SPINAL MECHANISMS INVOLVED IN SOMATIC ACTIVITIES 



941 



Concf/il of Myotdlic Unit 



The outstanding feature of the myotatic reflex 

 pathways, excitatory and inhibitory', is that the 

 afiferent fibers connect directly with the motoneurons. 

 The functional meaning of this fact is that the moto- 

 neurons inevitably are influenced whenever the mus- 

 cle spindles generate impulses in the Group I A afferent 

 fibers arising within them. In this the myotatic reflex 

 differs seemingly from all others. One can see readiU 

 that individual muscles could be isolated from in- 

 fluence of all other sorts by inhibition of the inter- 

 nuncial links interposed between the afferent influx 

 and the motoneuron. They cannot be isolated from 

 the self-engendered postural influence. Reflex stand- 

 ing (94) involves the play of a number of stretch re- 

 flexes so that the entire limb is held in the face of 

 gravity. But if, for purpose of argument, one were to 

 conceive of gravity acting upon but a single muscle, 

 say the medial gastrocnemius, the influence would 

 indeed be felt by the motoneurons of that muscle, 

 but also action by the lateral gastrocnemius and 

 soleus would be facilitated and by the tibialis anterior 

 and extensor longus, the antagonists, would be de- 

 pressed. Other muscles would remain uninfluenced. 

 Let the gravity play upon the tibialis anterior alone 

 and the situation would be reversed. The muscles at a 

 given joint are mutually dependent at the myotatic 

 level of postural performance in the sense that none 

 can be influenced independently of the others. Such a 

 group of mutually dependent muscles together with 

 the monosynaptic reflex connections that bind them 

 constitute a myotatic unit (58). 



Lengthening Reaction or Inverse Myotatic Reflex 



If weak conditioning volleys in the nerve to one 

 fraction of a synergic unit are employed in conjunc- 

 tion with testing monosynaptic reflexes engendered 

 by stimulating the nerve to the remainder of the 

 synergic unit, simple facilitation curves of the sort 

 illustrated in figure 14 usually can be obtained (20, 

 35, 50, 57, 58). On strengthening the conditioning 

 volleys, the action still being confined to Group I 

 afferent fibers, a sudden break in the direction of in- 

 hibition occurs, as exemplified b\- the difference be- 

 tween curves A and B to the left of figure 17. A la- 

 tency diiTerential of approximately 0.6 msec, between 

 facilitatory and inhibitory action bespeaks the exist- 



' The notion that monosynaptic inhibitory pathways arc 

 monosynaptic is not universally conceded at the present time 

 (22). This question is discussed in another connection. 



ence of an internuncial relay in the inhibitory path. 

 It may be noted that inclusion of the Group II fibers 

 in the conditioning acli\ity brings forth the further 

 change with still longer latency [characteristic for 

 Group II action (cf. fig. 19)] indicated by curve C. 

 Initial divergence from the simple facilitation is al- 

 ways inhibitory. The further change denoted by curve 

 C is inhibitory in an extensor nucleus Ijut facilitatory 

 in a flexor nucleus. 



The Group I inhibitory action in a synergic unit 

 is due to action by the Group IB fibers and has all the 

 proper attributes for designation as an expression of 

 the lengthening reaction. It is concluded that this 

 reaction is inediated through a disynaptic reflex path- 

 way. In line with these results and conclusion Granit 

 (31 ) has shown that large afferent fibers are concerned 

 with autogenic inhiljition (i.e. inhibition within the 

 synergic unit), and McCouch et al. (75) have observed 

 inhibition of the quadriceps by electrical stimulation 

 of the crureus tendon whereiit would lie Golgi tendon 

 organs. 



Associated with the lengthening reaction is an exci- 

 tation of antagonists. To the right of figure 17 one 

 sees in curve A the simple inhibition of an antagonist 

 due to Group lA action. In curve B the course of that 

 inhibitory action is interrupted by an excitatory action 

 that is the precise counterpart of Group IB inhibition 

 within the synergic unit. To complete the experi- 

 mental series, curve C illustrates the additional in- 

 hibitory influence of including the conditioning action 

 of the Group II fibers. 



The lengthening reaction thus appears to be but 

 one aspect of action of a reflex mechanism that is pre- 

 cisely opposed to that of the myotatic reflex mech- 

 anism and differing from it in central organization 

 only in the matter of possessing an internuncial relay. 

 For this reason the over-all reaction is appropriately 

 termed the inverse myotatic reflex. 



A disynaptic inhibitory connection exists between 

 certain muscles that are not co-members of a myotatic 

 unit (fig. 18). For this no associated excitatory action 

 has been found. The function of these connections is 

 not understood, although their existence accounts for 

 the inhibition of certain muscles concomitant with 

 elicitation of a tendon jerk in others, a phenomenon 

 described by Denny-Brown (17). 



Stretcli Flexor Reflex 



Stimulation of muscular afferent fibers in the Group 

 II and Group III bands produces effects in the pattern 

 of the flexor reflex. Because of Hunt's study (37) of 



