Shortland, P. J.; (1990) The morphology, somatotopy and plasticity of hindlimb low threshold cutaneous primary afferents in the dorsal horn of the rat lumbar spinal cord. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The morphology of the collateral arborizations of hair follicle, (HFA's, n=38), rapidly adapting, (RA, n=14) and slowly adapting type-I, (SAI, n=6) afferents innervating hindlimb skin were studied by intraaxonal injection of horseradish peroxidase (HRP) in adult rats. Each physiological class of afferent possessed complex, simple and blind-ending collaterals based on numbers of boutons and terminal branch pattern. Each afferent had a distinct morphology, laminar location and dimensions depending on their peripheral receptive field (RF) location. The location of the central terminal field of each afferent was reconstructed and somatotopic maps constructed for each afferent type. Overlap of central terminal fields was extensive between afferents within the same cutaneous nerve but it was restricted to blind, and on some occasions simple, collaterals between afferents from different cutaneous nerves. The spatial organization of the central terminals of cutaneous primary afferents formed a coarse somatotopic map of overlapping terminals whereby a region of dorsal horn had a maximal, but not exclusive, input from a particular skin area. This data was used to test the morphological and somatotopic consequences of neonatal deafferentation. Neonatal administration of capsaicin on the day of birth resulted in dorsally directed sprouting of HFA's, but not RA afferents, into lamina II without altering the gross morphology or somatotopic organization of primary afferent collaterals. Neonatal peripheral nerve section also resulted in sprouting, more complex than that following capsaicin treatment. Intact afferents adjacent to a denervated region sprouted from their normal terminal areas into denervated regions. When a particular functional class of afferent sprouted into an area containing terminals from another afferent class, the morphology of the sprouted terminal was appropriate to the new target area, rather than to its own functional class. This indicates that the central rather than the peripheral target determines the terminal growth pattern. In conclusion, the pattern and morphology of low threshold primary afferents is not fixed but can be altered by peripheral manipulations at birth.

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