Sensory neurons supplying touch domes near the body midlines project bilaterally in the thoracic spinal cord of rats.

Sensory neurons that project bilaterally to laminae III-V of the dorsal horns in the thoracic spinal cord of rats were described in a previous anatomical study. The electrophysiological experiments reported here show that these neurons innervate skin near the dorsal and ventral midlines. Three series of experiments were undertaken: the regions of skin supplied by subdivisions of thoracic nerves were mapped by recording from the subdivisions while mechanically stimulating the skin; subdivisions containing the axons of contralaterally projecting sensory neurons were identified by antidromic activation from the contralateral dorsal horn; the receptive fields of individual sensory neurons and dorsal horn neurons were characterized by single-unit recording methods. Contralaterally projecting sensory neurons had axons in subdivisions of the thoracic nerves that supply skin adjacent to the body midlines. Their receptive fields were located near the dorsal or ventral midline but did not extend onto the contralateral side of the body. Most of the neurons were slowly adapting "type I" units supplying touch domes. Dorsal horn neurons receiving input from contralateral sensory neurons had receptive fields that overlapped the dorsal or ventral midline. This bilateral representation of skin near the midlines may be important for the fusion of the sensory maps of the right and left sides of the body.