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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



approaching from different directions. Moreover, in a 

 patient with partial interruption of an ulnar nerve 

 associated with impaired sensiijilit}-, a shavina; of skin 

 3 cm^ from the dorsum of the hand in the hyper- 

 sensitive zone revealed in one area a single well- 

 stained nerve fiber amid other unstained and pre- 

 sumaiily degenerated fibers. This fiiaer terminated in a 

 net immediately beneath the epithelium, covering a 

 roughh- circular area of 0.75 cm in greatest diameter. 

 This distance corresponded to the limen of two-point 

 discrimination for pain in a similar normal area. The 

 observation suggests that the appreciation of the dual 

 nature of .such stimulus requires the separation of the 

 points by about the diameter of the terminal net of 

 each fiber supplying the zone in question. 



PERIPHERAL SENSORY NERVE FIBERS 



Single Fiber Studies 



Adrian (i, pp. 81-go) was the first to record electri- 

 cal impulses from individual sensory nerve fibers in 

 animals following a variety of peripheral stimuli. He 

 and Zotterman promptly established that the spike 

 potentials from a single axon are uniform in duration 

 and amplitude, i.e. that the axon's impulse has an 

 all-or-nothing character. Moreover the>- showed that 

 no particular frequency of the discharge is character- 

 istic for pain. Thus a needle prick evokes a discharge 

 which \aries between the usual limits for a number 

 of types of stimuli of around 5 to 100 per sec. in each 

 nerve fiber. Adrian pointed this out as e\idence that 

 pain is not the result of excessive stimulation of any 

 type of receptor; if it were, one would expect a uni- 

 formly high rate of discharge. In confirmation of the 

 conclusion that high frequenc\' of discharge is not 

 necessarily correlated with pain Adrian el al. (2) re- 

 ported that puffs of air at high frequency directed to 

 the skin of a frog would produce fiber discharges up 

 to 300 per sec. Such stimuli did not seem to hurt un- 

 anesthetized frogs. 



Echlin & Fessard (72) have also found in cats that 

 they can drive receptors at frequencies o\er 400 per 

 sec. so as to record synchronous afferent discharges 

 from proximal points on nerves. The effective stimulus, 

 a powerfully vibrating tuning fork placed against the 

 skin over the bone of the til)ia or ankle, would not 

 cause pain in man — further e\idence that high fre- 

 quency of discharge in a sensory receptor or nerve 

 need not give rise to pain in an afferent pathway not 

 ordinarily concerned therewith. 



Adrian however did note in animals that the dis- 



charge following; the painful stimulus of a heavy needle 

 prick was prolonged up to 20 sec. The discharge after 

 a light needle prick likely to evoke only a .sense of 

 touch in man lasted ijut 0.2 sec. or less (fig. 2). The 

 initial frequency of the discharge was however the 

 same with each type of stimulus. Although at the time 

 of his writing the naked terminations of nerves were 

 presumed to be exclusively receptors for pain, it 

 would, as he said, "make for economy if one and the 

 same nerve fiber could be used to signal nonpainful 

 stimulation by a brief discharge and painful stimula- 

 tion bv a much longer one." He would account for 

 the difference in sensation by a breakthrough of the 

 long discharge into areas of the central nervous 

 system inaccessible to its shorter counterpart. Such a 

 mechanism would not preclude another apparatus for 

 touch with particular receptors and fibers such as the 

 nerve roots around hair follicles. In addition to pro- 

 longed discharge the pain receptors and fibers, as 

 studied at the cornea for example, also show slow 

 adaptation, i.e. they continue to transmit pain im- 

 pulses as long as the noxious stimulus is present. 

 Studies of single afferent fibers in the cat by Maru- 

 hashi rl al. (185) are mentioned later. 



FIG. 1. .Action potentials in cat's cutaneous nei'\e in response 

 to touch and pain. Needle on weighed leser lowered on to the 

 skin and allowed to rest there. .-1. Weight on needle 3 gm, 

 very brief discharge. B. Weight on needle 43 gm, continued 

 discharge. C Weight on needle 99 gm, continued discharge. 

 The 3 gm weight on needle used by Adrian for tracing A would, 

 he says, be on human skin the stimulus for the sensation of 

 contact. The discharge of impulses lasted about 0.2 sec. At 

 weights above 20 gm a distinct prick' is felt on human skin. 

 Discharges as in tracings B and C lasted as long as 20 sec. 

 [From Adrian (i).l 



