title: Competitive interactions between retinal ganglion axons for tectal targets explain plasticity of retinotectal projection in the servomechanism model of retinotectal mapping.
abstract: The mechanism of topographic mapping of retinal ganglion cells to the midbrain was previously elucidated by the servomechanism model, which is based on the fact that cells expressing Eph-receptors respond specifically to surface expressing membrane-bound ephrin-ligands at a critical level. The retina has increased nasal-to-temporal gradient of Eph receptor-density, and the optic tectum/superior colliculus has increased rostral-to-caudal gradient of membrane-bound ephrin-ligand. An axon from the retina has an identification tag of a certain level of Eph-receptor density depending on its retinal position, and adheres to the site on the tectum/superior colliculus expressing ephrin-ligands at a critical ligand-density level. The servomechanism model rigidly defines positions of axon terminals on the midbrain. However, optic nerve regeneration experiments combined with halved retina or tectum show a plastic or flexible mapping (expansion, compression and transposition of tectal projections). To reconcile the discrepancy between the rigid model and the plastic behavior, competition between retinal axon terminals for a target site was introduced to the servomechanism. The servomechanism/competition model succeeded in computer simulations of the plastic mapping of retinal axons on the tectum. Recent experiments of upregulated ligand-density on the tectum during nerve regeneration and the role of axonal competition are discussed.
authors.affiliation: Hyogo University, Kakogawa, Hyogo 675-0195, Japan. hihonda@hyogo-dai.ac.jp
authors.forename: Hisao
authors.initials: H
authors.lastname: Honda
mesh_terms: D000465:Algorithms; D000818:Animals; D003198:Computer Simulation; D051379:Mice; D009473:Neuronal Plasticity; D012165:Retinal Ganglion Cells / Q000502:physiology*