464 



HANDBOOK OF I'HVSIGLOGV 



NEUROPHYSIOLOGY I 



group able to distinguish at least 8, possibly up to 16, 

 units of pain intensity when a chemical excitant of 

 pain is applied to the exposed base of a canthardin 

 blister. 



ANIMAL VERSUS HUMAN SUBJECTS IN PAIN STUDIES 



VVaterston (284) has pointed out that one's natural 

 repugnance to investigating pain in man must be 

 overcome because of " the \alue and importance . . . 

 of the information which can be thus obtained and by 

 this means only." The final and conclusive arbiter on 

 all questions relating to any sensation must i^e man 

 experiencing that sensation and able to describe it in 

 words; this is particularly true of pain which by defi- 

 nition must have .some degree of affective component 

 of unpleasantness. I should agree without reservation 

 with Hardy et al. (116, p. 2) that " the verbal report of 

 the instructed subject is the most reliable evidence that 

 the pain threshold has been reached." But it has 

 seemed logical to many to assume that a maneuver 

 which consistently evokes pain in man and pro\okes 

 in him some form of motor response when pain is felt 

 may Ije used in animals and the motor response of the 

 creature taken as an end point indicative of pain. Un- 

 fortunately this conclusion is not without pitfalls. 

 Such reasoning led Gerard (95, p. 335) to make a 

 series of successively more rostral incisions into the 

 spinal or descending trigeminal tract in cats, each 

 incision about 1.5 mm rostral to the previous one.' 

 She began at the midbulbar level at the obex and 

 ascended until stimulation of the cornea no longer 

 elicited the " pain reflexes' of struggling, pupillary 

 dilatation and rise of arterial pressure. Not until the 

 cut reached the midpontine area did corneal stimula- 

 tion fail to evoke such reflexes, .so she naturally con- 

 cluded that the pain fibers from the cornea terminate 

 just below this point. But in man all such fibers termi- 

 nate much lower, in fact all below the obex, because 

 cutting of all of the descending trigeminal tract at this 

 level produces enduring analgesia of the entire first 

 division trigeminal sensory zone (75; 296, p. 457}. It 

 is possible that collaterals from corneal pain fibers 

 may evoke reflexes without awareness of pain in both 

 cat and man, and that these come off at more rostral 

 levels than the obex; or it may be that the specific 

 anatomy in cats difiers from man. In either event 

 lesions needed to stop 'pain reflexes' in the cat were 



' The trigeminal nerve enters the upper pons and one bundle 

 of its fibers descends the length of the pons and medulla into 

 the uppermost cervical segments, so the more rostral the in- 

 cision the more toward the periphery the tract was being cut. 



decidedly diflx-rent from those required to stop pain in 

 man. 



Recently Goetzl c^/. (too) found upon stimulation 

 of the tooth pulp in unanesthetized cats and rabbits a 

 rise in arterial pressure and a decrease in volume of 

 the leg, spleen and kidney, whereas the reverse 

 changes in arterial pressure and organ volume oc- 

 curred when such stimuli were delivered to anes- 

 thetized animals. They concluded from these ob.serva- 

 tions that the ability of the stimulus to produce a rise 

 in arterial pressure depended upon actual perception 

 of pain by the animal. Such a conclusion has dubious 

 validity in \iew of Gerard's erroneous deductions from 

 the rise of arterial pressure in her cats. 



Animal experiments in the spinal cord have cor- 

 related even more mi.serably with work in man. 

 Cadwalder & Sweet (37}, after careful pre- and post- 

 operative studies in dogs, found behavior after antero- 

 lateral cordotomy which they considered evidence of 

 incomplete loss of cutaneous pain sensibilit\' along 

 with severe ataxia of the hind legs. Their post-mortem 

 material demonstrated incisions of the type which pro- 

 duce total cutaneous analgesia and no ataxia in man. 

 They quoted the work of six prexious groups who ob- 

 tained divergent results from similar animal work; 

 three of the other groups had been unable to demon- 

 strate any definite cutaneous sensory change in their an- 

 imals after anterolateral cordotomy. Even in monkeys 

 Mott (195) found no evidence of any loss of pain sensa- 

 tion after either unilateral or bilateral division of the 

 anterior halves of the cord. In the cat Karplus & 

 Kreidl (138) were unable to eliminate rostral response 

 to painful stimuli applied to the hind legs even by 

 complete hemisections, one on each side of the thoracic 

 cord, five or more segments apart. Only when the 

 incisions bisecting the cord were four segments apart 

 or less did noxa to the legs fail to e.xcite a response. 

 From this they deduced that pain is transmitted by 

 short chains of neurons crossing the cord from side to 

 side. The conclusion froin all these studies is that the 

 bulk of somatic pain-conducting axons in manv 

 mammals including monkeys do not maintain a fixed 

 position in the anterolateral columns of the cord. 

 Happily from the standpoint of easy surgical relief of 

 pain this position is usually the case in man. However, 

 if one had only reflex l)eha\ior in man as a guide one 

 might still be confu.sed. Thus, when one of my pa- 

 tients, after cordotomy, stepped on an upturned nail, 

 the analgesic leg briskly withdrew. C^urious as to why 

 the leg jumped, he discovered the heavy nail in the 

 sole of his foot; he was consciously aware only of some 

 local tina;linsf. 



