The Cerebral Cortex

The brain's wrinkled outer sheet: its folds, its six layers, its lobes and Brodmann areas, and how sensory, motor and association regions divide the work.

The cerebral cortex (Latin for "bark") is the outer layer of the cerebrum: a sheet of grey matter about 2 to 4 millimetres thick, holding roughly 16 billion neurons. In humans it is heavily folded into ridges, the gyri, and grooves, the sulci. The folding packs a large surface into the skull: about two-thirds of the cortex is hidden inside the folds.

Lobes and landmarks

Two deep grooves divide each hemisphere. The central sulcus separates the frontal lobe from the parietal lobe, and the lateral (Sylvian) fissure separates the temporal lobe below. The occipital lobe sits at the back. Folded deep inside the lateral fissure is the insula, sometimes counted as a fifth lobe.

Six layers

Most of the cortex — the neocortex — has six layers of neurons, numbered from the surface inward. Different layers have different jobs: layer 4 typically receives input from the thalamus, layers 2 and 3 connect to other cortical areas, and layers 5 and 6 send output to deeper structures and the spinal cord. Older types of cortex, such as the hippocampus, have fewer layers. The cortex is also organised into vertical columns of neurons with similar preferences.

Maps of the cortex

In 1909 the German anatomist Korbinian Brodmann divided the cortex into about 52 areas based on differences in the layering of cells under the microscope. Many of his numbers are still used: Brodmann area 17 is the primary visual cortex, areas 44 and 45 roughly correspond to Broca's area. In 2016 a project using MRI data from hundreds of people produced a map with 180 areas per hemisphere.

Three kinds of territory

Primary sensory areas receive information from one sense (visual, auditory, somatosensory). Primary motor cortex sends commands to muscles. The rest — most of the human cortex — is association cortex, which combines information across senses, memory and goals: planning, language, recognising objects, imagining and reasoning.