The primary somatosensory cortex of small rodents is an isomorphic representation of the body surface. Similar representations are characteristic of the subcortical pathways, leading from the periphery to the cortex, and these representations develop in a sequence that begins at the periphery, and that ends in the cortex. Furthermore, central representations at all levels of the neural axis are altered by perinatal perturbations of the peripheral surface. This has led to the hypothesis that the periphery plays an instructional role in the formation of central neuronal structures. The morphology of this discrete organization has been examined thoroughly during the development of the thalamocortical projections. The mechanism(s) that underlies the formation of these representations remains unclear although some recent evidence suggests the involvement of activity-dependent processes that are modulated by 5-HT.