Glial cell functions and activity-dependent plasticity of the mammalian visual cortex

The presence of highly ordered connectivities is one basic characteristic of the central nervous systems and is believed to be a prerequisite for its proper function. The elaboration of precise, topographic projections has been shown to include two subsequent steps: (1) The formation of exuberant projections with limited topographic accuracy, and (2) the activity-dependent refinement of the appropriate connectivities by synapse elimination and synapse stabilization. Although most current theories on the mechanisms underlying activity-dependent developmental plasticity focus on adaptive changes of the efficacy of synaptic transmission, the present overview deals with possible mechanisms underlying morphologic adaptations in the developing central nervous system. Especially the contribution of glial cells in the activity-dependent selection of neuronal projections in the thalamocortical visual system of higher mammals is elaborated and a unifying hypothesis on the involvement of nonneuronal cells in neuronal plasticity is formulated on the basis of the current knowledge on glial physiology and specific experimental data.