Submodality segregation and receptive-field sequences in cuneate, gracile, and external cuneate nuclei of the cat

The dorsal column nuclei of mongrel cats were mapped with low-impedance tungsten microelectrodes. Data concerning the nature of the adequate stimulus were collected and found to support the hypothesis that this region of the brain stem is composed of spatially segregated nuclear volumes that serve different clases of afferent fibers. The external cuneate nucleus and its extension beneath the cuneate nucleus serve a population of slowly adapting receptors located in deep structures, probably muscle spindles. A nuclear volume including parts of both the caudal cuneate and caudal gracile nuclei contains a specific cell population serving Pacinian afferent fibers from the entire body except the face. The middle portions of the cuneate and gracile nuclei receive input predominantly from cutaneous receptors. Both slowly adapting and rapidly adapting fiber populations project to this region. Each cutaneous submodality is restricted to spatially segregated nuclear regions that are longer in the rostral-caudal direction than in the mediolateral direction. When the electrode trajectory remained within one submodality region, sequences of receptive-field loci either remained on one body part or moved smoothly across a sequence of body parts without interruption. When braks in the smooth progression of receptive-field sequences occurred, they coincided with a change in submodality or modality. These observations support the hypothesis that within each submodality-specific region, the body is represented as a gradient or continuum of gradually shifting receptive fields and is interrupted only at boundaries with cell populations of another submodality or at nuclear boundaries. Submodality segregation and somatotopic organization are also present in the afferent fibers around the dorsal column nuclei.