The cerebral cortex is composed of an enormous number of neurons (1010). These neurons in the human are responsible for the high-order cognitive processing or the conscious mind. It is in the cerebral cortex that the sensory signals generated in the body terminate. These sensory inputs are then processed at several different levels and the integrated information is used in the generation of specific actions. In order to accomplish this highest order of processing, the cerebral cortex is composed of a relatively thin layer

Encyclopedia of the Human Brain Volume 1

Copyright 2002, Elsevier Science (USA).

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of cells and fibers, from 2 to 4 mm thick, that have multiple local interconnections. To receive information regarding the external and internal milieu and to generate commands to control the muscles and organs, the cerebral cortex has both direct and indirect connections with all other regions of the brain and spinal cord. The surface area of the cortex is extensive (approximately 250,000 mm2) due to the numerous folds in the cortical sheath and it accounts for approximately 40-50% of the mass of the brain. The folds permit the cerebral cortex to contain a large number of neurons and to develop many specialized regions that represent various parts of the body.

B. Cortical Layers

Although there are regional variations, the cerebral cortex is generally organized into six distinct layers based on cell types and the organization of the myelinated axon fibers when examined in stained sections (Fig. 1). The outermost layer, known as the molecular layer, is a fiber- and synapse-rich layer with few cell bodies. The cells in layer 1 are primarily neuroglial or nonpyramidal neurons. This layer contains many axons that run parallel to the surface of the cerebral cortex. Layer 2 is known as the external granular layer and contains primarily small pyramidal neurons with their apical dendrites directed toward the

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