The cerebral cortex is an approximately 5-mm-thick layer of gray matter covering the entire surface of the cerebrum. While glial cells and mesenchymal cells are naturally present, the cerebral cortex mainly consists of neuronal cell bodies, including gray matter neurons that project axons outside the cortical area and neurons that project axons within the cerebral cortex to transfer information. In addition, the information sent from the projected axons is received by dendrites.
From the phylogenetic perspective, Kappers divided the cerebral cortex into the archicortex, paleocortex, and neocortex. The archicortex consists of the hippocampus and nearby medial portion of the temporal lobe area and is a phylogenetically old area. The paleocortex is also an old cortical area in which olfactory fibers are projected. The remaining cerebral cortex is phylogenetically new and is called the neocortex. The term “phylogenetically new” refers to the fact that it developed in humans or in mammals evolutionarily close to humans.
Most cerebral cortexes consist of six layers, including layer 1 (molecular layer), layer II (outer granular layer), layer III (outer pyramidal layer), layer IV (inner granular layer), layer V (inner pyramidal layer), and layer VI (polymorphic layer) (beginning from the surface). However, the thickness of each layer, number of layers, and neuron density in the layers slightly differ depending on the site in the cerebrum. Based on this, Brodmann divided and coded the mammalian cerebral cortex into 52 areas: area 1 to area 52. Some of these areas do not exist in the human cerebral cortex, which is therefore divided into 47 areas.
However, the paleocortex and archicortex in Kapper’s classification do not have a six-layered structure. The portion that never goes through a six-layered stage is the allocortex, whereas the portion with a six-layered structure is classified as the isocortex. The former corresponds to the paleocortex and archicortex, while the latter corresponds to the neocortex.
The neocortex is divided into the motor area, sensory area, and association area. With regard to the motor and sensory areas, Penfield et al. examined the functional roles performed by different areas of the cerebral cortex on the basis of clinical symptoms on stimulating the cerebral cortex, and their results are easy to understand.
On the other hand, the association area is where sensory and motor information is regulated on a higher level. In other words, it is the portion that controls higher brain functions. The association area consists of the frontal association area (premotor area and prefrontal area), posterior association area (parietal association area, occipital association area, and temporal association area), and paralimbic association area.