Spatial convergence and divergence between cutaneous afferent axons and dorsal horn cells are not constant

We have proposed a quantitative model of the development of dorsal horn cel l receptive. fields (RFs) and somatotopic organization (Brown et al. [1997] Somatosens. Motor Res. 14:93-106). One component of that model is a hypoth esis that convergence and divergence of connections between low-threshold p rimary afferent mechanoreceptive axons and dorsal horn cells are invariant over skin location and dorsal horn location. The more limited, and more eas ily tested, hypothesis that spatial convergence and divergence between cuta neous mechanoreceptors and dorsal horn cell are constant was examined. Spat ial divergence is the number of dorsal horn cells whose RFs overlap the RF center of a primary afferent, and spatial convergence is the number of affe rent RF centers that lie within the RF of a dorsal horn cell. Innervation d ensity was determined as a function of location on the hindlimb by using pe ripheral nerve recording and axon counting. A descriptive model of dorsal h orn cell receptive fields (Brown et al. [1998] J. Neurophysiol. 31:833-848) was used to simulate RFs of the entire dorsal horn cell population in orde r to estimate RF area and map scale as a function of location on the hindli mb. Previously reported correlations among innervation density, map scale, and RF size were confirmed. However, these correlations were not linear. Th e hypothesis that spatial convergence and divergence are constant was rejec ted. The previously proposed model of development of dorsal horn cell somat otopy and RF geometries must be revised to take variable spatial convergenc e and divergence into account. J. Comp. Neurol. 420:277-290, 2000. (C) 2000 Wiley-Liss, Inc.