The topology of dermatomal projection in the medial lemniscal system

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Abstract
The topographic organization of first order afferent fibers in the lumbar, sacral and coccygeal dorsal roots, and in the fasciculus gracilis was studied in squirrel monkeys. At the entry zone, progressing from caudal to rostral, dorsal root filaments receive fibers from tail and hind-limb receptive fields which serially overlap and describe a spiral-shaped trajectory. The latter starts with tail, progresses post-axially towards the foot, crosses the foot from lateral to medial, and ascends the preaxial leg. In the fasciculus gracilis, this arrangement of fibers at the dorsal root entry zone is preserved in its entirety. It assumes the form of a fiber lamination, with the most caudal dorsal root fibers occupying a dorso-medial location; further rostral dorsal root fibers are more ventrolateral. Dorsum and sole of foot project in an overlapping and interdigitating manner to the fiber lamina of the 7th lumbar dermatome in the fasciculus gracilis. Thereby, dorsum and sole of foot behave in the projection as if they were one and the same surface. The argument is presented that the foot and its projection on to the cross-sectional plane of the dorsal funiculus are topologically equivalent and that the hind-limb as a whole and its projection are not. On the other hand, homotopic mapping of the foot together with the sequential fiber organization in the dorsal funiculus enable many more types of closed curves on the body surface to remain arc-wise connected in the projection than would otherwise be possible.