488 



HANDBOOK OF PHYS!OL(jnV 



NEUROPHYSIOLOGY 



The figures of other workers, such as Goldstein (103), 

 Foerster Sc Gagel (80, p. 24), Walker (276), Rasmus- 

 sen & Peyton (220), Gardner & Cuneo (87) and 

 Poirier & Bertrand (213), are in general agreement. 

 Although such degeneration has provided a subpial 

 signpost to the localization of the pain fibers, it has 

 failed utterly to intimate their full extent. This was 

 first shown by clinical plus post-mortem studies of 

 the lesions after thrombosis of vessels supplying the 

 bulbar brain stem. These deductions were fully 

 confirmed by the pioneering surgical work in man of 

 Schwartz & O'Leary (242, 243^, and of White (293) 

 and by subsequent surgeons, Crawford (50) and 

 D'Errico (64). Findings with respect to pain pro- 

 voked by stimulation at operation were checked 

 against depth of incision, postoperative analgesia 

 and, at times, later post-mortem studies. These show 

 that the pain fibers coming up from the cord occupy 

 a much wider area just dorsal to the inferior olives 

 extending 6 to 7 mm deep and continuing medially 

 in the midst of the bulbar reticular formation nearly 

 to the medial lemniscus. The quintoth^lamic or 

 secondary afferent trigeminal fibers tend to lie in 

 the more medial part of this area and to extend more 

 dorsally as well (D'Errico). McKinley & Magoun 

 (186) have shown from depth recording of action 

 potentials in cats that there is indiscriminate mixing 

 of the fibers from the three trigeminal divisions in 

 this area, whereas the grouping of fibers related to 

 trigeminal peripheral divisions is clearer in the de- 

 scending trigeminal tract and nucleus, as shown by 

 McKinley & Magoun (186) and Harrison & Corbin 

 (120). Subsequent work in man has also shown that 

 a discernible tendency to layering in the cord of 

 the fibers from specific sections of the body becomes 

 less consistent in the medulla. 



More work is especially necessary on the course 

 of pain fibers from face and head once thev start 

 up the brain stem There is some evidence from 

 Wallenberg (281) that these fibers separate into 

 two deep bundles as they move rostralh'. To the 

 illustration from Kuru (fig, g) has been added an 

 indication of Wallenberg's notion of the location of 

 the.se secondary fibers from the face at the mid- 

 bulbar level. 



Numerous fibers ascending from the cord move 

 medially to terminate in the reticular formation of 

 pons and medulla; their possible significance will i^e 

 considered in the next section. 



MESENCEPH.\LON 



In the upper pons and midbrain pain fibers again 

 become more superficial and hence more accessible 

 to special analysis and surgical section in animals 

 and man. Here their precise extent and location is 

 less well-known than in the cord and medulla be- 

 cause of the smaller numbers of studies. In general, 

 the fibers occupy a zone extending dorsallv and 

 medially for about i cm from the lateral messn- 

 cephalic sulcus. One example will suffice to indicate 

 some of the unresol\-ed discrepancies. Walker (278), 

 the major pioneer in this field, following a trigeminal 

 lesion in the monkey places the Marchi degenera- 

 tion in the lower midluain in a narrow zone 1 to 2 

 mm deep beginning right at the surface and ex- 

 tending dorsally a few millimeters from the lateral 

 mesencephalic sulcus (fig. 10). Wallenberg (281) 

 and van Gehuchten (271) working with the same 

 method in raijbits found the degeneration exclu- 

 sively in a much more medial position, and Wallen- 

 berg confirmed his impression in studies of degenera- 

 tion in a patient (281). Moreover the spinothalamic 

 tract demonstrable in Marchi stains at the level of 

 the superior coUiculus in man has dwindled to a 

 tiny bundle. Having identified the bundle in Marchi 

 stains, Glees & Bailey (99) then counted the fibers 

 in this region in normal Weigert preparations; they 

 found onl\ aijout 1500 fibers. Of these two-thirds 

 were 2 to 4 yu in diameter; most of the remainder 

 measured about 4 to 6 /x; they were all in a small 

 compact group only aijout 0.65 mm- in cross-sec- 

 tion. 



However, figure 10 also illustrates the area of 

 surgical destruction in the largest lesion figured by 

 Walker (278) which did not produce complete 

 analgesia on the opposite face and lower limb (al- 

 though it did on the torso and upper limb). Yet the 

 lesion essentially blankets all of the variously de- 

 scribed zones of niNelin degeneration. It is again 

 apparent that we need to know more about the 

 unmyelinated fibers and perhaps about the role of 

 relays of neurons in conduction of pain impulses. 



The marked decrease in size at the upper mid- 

 brain of the Marchi-stained ascending afferent 

 bundle following extensive cordotomy has long 

 been shown to be due to departure from it of ventral 

 spinocerebellar, spinoreticular and spinotectal fibers. 

 The earlier descriptions, such as those of Foerster & 

 Gagel (80), were confirmed by Morin el al. (190) 

 who were the first to suggest that the spinoreticular 



