Definition: the gray cellular mantle (1–4 mm thick) covering the entire surface of the cerebral hemisphere of mammals; characterized by a laminar organization of cellular and fibrous components such that its nerve cells are stacked in defined layers varying in number from one, as in the archicortex of the hippocampus, to five or six in the larger neocortex; the outermost (molecular or plexiform) layer contains very few cell bodies and is composed largely of the distal ramifications of the long apical dendrites issued perpendicularly to the surface by pyramidal and fusiform cells in deeper layers. From the surface inward, the layers as classified in K. Brodmann's parcellation are: 1) molecular layer [TA]; 2) external granular layer [TA]; 3) external pyramidal layer [TA]; 4) internal granular layer [TA]; 5) internal pyramidal layer [TA]; and 6) multiform layer [TA], many of which are fusiform. This multilaminate organization is typical of the neocortex (homotypic cortex; isocortex [TA] in O. Vogt terminology), which in humans covers the largest part by far of the cerebral hemisphere. The more primordial heterotypic cortex or allocortex (Vogt) has fewer cell layers. A form of cortex intermediate between isocortex and allocortex, called juxtallocortex (Vogt), covers the ventral part of the cingulate gyrus and the entorhinal area of the parahippocampal gyrus. On the basis of local differences in the arrangement of nerve cells (cytoarchitecture), Brodmann outlined 47 areas in the cerebral cortex that, in functional terms, can be classified into three categories: motor cortex (areas 4 and 6), characterized by a poorly developed internal granular layer (agranular cortex) and prominent pyramidal cell layers; sensory cortex, characterized by a prominent internal granular layer (granular cortex or koniocortex) and comprising the somatic sensory cortex (areas 1–3), the auditory cortex (areas 41 and 42), and the visual cortex (areas 17–19); and association cortex, the vast remaining expanses of the cerebral cortex.
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