David, after some searching, I finally found what the marginal cells that I think you were referring to. They must be the cells in the marginal layer (layer 1) of the substantia gelatinosa, mentioned in Gray's Anatomy
http://www.bartleby.com/107/185.html, and in the following studies:
• Galhardo V & Lima D (1999). Structural characterization of marginal (lamina I) spinal cord neurons in the cat. A Golgi study. Journal of Comparative Neurology, 414, 315-333.
• Lima D, Galhardo V & Coimbra A (1995) Morphological classification of marginal (lamina I) neurons in the cat spinal cord: a golgi study. Soc. Neurosci. Abstr., 21, 378.
and other references ... I posted them in the Pain Forum with comments.
Wise.