Abstract
As a test of the hypothesis that primary afferent cutaneous terminal neuropil has the same somatotopy as do dorsal horn cells, the projections of dorsal roots L3-S2 were studied in cats. Specifically, the projections of these roots to laminae III-IV in segments L2-S3 were depicted as schematic dorsal-view maps. These laminae are believed to receive predominantly cutaneous light-touch afferent projections. The dorsal roots projected only to those areas of dorsal horn that are somatotopically appropriate, in that a root''s dermatome overlapped the receptive fields of dorsal horn cells in the root''s projection zone. Although no simple geometrical rule can be used to describe uniformly the projections of all the roots studied, the projections can be described according to the presynaptic somatotopy hypothesis. For example, although the L5 projection is shifted laterally as it goes caudal through L6 and L7 and the S1 projection is shifted laterally as it goes rostral through L6 and L7, both shifts can be described as a displacement of fibers from relatively proximal skin by the enlarged representation of relatively distal skin in medial L6 and L7. Presynaptic primary afferent neuropil in laminae III-IV apparently has the same somatotopic organization as dorsal horn cells.