The cerebral cortex represents the GREY MATTER at the outer surface of the CEREBRAL HEMISPHERES. Evolutionarily, it is the most recent development of the CENTRAL NERVOUS SYSTEM. The human cerebral cortex contains billions of NEURONS and represents the seat of higher psychological functions. As a result of rapid increase in size, the cerebral cortex in higher mammals shows extensive foldings called convolutions. Unfolded, the human cerebral cortex measures about 1600–2500 cm2 in area. The crest of a fold is called a GYRUS, while a groove that separates gyri is called a SULCUS. Deep sulci are often called FISSURES. The longitudinal fissure separates the two cerebral hemispheres at the midline. Although there are individual differences in detail, certain gyri and sulci are more constant than others, and are used as landmarks. Beneath the cerebral cortex is the WHITE MATTER which consists of descending and ascending fibres, association fibres interconnecting different cortical regions, and commissural fibres connecting the two sides of the hemisphere. The commissural fibres form the CORPUS CALLOSUM.
The cerebral cortex can be divided into four lobes that are named after the bones that cover them: frontal, parietal, occipital and temporal lobes. The central sulcus separates the FRONTAL LOBE from the PARIETAL LOBE. Two important gyri lie anterior and posterior to the central sulcus: the pre-central gyrus, containing the primary motor area, and the post-central gyrus, representing the primary somatosensory area. The parieto-occipital sulcus separates the OCCIPITAL LOBE from the parietal lobe. The TEMPORAL LOBE is located laterally and mostly inferior to the other three lobes, and is separated from the frontal lobe by the lateral fissure. A fifth part of the cortex is the insula (see INSULA CORTEX), which lies deep to the lateral fissure.
The cellular arrangement or CYTOARCHITECTURE of the cerebral cortex has been studied by inspecting sections stained for NISSL BODIES and fibres. Most (90%) regions of the cortex have six layers, and as such, represent the ISOCORTEX or NEOCORTEX. The six layers, numbered from 1 to 6 from the surface, have distinct connections. Layer 4 is the input region and is highly developed in the primary sensory areas (see below), whereas layer 5 is the output region that sends information to subcortical structures, and is well developed in the primary motor area. Layers 2 and 3 provide associative connections. In addition to this layering, the neocortex exhibits a vertical radial arrangement of the cells across the thickness of the cortex which is distinct in all the four lobes except for the frontal lobe. In the primary visual area, these columns represent OCULAR DOMINANCE and orientation columns. In contrast to the isocortex, the thickness and density of individual layers in the ALLOCORTEX are so variable that the typical six-layer arrangement is practically obscured. The allocortex includes the archicortex (HIPPOCAMPAL FORMATION and DENTATE GYRUS), and the paleocortex (PYRIFORM CORTEX), which are all part of the LIMBIC SYSTEM. All of these cytoarchitectonic features are used to map the entire cerebral cortex, and the chart of 52 areas described by Brodmann for the human brain is widely used as a reference (see BRODMANN’S AREAS).
The cerebral cortex can be divided into sensory, motor and association areas. Generally, sensory areas are located in posterior regions of the cortex whereas the motor areas occupy the anterior part. The primary somato-sensory area is located in the post-central gyrus (Brodmann’s areas 1, 2 and 3); the primary visual area (area 17) in the medial surface of the occipital lobe; the primary auditory area (areas 41 and 42) in the superior part of the temporal lobe; the primary gustatory area (area 43) at the base of the postcentral gyrus (see GUSTATION); and the primary olfactory area in the medial aspect of the temporal lobe. The primary motor area (area 4) is located in the precentral gyrus. Adjacent to the base of the precentral gyrus is the motor speech area (area 44), also called BROCA’S AREA. This area is concerned with the production of speech and is usually located on the left side only. Lesions in Broca’s area result in APHASIA.
The association areas occupy the largest part of the cortex in the human, and are concerned with higher functions including perception and voluntary motor control as well as cognitive and other intellectual functions. Not all of these cortical functions are bilaterally represented, and LATERALIZATION of functions occurs. For example, language functions are represented mostly in the left hemisphere (in right-handed people), and musical and artistic awareness and spatial and pattern perception are represented predominantly in the right hemisphere. The brain imaging techniques that allow metabolic mapping of the human brain during different mental activities have been rapidly advancing the understanding of neuroanatomical localization of psychological functions in the human brain.
