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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



the peculiar sensation so extraordinary and un- 

 matched by any of their previous experiences. Vet 

 many so afflicted find the feelings sufficiently intoler- 

 able to seek major surgery for relief Certainly such 

 sensations are reasonably classified as an unusual form 

 of pain, and the elucidation of the mechanism of 

 severe unpleasantness referred to an anesthetic area in 

 the presumed absence of an organic central lesion re- 

 mains one of many challenE^es to the neurophysi- 

 ologist. 



Pain may be arbitrarily divided into two main 

 elements, the initial sensation and the reaction to that 

 sensation. As Beecher (15) has emphasized, the signifi- 

 cance of the pain to the individual plays a major role 

 in determining the extent of the second reactive com- 

 ponent of the feeling. Thus he found that only 32 per 

 cent of 1 50 war-injured men had pain severe enough to 

 require a narcotic, whereas 83 per cent of 150 male 

 civilians undergoing surgery involving much less 

 trauma required narcotics. For the soldier the war 

 wound marked the end of a gravely hazardous form of 

 life, whereas no such compensation and, at worst, 

 serious problems beset the operated civilian. Beecher 

 interpreted the differing degree of complaint in these 

 two settings as indicating "that the reaction or 

 processing phase is very often of more importance in 

 suffering than is the original sensation." Indeed the 

 original sensation as well may appear as a symptom 

 of protest, as from the person who develops a ' sick 

 headache' at will; or the pain may come as a conse- 

 quence of previous conditioning as in 'a painful 

 memory.' Contrariwise, various cerebral deficiency 

 states result in a reduced reaction to afferent impulses 

 for pain which would normally evoke a lively response. 



In this essay we shall devote most of the discussion 

 to the sensation of pain as evoked by specific stimuli 

 to sensory endings or pathways, recognizing, how- 

 ever, the vast importance of the psychological com- 

 ponent of the response and the virtual impossibility of 

 separating this from the primary awareness of pain. 



PAIN AS A SENSATION WITH ITS OWN CENTRAL 

 AND PERIPHERAL APPARATUS 



The validity of searching for a special sensory 

 mechanism concerned wholly or mainly with pain 

 requires inquiry. The physiologist has not sought out 

 specific nervous pathways subserving pleasure, the 

 philosopher's antipode to pain; is it sensible to look 

 for pain pathways? One can answer promptly that it 

 is and has proved eminently fruitful to do so because 



certain stimuli to certain areas almost invariaiilv 

 bring on pain in man, whereas the same constancy of 

 relationship in no way applies to pleasure. One may 

 venture to state that even the amorous male with the 

 most Ca.sanovian success has not developed a form and 

 site of stimulus which constantly evokes pleasure in 

 his partner, thous^h in the absence of a criticallv re- 

 ported series this can be no more than the author's 

 disgruntled surmi.se. 



Erasmus Darwin (53, pp. 121 and 125) thought pain 

 to be the consequence of any excessive stimulation and 

 a result of exaggeration of sensations of heat, touch, 

 sight, taste or smell. This intensive theory of pain in 

 one modification or another has found many sup- 

 porters. And we can scarcely disagree with William 

 James' (134) conclusion that it is certain that sensa- 

 tions of every order which in moderate degree are 

 rather pleasant than otherwise become unpleasant 

 when their intensity grows too strong. For example, in 

 1934 Nafe (198) drew attention to the fact that when 

 smooth muscle was in spastic contraction at the 

 extremes of heat and cold, 52°C and 3°C respectively, 

 there was pain. At levels intermediate between these 

 there was only a sense of warmth or coolness. As 

 stimulation became more intense with a rise in tem- 

 perature the quality of the sensation was altered from 

 warmth, to heat, to pain; all mediated he thought bs- 

 the same peripheral equipment and integrated at the 

 thalamocortical level. More recently Gooddy (104) 

 has argued that "any nervous pathways are potential 

 pain pathways," i.e. that any pathway may provide 

 "the impulse patterns that are associated with the 

 perception of pain. " In certain patients successive 

 operations on peripheral nerves, posterior roots, spinal 

 cord, thalamus and cerebral hemispheres may all fail 

 to give permanent relief from pain. From such series 

 of events, infrequent though they are, Gooddy reaches 

 the extreme point of view that "unless the whole 

 nervous system is destroyed, the abnormal patterns 

 (evoking pain) gradually establish themselves anew. 



The most clear-cut evidence to the contrary, that 

 at least some pain is to be regarded as a particular 

 form of sensation with its own pathways and not 

 merely an intensification of other forms, is provided 

 by patients with a lesion confined to the anterior 

 quadrant of the spinal cord. This usually deprives 

 them of the capacity to feel pain in response to a wide 

 variety of noxa previously painful, yet proprioceptive 

 and light touch sensibility are virtually unimpaired. 

 This is the typical finding after the operation of 

 anterolateral cordotomx. Although thermanesthesia 



