PAIN 



485 



fibers. Furthermore, although the experience of 

 pain is a more compelling usurper of conscious at- 

 tention and tends to evoke more obvious motor 

 activity than touch, pressure or the movement of a 

 limb, these latter stimuli are accompanied by far 

 more conspicuous signs of electrical activity appear- 

 ing at a lower threshold. This is of course due to the 

 fact that the latter modalities tend to traverse larger 

 fibers and the recorded potential is in approximately 

 linear relation to the diameter of the fiber, as shown 

 by Gasser & Grundfest (92). Hence but few workers 

 have carried their studies into the difiicult realm of 

 analysis of the smaller late potentials from noxious 

 stimuli. 



There are several facts which indicate that pain 

 impulse conduction within the cord ma\' invoke 

 unmyelinated as well as myelinated fibers. In the 



painstaking study of Haggqvist (i 14) 17,000 fibers in 

 a single cross section of a young woman's cord at 

 the T3 .segment were measured. Samples were 

 counted from each of the zones numbered in figure 

 8. Forty-two per cent of all the fibers in the whole 

 cross section measured 2 ixor less in diameter, whereas 

 in the anterolateral zones 6 and 7, the division of 

 which would yield contralateral analgesia, 55 per 

 cent and 61 per cent, respectively, of the fibers were 

 of this small diameter. 



Most of the histologic studies of fiber tracts in- 

 cluding those for pain have relied on myelin or its 

 degeneration products, but it would be a coincidence 

 if the unmyelinated pain pathways were coexten- 

 sive with pathways we can see in such stains as the 

 Swank-Davenport (258) modification of the Marchi 

 method. And indeed we shall see that in the brain 



. .y?^. 



1 i M S 6 T S t t& 11 a II n IS ti 17 IS // 20 Vfi 



FIG. 8. Fiber sizes in the spinal cord. Cross section at T3 segment in which Haggqvist measured 

 diameters of 17,000 fibers and subdivided the white matter into 14 zones on the basis of differing 

 constellations of fiber sizes. The histogram below and to left indicates the percentage of fibers of 

 each diameter in the entire hemisection. In the regions ventral and ventromedial to the anterior 

 horn fibers less than 2 n constitute 43 to 45 per cent of total. Since this is about the general average, 

 these studies give no clue that the pain fibers lying here preponderate over tiny fibers with other 

 functions. See te.xt for different deductions with respect to zones 6 and 7. [From Haggqvist (i 14).] 



