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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



greater length. Figure lo clearly shows another fea- 

 ture of chain activity; as input increases, the reflex 

 progressively prefers the shorter paths. In a study of 

 single motoneurons subjected to the same experi- 

 mental conditions, Alvord & Fuortes (2) have shown 

 that an individual motoneuron will respond with 

 progressively diminishing latency and that it rarely 

 responds more than once in the course of the inter- 

 nuncial bombardment, whatever may be the actual 

 latency of the response it does yield. Thus there is 

 clear proof that the paths of various lengths do in 

 fact converge upon the individual motoneurons, as 

 supposed in the diagram of figure gA/, and that 

 increment in the early response may take place at the 

 expense of later response by reason of refractoriness. 



Reflex discharges, transmitted through internun- 

 cial chains, are frequently less simple than those 

 recorded in figure lo. Employing the same aflferent 

 source, namely the sural nerve, or a comparable 

 source, the course of excitability change in the motor 

 nucleus (55, 59) and the motoneuron discharge re- 

 sulting from internuncial activity display two or 

 even three peaks (7, 54). Figure 11 illustrates the 

 effect; it is from an experiment entirely comparable 

 to that from which figure 10 was taken. It is seen 

 that the two groups of discharges grow more or less 

 in parallel with incrementing stimulation. The first 

 peak of discharge compares with the total discharge 

 in figure 10. Although there is no rigorous proof, it 

 is generally considered that discharges of this char- 

 acter may indicate the operation, within the inter- 

 nuncial system, of closed chains. Recently an impor- 

 tant and detailed study of the exact behavior of 

 interneurons in a variety of circumstances has been 

 commenced by Kolmodin & Skoglund (43) em- 

 ploying direct recording and a statistical approach. 

 This eventually will provide a much needed direct 

 picture of internuncial organization. 



REFLEX ACTION OF MUSCULAR ORIGIN 



Monosynaptic Myotatic Reflex 



An important datum concerning the monosynap- 

 tic reflex elicited by stimulating the Group I afferent 

 fibers of a given muscle nerve is that the reflex dis- 

 charge returns to that nerve (flg. 12), which is to 

 say to the muscle from which the aff"erent fibers con- 

 cerned originate (54) and in all usual circumstances 

 not to other nerves (64). In this the monosynaptic 

 reflex has the quality of the myotatic reflex (52, 94) 

 which fact led to the hypothesis that the inonosynap- 



J 



FIG. 10 ilefl). Flexor reflex discharges into the nerve of the 

 semitendinosus elicited by single shocks of progressively incre- 

 menting strength to the sural nerve. Time, i msec, at bottom. 

 [From Lloyd (62). ■ 



FIG. II {righlt. Flexor reflex discharges elicited in nerve of 

 semitendinosus by sural nerve stimulation with single shocks of 

 incrementing strength. In this experiment the flexor reflex 

 discharge occurs in two distinct peaks which grow with increas- 

 ing stimulation in a more or less parallel manner. The first 

 peak corresponds approximately to the entire discharge re- 

 corded in flg. 10. Note slower time base line. [From Lloyd (62).] 



tic connections within the spinal cord indeed are the 

 mechanism for transmission of the stretch reflex (54, 

 55). Confirmation of the hypothesis required proof 

 that the reflex elicited by natural stimulus, that is 

 by stretch, is in fact monosynaptic, proof of which is 

 seen in figure 13. Another important correlation is 

 that the monosynaptic reflex elicited by nerve stimu- 

 lation just like the natural myotatic reflex (52, 94) 

 does not display after-discharge. 



The monosynaptic reflex as evoked by single- 

 shock nerve stimulation resembles closely the 'tendon 

 jerk" which is a fractional manifestation of the myo- 

 tatic reflex (52), the phasic component, in effect, 

 rather than the static component. In a static, or 

 maintained, stretch response, which is seen to best 

 advantage in the extensor muscle of the decerebrate 

 preparation, other factors come into operation, 

 although the mechanism proper is purely spinal (18). 

 Impulses descending from supraspinal levels support 

 the reaction and the small motor fiber system plays 

 upon the muscle spindles to control the afTerent 

 response therefrom (32, 44, 45, 51). It is important 



