936 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



Locnl Sign in Reflex Action 



There are in the nervous system many examples 

 of what is called point-to-point localization (or 

 projection). The relation of points in the occipital 

 cortex to points in the retina, or points on the somatic 

 sensory area to points on the body surface (4) come 

 to mind in this connection. Something of this sort 

 is seen in the myotatic refiex pathways and in these, 

 serving as they do reflex actions narrowly limited to 

 the zone of afferent origin, the localization is deter- 

 mined by anatomical limitation of connection (58, 

 67). However, local sign is preserved, in most 

 circumstances, within a fraction of the synergic 

 unit. Hence, and despite the fact that anatomical 

 limitation of connection establishes the ultimate 

 boundary of the field of action, a functional limita- 

 tion of action to a fraction of that field is usual. 

 Specifically an afferent nerve volley in the entire 

 Group I supply to the gastrocnemius can invol\e the 

 entire motoneuron pool of the gastrocnemius in 

 monosynaptic reflex discharge. That no other moto- 

 neurons are discharged or excited subliminally (57, 

 58) implies anatomical limitation of monosynaptic 

 reflex connection. In all ordinary circumstances a 

 similar volley confined to the afferent fibers of the 

 medial gastrocnemius will discharge only the medial 

 gastrocnemius motoneurons (57), there being certain 

 notable exceptions (i, 33, 41, 42, 64, 84). Action by 

 the lateral gastrocnemius motoneurons is facilitated 

 however (20, 57), which implies anatomical connec- 

 tion between them and medial gastrocnemius affer- 

 ent fibers and hence limitation of the discharge zone 

 by nature of connection rather than by absence ot 

 connection. In this instance the factor involved ap- 

 parently is quantitative rather than qualitative (64). 



A classical example of local sign, or reflex frac- 

 tionation, is provided by flexion reflexes (16) which 

 in all of their pathways involve internuncials (54). 

 However set into action, the flexion reflex is not 

 confined to a single flexor muscle nor to the synergist 

 flexor muscles of a given joint but in \-aried intensity 

 extends to the muscles of hip, knee and ankle. Table 

 I exemplifies the argument. In it the tensions de- 

 livered by flexor muscles of the several joints are 

 expressed as percentages of the strongest contraction, 

 and the plurimuscular reflexes elicited by stimulation 

 of each of three afferent nerves are represented. A 

 study of table i will convince one that the limb left 

 to itself would assume different final positions de- 

 pending upon the afferent nerxe stimulated. 



From the foregoing one may infer something of 



T.ABLE I. I'arialion in Pattern of Reflex Flexion Involving 

 Several Joints Depending upon the Nerve Stimulated*'\ 



Afferent Xerve 



Internal saphenous 

 Popliteal (tibial) 

 Peroneal (distal to ant. 

 tibial n.) 



* From Creed et al. (15); after Creed & Sherrington (16). 



t In this table, the strength of contraction of the muscle 

 responding most vigorously to stimulation of one afferent 

 nerve is arbitrarily called 100 per cent. The contractions of 

 the other muscles are expressed as percentages of that con- 

 tracting most vigorously. 



the organization of the internuncial systems of the 

 spinal cord, in this instance with specific reference to 

 local sign. It seems quite certain that local sign in 

 the flexion reflex mechanism cannot imply a "private 

 line' system as is, to a considerable extent, the case 

 in monosynaptic systems. 



The internuncial mediate system of the flexion 

 reflex clearly extends longitudinally to bind together 

 the several motoneuron pools of the flexor muscles 

 in a given limb. It is probable, could the system be 

 activated uniformly, that the motor product would 

 displav a fixed pattern, which it manifestly does not 

 in the circumstance of afferent stimulation. .Ac- 

 cording to table I, and the more complete original 

 data of Creed & Sherrington (16), there is a tendency 

 for the flexion reflex to be greatest when the seg- 

 mental level of afferent inflow and motor outflow is 

 in greatest approximation. This in general is true of 

 the polysynaptic reflex discharges recordable in 

 ventral roots on stimulation of various cutaneous 

 nerves (54). Those cord potentials that signify inter- 

 nuncial activity (8) too are greatest at the level of 

 afferent inflow. Considering these several evidences, 

 the simplest conclusion would be that each longitudi- 

 nal level of the internuncial system is engaged more 

 or less in proportion to the density of impinging 

 afferent collaterals and that the motoneurons in 

 turn are proportionally dri\cn by the interneurons. 



After-Discharge 



According to the general definition after-discharge 

 is a discharge that continues after withdrawal of an 

 external stimulus. With change of technique from 

 the recording of muscle contraction to the recording 



