Modality representation in the lumbar and cervical fasciculus gracilis of squirrel monkeys

The first-order fibers comprising the fasciculus gracilis in the spinal cord of squirrel monkeys were studied with electrophysiological techniques. The data indicate that the modality composition of the fasciculus is modified systematically as it ascends to the nucleus gracilis in the brain stem. At lumbar levels of the spinal cord the fasciculus is comprised of modality-specific fibers, some of which supply cutaneous receptive fields in the periphery; and others which innervate the deeper-lying tissues such as the joint capsules, periosteum, fascia, and muscle. Within the lumbar portion of the tract the 'skin' and 'deep' fibers are completely intermingled in their topographical distribution and, to a first approximation, the fasciculus contains equal numbers of the two classes of fibers. At high cervical levels the fasciculus gracilis contains only fibers which supply cutaneous receptive fields. This change in the modality composition of the fasciculus between lumbar and cervical levels provides a firm basis for the previous reports that total destruction of the primate cervical dorsal columns fails to alter hindlimb kinesthetic function.