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HANDBOOK OF I'HYSIOLOGY 



NEUROPH-l'SIOLOGY 1 



to have pain in his right upper limb, maximal in 

 thumb and axilla. This on examination proved to be 

 accompanied by "hyperalgesia to pin prick, hyper- 

 esthesia to warm and cold" and intense pain from a 

 vibrating tuning fork when applied over sharply 

 defined areas of the limb and chest. Both the spon- 

 taneous pain, and the abnormal responses to pinprick, 

 temperature and vibration were completely and im- 

 mediately stopped by pressure on the anterior surfaces 

 of the tips of the medial four right fingers; later, pres- 

 sure only on the tips of fingers four and five sufficed 

 to stop the pain. The inhibitory mechanism did not 

 tend to fatigue and the patient could and did keep 

 away the pain by keeping his fingers clenched to a 

 fist. The purely clinical observations gave no clue to 

 the mechanism of such inhibition. 



The converse situation in which pain exerts an 

 inhibitory effect on simultaneous perception of non- 

 painful stimuli has been studied by Benjamin (19). 

 He found that several forms of experimental pain all 

 increased the thresholds of hearing over the total 

 tonal range, flicker fusion, vibration at 60 cps and 

 contact heat. The mean threshold raising efifect was 

 generally proportional to intensity of pain. 



REFERRED P.iiIN 



Under a variety of circumstances pain arising from 

 impulses in one structure, usually deeply placed such 

 as a viscus, is referred wholly or partly to some other 

 area, usually superficial. The paucity of nerve endings 

 in the deep tissues and the small volume of conscious 

 sensation normally arising from these protected areas 

 allow lesser opportunity for the cerebral cortex to 

 build up a pattern of the internal image of the body 

 as detailed as that of the surface. And indeed the 

 cerebral mechanism for so doing is much smaller, as 

 witness the tiny cerebral cortical area of splanchnic 

 representation found by Amassian (5). If then noxious 

 stimuli arising from deep structures converge upon the 

 same neuron as such stimuli from the skin, the sensory 

 centers may refer the origin of the stimulus to the far 

 more frequent site of such origin — namely the skin. 



That the peripheral neuron itself may be one of the 

 sites of the convergence has been suggested by Sin- 

 clair et al. (251). The bifurcation of a single parent 

 axon into two limbs, each passing into a different 

 nerve trunk, has been demonstrated in fish by Wern0e 

 (292), in amphibia by Adrian et al. (2) and probably 

 in mammals by Lloyd (179), but not yet in man. 

 However, the phenomena of summation, inhibition 



and irradiation — all demonstrable in connection with 

 referred pain — are more readily explicable on a 

 central basis. Hence Weddell himself (287) is inclined 

 to place the mechanism for referred pain mainly in 

 the central nervous system. 



Ruch (231) has drawn attention to one likely site 

 of convergence of visceral and cutaneous afferents, 

 namely the cells of the secondary afferent neuron in 

 the posterior horn, because he finds many more 'pain 

 fibers' in the posterior roots than axons in the spino- 

 thalamic tracts. If it is true that there are more 

 primary than secondary afferent fibers potentially 

 concerned with pain, then two of the former, one 

 from a viscus, the other from skin, may well terminate 

 in relation to a single dorsal horn cell. E.\citation of 

 the pool of such cells from a viscus ma\- then result in 

 erroneous reference to the skin. 



Physiologic evidence for confluence of cutaneous 

 and deep afferent pathways upon a single neuron 

 has been acquired by demonstration of firing of the 

 central neuron by either the cutaneous or the deep 

 sensory nerve. Proof that it is indeed the same neuron 

 responding is enhanced by the finding of 'occlusion,' 

 i.e. that after excitation of the central neuron from 

 one peripheral source there elapses an 'unresponsive 

 period' during which it cannot be excited from the 

 other peripheral source. Such convergence upon single 

 neural units in the thalamic nucleus ventralis postero- 

 lateralis of the cat has been found by MacLeod (181) 

 specifically related to delta afferent fibers in the 

 splanchnic nerves and hence presumptively related 

 to pain. In fact the majority of the cells responding to 

 splanchnic delta afferents also responded to stimulation 

 of the skin, usually that of the trunk but at times that 

 of limbs or tail. Such thalamic cells were present both 

 ipsilateral and contralateral to the splanchnic 

 stimulus. The duration of the 'unresponsive period' 

 of the pathway depended upon which peripheral 

 field was stimulated first, and the response to stimu- 

 lation of one of the fields was at times intermittent 

 while that from the other field was consistent. Hence 

 MacLeod, cited by Gordon (105), considered it un- 

 likely that the confluence occurred peripherally in 

 branches of the primary afferent neuron. 



Widen (298) has also studied in similar fashion 

 delta afferent fibers projecting to the anterior lobe of 

 the cerebellum. This region was excited by stimu- 

 lation of either a lower intercostal nerve or the 

 splanchnic delta fibers and a high degree of occlusion 

 between them was found. Although these studies are 

 less clearly related to referred pain because of the 

 lack of correlation between the cerebellum and con- 



