Cerebral cortex

The cerebral cortex, also known as the cerebral mantle, is the outer layer of neural tissue of the cerebrum of the brain in humans and other mammals. It is the largest site of neural integration in the central nervous system, and plays a key role in attention, perception, awareness, thought, memory, language, and consciousness.

The six-layered neocortex makes up approximately 90% of the cortex, with the allocortex making up the remainder. The cortex is divided into left and right parts by the longitudinal fissure, which separates the two cerebral hemispheres that are joined beneath the cortex by the corpus callosum and other commissural fibers. In most mammals, apart from small mammals that have small brains, the cerebral cortex is folded, providing a greater surface area in the confined volume of the cranium. Apart from minimising brain and cranial volume, cortical folding is crucial for the brain circuitry and its functional organisation. In mammals with small brains, there is no folding and the cortex is smooth.

A fold or ridge in the cortex is termed a gyrus (plural gyri) and a groove is termed a sulcus (plural sulci). These surface convolutions appear during fetal development and continue to mature after birth through the process of gyrification. In the human brain, the majority of the cerebral cortex is not visible from the outside, but buried in the sulci. The major sulci and gyri mark the divisions of the cerebrum into the lobes of the brain. The four major lobes are the frontal, parietal, occipital and temporal lobes. Other lobes are the limbic lobe, and the insular cortex often referred to as the insular lobe.

There are between 14 and 16 billion neurons in the human cerebral cortex. These are organised into horizontal cortical layers, and radially into cortical columns and minicolumns. Cortical areas have specific functions such as movement in the motor cortex, and sight in the visual cortex. The motor cortex is primarily located in the precentral gyrus, and the visual cortex is located in the occipital lobe.

The cerebral cortex is the outer covering of the surfaces of the cerebral hemispheres and is folded into peaks called gyri, and grooves called sulci. In the human brain, it is between 2 and 3-4 mm. thick, and makes up 40% of the brain's mass. 90% of the cerebral cortex is the six-layered neocortex whilst the other 10% is made up of the three/four-layered allocortex. There are between 14 and 16 billion neurons in the cortex. These cortical neurons are organized radially in cortical columns, and minicolumns, in the horizontally organized layers of the cortex.

The neocortex is separable into different regions of cortex known in the plural as cortices, and include the motor cortex and visual cortex. About two thirds of the cortical surface is buried in the sulci and the insular cortex is completely hidden. The cortex is thickest over the top of a gyrus and thinnest at the bottom of a sulcus.

The cerebral cortex is folded in a way that allows a large surface area of neural tissue to fit within the confines of the neurocranium. When unfolded in the human, each hemispheric cortex has a total surface area of about 0.12 square metres (1.3 sq ft). The folding is inward away from the surface of the brain, and is also present on the medial surface of each hemisphere within the longitudinal fissure. Most mammals have a cerebral cortex that is convoluted with the peaks known as gyri and the troughs or grooves known as sulci. Some small mammals including some small rodents have smooth cerebral surfaces without gyrification.

The larger sulci and gyri mark the divisions of the cortex of the cerebrum into the lobes of the brain. There are four main lobes: the frontal lobe, parietal lobe, temporal lobe, and occipital lobe. The insular cortex is often included as the insular lobe. The limbic lobe is a rim of cortex on the medial side of each hemisphere and is also often included. There are also three lobules of the brain described: the paracentral lobule, the superior parietal lobule, and the inferior parietal lobule.