The wrinkled grey rind of the brain, only a few millimetres thick
Cerebral cortex — 2–4 mm thick.
Anatomy · Brain & nerves
Cerebral cortex
Latin cortex, 'bark'
Brain · outer grey matter
The thin outer layer of grey matter covering the surface of the cerebrum, only 2–4 mm thick but folded into ridges and grooves so that a large sheet fits within the skull. It is where conscious perception, voluntary movement, language, planning and thought arise. Most of it is neocortex arranged in six layers; its neurons number roughly 16 billion.
How to spot them
- ▸The wrinkled grey rind of the brain, thin in section, with pale white matter beneath
- ▸Folded into ridges (gyri) and grooves (sulci): about two-thirds of it is hidden in the grooves
- ▸In microscopic section, six horizontal layers of cells, numbered I to VI from the surface inwards
- ▸Divided into functional territories by the Brodmann map, drawn from cell layout
What to know
Its name is Latin for 'bark' or 'rind'
Cerebral cortex — from cortex, bark.
Most of it consists of six horizontal layers of neurons, numbered from the surface inwards
Cerebral cortex — the neocortex (isocortex) is six-layered.
A German neurologist in 1909 mapped it into 52 areas by the arrangement of cells, still used today
Cerebral cortex — Korbinian Brodmann's areas.
A Canadian neurosurgeon stimulated it in conscious patients and drew a distorted 'little man' spread across its surface
Cerebral cortex — Wilder Penfield's homunculus of the motor and sensory strips.
About two-thirds of it lies buried in grooves; flattened out, it would cover roughly 2,000 to 2,500 square centimetres
Cerebral cortex — gyrification packs a large area into a small skull.
Its grey colour comes from the cell bodies of some 16 billion neurons, while the pale layer beneath is made of myelinated fibres
Cerebral cortex — grey matter over white matter.
In rats and mice it is smooth; in humans, whales and dolphins it is deeply folded
Cerebral cortex — lissencephalic versus gyrencephalic brains.
Key ideas
- Folding is a way of packing area into volume: cortical thickness varies far less between species than surface area does.
- Neurons in the same vertical column tend to respond to the same feature, so the cortex is organised in both layers and columns.
Quick-fire facts
- •Only 2–4 mm thick, but roughly 2,000–2,500 cm² in area when flattened, about a quarter of a square metre
- •Contains about 16 billion neurons out of the brain's roughly 86 billion
- •About 90 per cent is neocortex, with six layers; older archicortex and paleocortex, such as the hippocampus and olfactory cortex, have three to five
- •Korbinian Brodmann published his map of 52 cytoarchitectural areas in 1909; area 4 is the primary motor cortex, areas 3, 1 and 2 the primary somatosensory cortex
- •Wilder Penfield's homunculus, mapped in the 1930s–1950s at the Montreal Neurological Institute, shows the disproportionate space given to the hands and face
- •Gyri are the ridges and sulci the grooves; the deepest grooves are called fissures
- •Grey matter refers to the layer of cell bodies; white matter to the myelinated axons beneath
- •The corpus callosum links the cortices of the two hemispheres
- •Cortical thickness declines with age and in Alzheimer's disease
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