Mediolateral and dorsoventral projection patterns of cutaneous afferents within transverse planes of the mouse spinal dorsal horn.

The central projection patterns of cutaneous afferents from the forelimb and shoulder of mice were studied in the spinal dorsal horn after intracutaneous injection of AlexaFluor 488-conjugated and/or 594-conjugated cholera toxin subunit B (CTB). Based on their dermatomes, the following eight skin regions are thought to be innervated by spinal nerves from the sixth to eighth cervical spinal nerve roots: the dorsal surface of the shoulder, brachium, proximal forearm, distal forearm, hand, palmar surface of the second and third digits, and palm. The termination areas of afferents from the dorsal surface of the shoulder and forearm were narrow, distributed in a dorsoventral direction, and aligned in order from lateral to medial within the sixth to eighth cervical dorsal horns. By contrast, the termination areas of the palmar surface of the second and third digits largely overlapped. We also injected CTB into the dorsal surface of the hindlimb and pelvic regions. Skin regions there are thought to be innervated by nerves from the third to fifth lumbar spinal nerve roots. The observed projection patterns in the lumbar dorsal horn were similar to the cervical patterns. Injection of a mixture of CTB and wheat-germ agglutinin-conjugated horseradish peroxidase (WGA-HRP), which are thought to label Aβ and Aδ/C fibers, respectively, showed segregated termination areas of CTB- and WGA-HRP-labeled afferents. Moreover, alignment of the termination areas was in the dorsoventral direction. These results suggest there is fine somatotopic (mediolateral axis) and modality-specific (dorsoventral axis) organization within the spinal dorsal horn.