Neocortex: Cortical Areas

Cortical Areas

All regions of the neocortex develop in a similar way. First, a wide cell layer, the cortical plate, forms on the surface of the hemisphere which then divides into six layers. Because of this similar development, the neocortex is also known as isogenetic cortex, or briefly, isocortex, or as homogenetic cortex.

Nevertheless, the adult neocortex exhibits considerable regional variations, and we distinguish several regions of different structure, the cortical areas. The individual layers may vary considerably in these areas: wide or narrow, with densely or loosely packed cells. The cells may vary in size, or a specific cell type may predominate. The definition of individual areas according to such criteria is called architectonics. Depend-ing on the staining method employed, the following terms are used: cy-toarchitectonics, myeloarchitectonics, or pig-ment architectonics. A map of cortical areascan be reconstructed on the surface of the hemisphere, similar to a geographic map. The cytoarchtectonic map of cortical areas established by Korbinian Brodmann (A, B) has been confirmed repeatedly and is gen-erally accepted.

Types of cortices.A special feature of pro-jection areas (terminals of ascending sensorypathways) is the prominent development of their granular layers. In the sensory cortex (area 3) as well as in the auditory cortex (areas 41 and 42, temporal transverse gyri), the granular layers (layers II and IV) are wide and rich in cells, while the pyramidal cell layers are less well developed. This type of cortex is referred to as koniocortex, or granular cortex. The visual cortex (area 17,striate area) even exhibits a duplication of layer IV. These sensory cortical areas, which are the terminals of afferent projection fibers, are involved in associative processes, where relay neurons with short axons play an important role. On the other hand, the granular layers in the motor cortex (areas 4 and 6) are largely reduced in favor of the pyramidal layers (agranular cortex). The pyramidal cells are projection neuronswith long axons; for example, in the motor cortex, their axons form the corticospinal tract (pyramidal pathway).

Border zones.Wherever the isocortexborders on the archicortex or paleocortex, its structure becomes simpler. The transi-tional formation with the simpler structure is referred to as proisocortex. The proisocor-tex includes the cortex of the cingulate gyrus, the retrosplenial cortex (lying around the posterior end of the corpus callosum), and parts of the insular cortex. The proisocortexis phylogenetically older than the neocor-tex.

Paleocortex and archicortex are also sur-rounded by a border zone, the structure of which approximates that of the neocortex. The border zones are known as periarchicor-tex and peripaleocortex. The periarchicortexincludes, for example, the entorhinal area bordering on the hippocampus.

Allocortex.The allocortex is often con-trasted with the isocortex. The term refers to the paleocortex and the archicortex. As both are completely different parts of the telencephalon, genetically as well as struc-turally and functionally, this collective term is not justified. The only thing they have in common is that they are different from the isocortex. Their simpler structure (for ex-ample, no stratification into six layers as it is characteristic for the neocortex) does not justify to label them as primitive regions. Rather, they are phylogenetically old, highly specialized structures.