ASSEMBLY OF THE DORSAL HORN SOMATOTOPIC MAP

We hypothesize: (a) peripheral innervation densities determine map sca les in dorsal horn, (b) dorsal horn cell (DHC) receptive field (RF) ge ometries are determined by map scales, and (c) morphologies of primary afferents (PAs) and DHCs reflect their developmental history. We sugg est the following sequence: (A) PAs project in a somatotopic mediolate ral sequence. (B) DHCs assemble prototype RFs by sampling presynaptic neuropil with their dendrites. (C) PAs then project to all levels wher e their RFs are contained within prototype RFs of DHCs. (D) A competit ive mechanism produces the adult form of DHC RFs. (E) Adult distributi ons of PA terminals and DHC dendrites reflect this developmental histo ry. (F) Mediolateral somatotopic gradients are determined by RF densit ies of axons entering at the same levels. (G) Map scales at different rostrocaudal levels are determined by somatotopic gradients. (H) Geome tries of DHC RFs are determined by constant convergence and divergence of monosynaptic connections. (I) Secondary processes further modify g eometries of DHC RFs. (J) Residual self-organizing capacity supports m aintenance and plastic mechanisms. We adduce the following evidence: ( 1) agreement between monosynaptically coupled inputs and cells' excita tory low threshold mechanoreceptive fields; (2) the temporal sequence of events during penetration of the gray matter by PAs; (3) variation of PA terminal and DHC dendritic domains as a function of map scale; ( 4) somatotopic gradients and geometries of DHC RFs in adult dorsal hem ; (5) calculations of peripheral innervation densities and dorsal horn map scales; and (6) constant divergence and convergence between PAs a nd DHCs.