The neocortex is approximately 1-4 mm thick and is often described as consisting of six layers. The layers were initially distinguished on the basis of their appearance in Nissl stained sections, which is primarily sensitive to the density and size of neural cell bodies. Layer I, the "plexiform" layer (closest to the pial surface) has few cell bodies, and consists almost entirely of axonal and dendritic ramifications. Layer IV consists predominantly of densely packed spiny stellate cells (also known as "granular" cells), with an accompanying absence of pyramidal cells. This layer is most evident in the primary sensory areas of the cortex (where it may have several distinguishable sublaminae) while it is nearly absent in motor areas. Layers II and III contain many pyramidal cells. Layer II is distinguished from layer III by its slightly smaller cell bodies; however, Braitenberg argues that this is essentially a continuum, with the smaller size of the layer II pyramids possibly accounted for by the smaller size of the dendritic arborization required to reach layer I. Similarly, layers V and VI contain pyramidal cells which tend to be larger in layer V, while the cell bodies of layer VI may be more irregularly shaped than those of layer V; but functionally layers V and VI may essentially represent a continuum.
From http://home.iprimus.com.au/
