The cerebral cortex is the outer surface of the cerebral hemispheres. It is the highest level of the brain and has about 20 billion neurons in the human brain which carry out the highest levels of mental functioning. The cerebral cortex is a layer of grey matter up to about 1/2 cm thick. Figure 3-2d shows a cross section of the cerebral hemispheres with the cortex shown as the pink layer on their outer surface. Figure 4-2d shows a highly magnified slice through the cortex. It is stained with a dye that colors (only) the cell bodies of neurons and glia blue. This shows the densely packed neurons form six layers, each with different functions and connections.
Figure 3-2d. Cross-section of cerebral hemispheres, just in front of the brain
stem. The pink outer layer is the cerebral cortex. The pink areas inside the hemispheres are areas of
grey matter called nuclei. The white matter between these nuclei and the cortex contains the axons of
neurons connecting the neurons in these different areas.
Figure 4-2d. A small piece of cortex showing the many neurons (light blue
shapes) packed into it. The cortex here is about 5 mm thick. The six layers marked at the right refer to
six layers of neurons that have different anatomical and functional properties.
To pack more grey matter into the limited space inside the skull, animals with big brains, especially humans, the cerebral cortex is wrinkled. The grooves that make these wrinkles are called sulci and the ridges between them are called gyri. The human cerebral cortex is very deeply wrinkled, so it has a lot of grey matter, which has the large majority of all the neurons in the CNS. In Figure 5-2d, sulci (singular = sulcus) are shown by the curving lines on the cerebral hemispheres. The ridges between the sulci are the gyri (singular = gyrus). Figure 6-2d shows the "typical" pattern of gyri and sulci, though the pattern varies among individual brains.
Figure 5-2d. Left cerebral
hemisphere, showing the sulci (black lines) with the gyri between them and the major primary
areas