The cerebral cortex is the thin, outermost layer of the brain of any animal with a cerebrum. It is approximately two to three millimeters thick and includes only the outer layer of the brain proper. In humans, it is where approximately 19 percent of all neurons’ cell bodies exist in the brain.
The cerebral cortex is also known as “gray matter.” This is based on how it looks to the naked eye, as the cell bodies are not myelinated. Thus the cerebral cortex does not include the underlying “white matter” through which the neurons’ axons project. The cerebral cortex is the most easily visible part of the brain and has a highly characteristic folded appearance, particularly in humans. Its folded nature accommodates a tremendous amount of surface area. In fact, the human cerebral cortex contains three times the amount of surface area compared to that of the chimpanzee, the nearest primate relative of humans. If unfolded completely, this part of the brain would cover three sheets of notebook paper.
The cerebral cortex appears to have discrete, functional units. Several research experiments have identified distinct regions of the cerebral cortex that handle their own discrete tasks, such as visual processing, understanding verbal communication, and so on. In some cases, however, if one of these areas becomes damaged, studies have shown that other regions of the cortex may be able to learn new tasks and help the person compensate for the loss of that ability. This action indicates that the cerebral cortex might be somewhat modular and plastic, meaning it has the ability to adapt.
In the human brain, most of the cell-to-cell communication that exists in the cerebral cortex occurs between different regions of the cortex itself, but there is also communication with other structures buried deep underneath, including those that regulate emotion. This may help to explain why emotions can impinge strongly on thoughts and actions.
Histology and Function
The cerebral cortex is made up of three regions: archicortex—meaning the “ancient outer layer,” paleocortex—which translates to the “old outer layer,” and lastly, neocortex—which is the “new outer layer.” These layers are named according to their phylogeny. Specifically, the archicortex is the most phylogenetically conserved across all animals with a cerebrum while the neocortex is the most recently developed portion of the cerebral cortex and is only conserved through mammals. The paleocortex shows intermediate conservation between the other two cortical regions.
The cellular organization is different within each region. The archicortex, which forms the hippocampal formation, has three layers, and the paleocortex has a variable number of cell layers. The neocortex is a six-layered structure with each layer being numbered with the corresponding Roman numeral: I—VI. As the neocortex is common in all mammals, this entry will focus on its distinct characteristics.
The neocortex’s outermost layer is the “molecular layer” and is also called layer I. Alternating layers of neurons that have either a granular or pyramidal shape follow the molecular layer, thus these layers are called granular and pyramidal layers. Layer VI is polymorphic, meaning it consists of several cell types and is the deepest layer. In mammalian brain development, these layers develop in an “inside out” migration, meaning higher numbered layers are formed first, and then lower numbered layers migrate past them. For example, layer V forms, then layer IV forms and moves past layer V into position, followed by the formation of layer III that moves past both of these layers into position. The exception is layer I, which forms first and demarcates the boundary of the cortex. Within layers II—V, the granular cells act as the principal interneurons, and their projections do not leave the local cortex. The pyramidal cells act as the primary output neurons, and typically do send projections out of the cortex. These projections frequently contact other cortical areas of the brain or subcortical areas. Sometimes, the inputs and outputs are fairly consistent by layer. For example, the afferent inputs into layer IV frequently come from thalamic nuclei, while efferent output from layer III is predominantly corticocortical fibers, meaning fibers from one part of the cortex to another. Although this layered architecture is the general layout for the neocortex, it is not always clearly visible. The motor cortex, for example, is so dominated by large pyramidal cells that individual layers are difficult to identify.
The neocortex is profoundly enlarged in humans compared to all nearby species. In human fetal development, the neopallium gives rise to the neocortex, and this is initially a smooth structure. As development continues, however, the neopallium becomes the largest and fastest growing cortical area, eventually covering more than 90 percent of the total cortical area. By six months in development, the cortex begins to invaginate to accommodate the profound cortical growth and to increase its surface area. These folds are called gyri while the grooves are named sulci.
The cerebral cortex can be further subdivided into regions based on function or by cellular organization. In 1909, the German neurologist Korbinian Brodmann (1868—1918) established a map of the human brain based on cytoarchitectonics, which is defining how cells are organized in a tissue. Since then, this map has been closely correlated to functional areas of the brain. This means that experiments investigating regional function, such as brain imaging, have found close overlap of Brodmann areas with discrete, functional units. For instance, Brodmann area 17 has axons from the retina ending in the occipital cortex, which is considered the visual cortex. From brain imaging experiments, Brodmann area 17 does appear to be the primary visual cortex, although these are approximations because it is currently not possible to obtain cellular-level resolution with current imaging techniques.
See also: Brain Anatomy; Entorhinal Cortex; Occipital Lobe; Parietal Lobe; Somatosensory Cortex; Somatosensory System
Creutzfeldt, O. D. (1995). Cortex cerebri: Performance, structural and functional organisation of the cortex. Oxford Scholarship Online. Retrieved from http://www.oxfordscholarship.com/view/10.1093/acprof:oso/9780198523246.001.0001/acprof-9780198523246
Herculano-Houzel, Suzana. (2009). The human brain in numbers: A linearly scaled-up primate brain. Frontiers in Human Neuroscience, 3(31), 1—11.