The subplate, a transient neocortical structure: its role in the development of connections between thalamus and cortex.

The functioning of the mammalian brain depends upon the precision and accuracy of its neural connections, and nowhere is this requirement more evident than in the neocortex of the cerebral hemispheres. The neocortex is a structure that is divided both radially, from the pial surface to the white matter into six cell layers , and tangentially into more than 40 different cytoarchitectu­ ral areas (Brodmann 1909). For instance , within the cerebral hemispheres , sets of tangential axonal connections link neurons within a given cortical layer to each other and also link neurons of different cortical areas; sets of radial connections link neurons of different layers together. In addition, the major input to the neocortex arises from neurons in the thalamus , which in tum receive a reciprocal set of connections from the cortex. These connections are highly restricted: In the radial domain, thalamic axons make their major projection to the neurons of cortical layer 4, and the neurons of cortical layer 6 project back