Somatotopic reciprocal connections between the somatosensory cortex and the thalamic Po nucleus in the rat.

Retrogradely labeled neurons are observed in the posterior group of the thalamus (Po) after injection of wheatgerm agglutinin-horseradish peroxidase in the rat somatosensory cortex. These neurons are organized in rods elongated rostrocaudally, defining a clear somatotopic map. Injections of tritiated leucine in the somatosensory cortex indicate that these somatotopically organized connections are reciprocal. Injections of tritiated leucine in the dorsal column nuclei label afferent fibers in a small area dorsal to Po but not in the core of the nucleus. Po does not receive direct projections of ascending somatosensory afferents. It is hypothesized that this thalamic area participates in a thalamo-cortico-thalamic loop.