SPINAL MECHANISMS INVOLVED IN SOMATIC ACTIVITIES 945 



FIG. 21. Ventral root potentials resulting from single shock 

 stimulation of an ipsilateral cutaneous nerve. The recording 

 leads were arranged so that electrotonic potentials extending 

 into the root from central structures as well as impulse dis- 

 charges were recorded. A. Weak stimulus (possibly of Group 

 II) 'Viot followed by after-discharge. B. Stronger stimulus 

 (possibly of Group III) resulting in withdrawal of the limb and 

 marked after-discharge. [From Brooks & Fuortes (13).] 



4^u^u-u-ij[jl4i-vUu4. 



-^44A 



Skin pinched, 

 dorsal aspect of leg. 



^_;_;^^,^^ tec 



Same, ventral aspect 



FIG. 23. Conditioning inliuence upon monosynaptic reflex 

 of the gastrocnemius, an ankle extensor, of "pinch" stimuli 

 applied to various points on the skin of the hind limb. The 

 upper record in .-1 shows enhancement of rhythmically elicited 

 monosynaptic reflexes when the crosshatched area, as repre- 

 sented in B, was stimulated. The lower record in A shows inhibi- 

 tion when other areas of the limb were stimulated. In B, -\- and 

 — indicate areas facilitating and inhibiting the extensor test 

 reflex. [From Hagbarth (34).] 



discharge is the correlate of elaborated reflex activity 

 capable of giving rise to organized and purposeful 

 movements." This concept is not very clear, the more 

 so because Tureen's (88) repetitive stimulation at 

 Group II strength produced a better maintained con- 

 traction (without after-discharge) than did the 

 stronger stiinulation which revealed evidence of 

 'concealed inhibition' followed by after-discharge. 

 Because natural reflexes would surely be evoked by 

 repetitive activity one cannot agree that the Group 

 II flexor reflex pathways would not represent a mech- 

 anism for organized purposive reflex action. 



Because there is no positive information concerning 

 the upper limit in diameter of "pain" fibers (28), one 

 cannot state dogmatically that the Group II reflex 

 is not a nociceptive reflex; but it seems improbable. 

 The Group III reflex, on the contrary, cannot incon- 

 testibly be considered a nociceptive reaction; but 

 'pain' fibers aie concentrated in this group and in the 

 C fiber group, .so that the likelihood in this instance is 

 great. 



Special Effects From Specific Regions 



Many exceptions to the rule of ipsilateral flexion 

 exist (26, 34, 91, 96) either as responses of extension 



or in the form of concealed reflexes. The extensor 

 thrust (89) is a well-known example. It is elicited by 

 pressure between the toe pads. The afferent fibers for 

 this reflex are restricted to the plantar nerves (89), 

 but stimulation of the plantar nerves themselves in- 

 duces a flexor reflex despite the necessary presence 

 within them of fibers that cause ipsilateral extension. 

 A systematic basis for some of the mixed effects 

 elicitable by stimulation of cutaneous nerves has been 

 put forward recently by Hagbarth (34) who utilized 

 monosynaptic reflexes of various muscles as test sys- 

 tems and conditioned these with action initiated not 

 by electrical stimulation but by pinching the skin at 

 various loci. Such stimulation applied over most of 

 the limb facilitated the flexor monosynaptic test re- 

 flexes and inhibited the extensor tests. However, a 

 flexor monosynaptic reflex test was inhibited rather 

 than facilitated if a skin area over its antagonist was 

 stimulated, and an extensor test was facilitated rather 

 than inhibited when the skin overlying the extensor 

 itself was stimulated. This latter observation is ex- 

 emplified in figure 23. The rule then is that the skin 

 area over a given antagonist pair, unlike the re- 

 mainder of the skin of the limb, is for that pair re- 



