Cortical layers · Lobeworks/17

The cortical layers are the six horizontal sheets, numbered I to VI from the surface inwards, into which the cells of the neocortex are arranged, each with its own mix of cell types and its own inputs and outputs; they are the cortex's radial axis, the way a thickness of two to four millimetres is organised.

Cortical layers. The cortical layers are the six horizontal sheets, numbered I to VI from the surface inwards, into which the cells of the neocortex are arranged, each with its own mix of cell types and its own inputs and outputs; they are the cortex's radial axis, the way a thickness of two to four millimetres is organised.

Layer I, at the surface, holds few cell bodies and many fibres, among them the tufts of dendrites from cells below. Layers II and III hold small pyramidal neurons whose axons run to other cortical areas, the cortex talking to itself. Layer IV is the main entrance: it receives most of the input from the thalamus, which is why it is thick in primary sensory areas (the visual cortex has it subdivided) and almost absent in motor cortex. Layers V and VI send the output out of the cortex: layer V's large pyramidal cells project to the brainstem and spinal cord (the motor cortex's giant Betz cells sit here), layer VI back to the thalamus. Brodmann's 52 areas were drawn by how these layers differ in thickness and density under the microscope.

Layers and lobes are different axes. The four lobes are a map of the surface, drawn by grooves; the six layers run through the depth of any patch of it. Every lobe has the six layers, and a confusion between six layers and six lobes mixes the two maps.

Six layers are a mammalian design. They have been found in all mammals and in no other animals; birds solve comparable problems with a pallium of clustered nuclei instead of layers, and evidence suggests it does equivalent work.

The scale is large. Neocortex makes up about 90 % of the human cerebral cortex and about 76 % of the whole brain's volume, the older three-layered cortex of the hippocampus and smell areas being the rest of the cortex.

Input arrives in the middle, output leaves from the bottom, and the top layers wire areas together.

That one plan, repeated across the whole sheet, is the column the cortex is built from.

Questions: Does the cortex have six layers or four lobes? Both, because they are maps along different axes. The four lobes (frontal, parietal, temporal, occipital) divide the surface of the cortex, separated by grooves. The six layers run through its depth, two to four millimetres from the surface inwards, and every lobe has all six: layer IV takes the input from the thalamus, layers V and VI send the output away, the upper layers link cortical areas.