A few millimetres of folded grey run the mind
The cerebral cortex—also called the cerebral mantle—is the cerebrum’s outer neural sheet and the largest integration site in the mammalian CNS. Attention, perception, language, and memory lean on it. In humans it packs some fourteen to sixteen billion neurons into layers only a few millimetres thick.
About ninety percent is six-layered neocortex; allocortex fills the rest. A longitudinal fissure splits left and right hemispheres, bridged beneath by the corpus callosum and other commissures. Ridges (gyri) and grooves (sulci) arise through fetal and postnatal gyrification, hiding much of the surface in folds so that what looks like a wrinkled walnut is mostly buried wall.
Neurons stack into horizontal layers and radial columns or minicolumns. Specialised patches handle motion in motor cortex or sight in visual cortex. Thickness runs roughly two to about three or four millimetres and accounts for around forty percent of brain mass—an outer film that nonetheless dominates computational real estate.
About two thirds of the surface lies hidden in sulci, and the insular cortex is tucked away entirely; unfolded, each hemisphere's cortex would cover about 0.12 square metres. The biggest grooves mark out the frontal, parietal, temporal and occipital lobes, with insular and limbic lobes often added. Across mammals, bigger brains tend to carry thicker cortex: shrews manage about 0.5 millimetres, while humans and fin whales reach 2 to 4. In people, sensory cortex is generally thinner than motor cortex, and one study linked thickness modestly to intelligence; a claim that migraine sufferers have a thicker somatosensory cortex was not borne out by a later, larger study. The neocortex's six layers run from I near the pia mater to VI beside the white matter, and in the visual cortex a pale band called the line of Gennari, visible to the naked eye, carries thalamic input into layer IV. Korbinian Brodmann's 1909 work established this consistent six-layer plan across mammals.
Source: Cerebral cortex
