Somatotopic maps are disorganized in adult rodents treated neonatally with capsaicin

The somatosensory system is characterized at each stage of the projection pathways by the presence of maps formed by an orderly arrangement of cells and of the incoming fibres. Each cell responds to a definite area of skin, its receptive field (RF). If cell locations and their RFs are plotted side by side, they make up a continuous map of the body surface. Given the stability and repeatability of these maps, it was of interest to find that lesions which destroyed the input to one part of the map of the spinal cord in adults were followed by a readjustment of the RFs of cells which had lost their normal input¹. It was more surprising to discover in adult cats and rats that peripheral nerve lesions which do not produce an anatomical destruction of spinal cord afferents were followed by the appearance of novel RFs in cells deprived of their normal physiological input^2,3. Capsaicin given to neonatal mice or rats destroys unmyelinated afferents. We report here that this is followed by an expansion of the normal receptive fields supplied by myelinated afferents^2,3. Our results suggest that unmyelinated fibres may be involved in controlling the connectivity of myelinated afferents with first central cells.