Cerebral cortex

The cerebral cortex is the outer sheet of the cerebrum, the largest part of the human brain. It is only a few millimeters thick and consists of gray matter: densely packed neuron cell bodies, dendrites and synapses, with little of the myelin that makes deeper brain tissue look white. Its surface is creased into ridges called gyri and grooves called sulci, which let a large area fit inside the skull. A deep cleft, the longitudinal fissure, divides it into left and right hemispheres, linked underneath by the corpus callosum. Most of it, the neocortex, is built of six layers of cells.

Each hemisphere has frontal, parietal, temporal and occipital lobes, and particular tasks are concentrated in particular areas, a principle called localization of function. The primary motor cortex in the frontal lobe issues commands to move, the somatosensory cortex in the parietal lobe receives touch and body position, the temporal lobe handles hearing and the occipital lobe vision. Large association areas combine these streams for language, planning, reasoning and memory, and each hemisphere mainly serves the opposite side of the body.

Damage to one cortical area, often from a stroke, causes a matching loss, such as weakness of the opposite arm or difficulty producing speech when Broca's area is affected.