SPINAL MECHANISMS INVOLVED IN SOMATIC ACTIVITIES 



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FIG. 8. Electrical (f) and mechanical (ml records of re- 

 sponses by the tibialis anterior muscle. Time, lo msec, (a) 

 Flexor reflex response to single shock stimulation of popliteal 

 nerve at, presumably, Group II strength, (i) Maximal motor 

 twitch elicited by a single break-shock to peroneal nerve, (c) 

 Flexor reflex response obtained employing a strong stimulus 

 (probably including Group III afl'erent fibers) to the popliteal 

 nerve. [From Creed et al. (15).] 



of motor nerve impulses this definition encountered 

 difficulties. To exemplify: the 'single-shock" flexor 

 reflex of figure 8a is essentially like the motor twitch 

 recorded in figure Qh. One would not have said that 

 the reflex displayed after-discharge and yet it is now 

 known that the flexor reflex motor nerve impulses in 

 just this situation are dispersed over 10 or more 

 msec, greatly outlasting the external stimulus or the 

 (virtually synchronous) afferent volley caused by it. 

 The flexor reflex to stronger stimulation displayed in 

 figure Qc would be said to show after-discharge. It is 

 now known (88) that participation of Group III 

 (delta) afferent fibers in the afferent \olley is the 

 condition for securing the reflex of figure 8r. After 

 repetitive stimulation, contraction may continue for 

 long periods. The fact has provoked much specula- 

 tion as to the mechanism. Forbes (25) postulated 

 'long delay paths,' which is to say internuncial 

 chains, as the mechanism, others an enduring state at 

 the motoneuron. The best operational definition of 

 after-discharge today would regard it as a discharge 

 resulting from the arrival of presynaptic impulses but 

 not immediately dependent upon the continued ar- 

 rival of such impulses. So defined, after-discharge 

 has been demonstrated in sympathetic ganglia (11) 

 and may occur in the central nervous system. 



M 



TTT 



FIG. 9. Diagrams of the two fundamental patterns of inter- 

 nuncial circuits according to Lorente de No. M, multiple 

 chain. C, closed chain. [From Lorente de No (71).] 



Most of the delayed discharge encountered in the 

 spinal cord can be traced to continued arrival at 

 the motoneurons of internuncial impulses. The 

 internuncial net then is a mechanism for temporal 

 dispersion as well as spatial diffusion of action. The 

 organization of interneurons to this end is next con- 

 sidered. 



Multiple and Closed Internuncial Chains 



The internuncial systems are not only divergent 

 and convergent in connection but also are linked in 

 other manners made possible h\ the fact that they 

 form chains of varying numbers of links. According 

 to Lorente de No (70, 71) the infinitely complex 

 internuncial paths of the central nervous system can 

 be reduced, for sake of argument, to patterns, in 

 repetition and combination, of two fundamental 

 types of circuits — the multiple chain {M in fig. 9) and 

 the closed chain (C in fig. 9). The closed chain is in 

 essence the structure postulated by Forbes in his 

 delay path hypothesis and liy Ranson & Hinsey 

 (82). 



Temporal Characteristics uf Action Through 

 Internuncial Chains 



Figure 1 o illustrates the manner in which moto- 

 neurons respond under the influence of internuncial 

 barrage set in action by stimulation of a cutaneous 

 nerve at incrementing intensities (62). The behavior 

 exemplified is that to be expected of a multiple 

 chain type of internuncial organization [cf. Lorente 

 de No (70, 71 1]. Although the afferent input to the 

 internuncial net is synchronous, the product of the 

 internuncial action is dispersed over 6 or 7 msec. 

 The subliminal influence exerted through the chain, 

 which is evaluated by monosynaptic reflex tests of 

 motoneuron excitability, begins earlier than and 

 far outlasts the period of discharge. Thus it may be 

 said that the paths of intermediate length in the 

 chain are more powerful than those of lesser or 



