THE zyxwvuts JOURKAL OF COMPAKATlVE NEUROLOGY 329~491-511 (1993) Morphology and Somatotopy of the Central Arborizations of Rapidly Adapting Glabrous Skin Afferents in the Rat Lumbar Spinal Cord PETER SHOKTLAND AND CLIFFORD .J. WOOLF Department of Anatomy and Developmental Biology, University College London, London WClE 6BT, England ABSTRACT The central arborizations in the dorsal horn of the spinal cord of 23 rapidly adapting (RA) A-beta primary afferent neurons innervating different regions of the glabrous skin of the hindpaw were studied by the intra-axonal injection of horseradish peroxidase in adult rats. A total of 284 arbors of the complex, simple, and blind-ending variety were recovered. The arbors of RA afferents innervating the toes, paw pads, and non-pad hindpaw differed from each other in branch pattern and dimensions. The simple and complex arbors, which are both bouton- containing, were distributed mainly in laminae 111-V, although some complex arbors projected dorsally into lamina IIi. The hindpaw glabrous skin afferent terminals were located in the lumbar enlargement from caudal L3 to rostra1 L6. A crude somatotopic organization was observed such that toes 1-5 were represented successively in more caudal positions from mid-L4 to caudal L5. The paw pads were organized in a rostrocaudal sequence moving from the paw pads proximal to toe 1 across the foot to the paw pads proximal to toe zyxwvut 5, from caudal L3 to mid-L5. Non-pad hindpaw afferents were located in caudal L5. Overlap between toe, paw pad and non-pad afferent central fields was present, however, and the central terminals of afferents with non-adjacent peripheral receptive fields were shown to occupy the same region of the dorsal horn. zyxw r 19% I+'iky-Liss, Inc Key words: dorsal horn, primary afferent neuron, somatosensory system The glabrous skin of the hindpaw of the rat is innervated predominantly by the tibial nerve (Swett and Woolf '85). The central terminals of this nerve have been shown by bulk labelling with free or conjugated horseradish peroxi- dase (HRP),to be distributed to the medial quarter to third of the dorsal horn in lumbar segments L3 to L5, (Swett and Woolf, '85; Molander and Grant, '86; Woolf and Fitzgerald, '86). Bulk labelling methods give no detail, however, about the organization of the central arborizations of individual afferents with different peripheral receptive fields within a single nerve territory. This can only be elucidated by using intracellular dye injections into single, functionally charac- terized afferents. The injection of HRP into the central axons of primary afferent neurons has shown that axon collaterals of coarse and fine primary afferents terminate in different laminar regions of the spinal cord. Low threshold mechanosensory A-beta and A-delta myelinated fibres terminate mainly in laminae 111-IV (Light and Perl, '79b; Brown, '81; Woolf, '87; Shortland et al., '89a; Brown et al., '91) while A-delta mechanical nociceptors terminate in laminae I and zyxwvu V (Light and Perl, '79a) and C fibre nociceptors and thermoceptors in laminae I and I1 (Sugiura et al., '86). Intra-axonal staining has also shown that the collateral arbors of different functional classes of primary afferents exhibit morphological specialization (Brown, '81; Ralston et al., '84; Maxwell and Rethelyi, '87). Hair follicle afferent neurons (HFAs), for example, possess characteristic flame shaped arbors in cat, rat, and monkey, forming an uninter- rupted narrow rostrocaudal sheet in the dorsal horn (Brown et al., '77; Light and Perl, '79b; Woolf, '87; Shortland et al.. '89a). Rapidly adapting (RA) glabrous skin afferents have been found to have a different morphology, without flame shaped terminals and collateral arbors which form inter- rupted sheets of terminals in the dorsal horn (Brown et al., '80; Woolf, '87). Accepted November 13,1992. Peter Shortland is now at Department of Anatomy and Ncurobiology, St. Louis University Medical School, St. Louis, MO 63104. Address reprint requests to Clifford J. Woolf, Department of Anatomy and Developmental Biology, University College London, Goiver Street, London WCIE 6BT. England. zyxwvu c 1993 WILEY-LISS, INC.