Assembly of the dorsal horn somatotopic map

We hypothesize: (a) peripheral innervation densities determine map scales in dorsal horn, (b) dorsal horn cell (DHC) receptive field (RF) geometries are determined by map scales, and (c) morphologies of primary afferents (PAs) and DHCs reflect their developmental history. We suggest the following sequence: (A) PAs project in a somatotopic mediolateral sequence. (B) DHCs assemble prototype RFs by sampling presynaptic neuropil with their dendrites. (C) PAs then project to all levels where their RFs are contained within prototype RFs of DHCs. (D) A competitive mechanism produces the adult form of DHC RFs. (E) Adult distributions of PA terminals and DHC dendrites reflect this developmental history. (F) Mediolateral somatotopic gradients are determined by RF densities of axons entering at the same levels. (G) Map scales at different rostrocaudal levels are determined by somatotopic gradients. (H) Geometries of DHC RFs are determined by constant convergence and divergence of monosynaptic connections. (I) Secondary processes further modify geometries of DHC RFs. (J) Residual self-organizing capacity supports maintenance and plastic mechanisms. We adduce the following evidence: (1) agreement between monosynaptically coupled inputs and cells' excitatory 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 horn; (5) calculations of peripheral innervation densities and dorsal horn map scales; and (6) constant divergence and convergence between PAs and DHCs.