Direct somatosensory projections from the spinal cord to the hypothalamus and telencephalon.

Somatosensory input to the hypothalamus has been thought to ascend via an indirect, multisynaptic pathway. However, we have antidromically identified nociceptive spinal cord neurons that project directly to the lateral hypothalamus in rats. Retrograde tracers injected into the lateral hypothalamus labeled many spinal neurons bilaterally within the marginal zone, the lateral reticulated area, the lateral spinal nucleus, and the area surrounding the central canal. An anterograde tracer injected into these areas of the spinal cord labeled fibers and terminals in the lateral hypothalamus and, surprisingly, in a number of telencephalic areas. These findings demonstrate a direct somatosensory projection from the spinal cord to the hypothalamus and several telencephalic regions.