . while the after- 



