The structure and organization of the cerebral cortex The lobes of the cerebral cortex

A variable pattern of fissures (sulci) and folds (gyri), many of which have specific names, extensively groove the surface of the cerebral hemisphere. A few are relatively constant and are used to subdivide the cerebral hemisphere into lobes, named for the bones of the skull which they underlie ( Fig 1).

Fig. 1 The lobes of the cerebral cortex.

The deep lateral sulcus, also called the Sylvian fissure, extends from the uncus, anteriorly and medially, to the parietal lobe, posteriorly and medially. It has a short stem, and anterior, ascending, and posterior rami. The anterior and ascending rami embrace the pars triangularis of the frontal lobe, which houses Broca's motor speech area. The much longer posterior ramus is used in defining the lobes of the hemisphere. The central sulcus is prominent approximately midway along the anteroposterior extent of the lateral surface of the hemisphere and, most commonly, extends over the medial margin, where its inferomedial tip is embraced by the U-shaped paracentral lobule. On the lateral surface, it passes from the medial margin, forwards and laterally, to reach the lateral sulcus. The line of the central sulcus closely approximates the line of the coronal suture of the adult skull, i.e. the junction between the frontal and parietal bones; consequently, the sulcus separates the frontal and parietal lobes. The demarcation of the occipital lobe is the parieto-occipital sulcus dorsally and medially, and the pre-occipital sulcus ventrally and laterally, with an imaginary line connecting the two and intersecting the posterior tip of the lateral sulcus. The temporal lobe lies anterior to this line and inferior (ventral) to the lateral sulcus. The deep lateral sulcus broadens out at its fundus, with an area of cortex forming the extensive floor of the sulcus, particularly in its anterior two-thirds. This cortex is the insula, which does not form part of any of the lobes mentioned above. The insula is surrounded by the circular sulcus, and is overhung by the frontal and parietal opercula superiorly, and the temporal operculum inferiorly (ventrally). The anatomical borders of the lobes of the cerebral cortex, and other sulcal and gyral landmarks, are only loosely paralleled by functional boundaries. However, lobar terminology is so firmly embedded in clinical and non-clinical neuroscience that consideration of their anatomical features is essential.

The precentral gyrus, immediately in front of the central sulcus and continuing onto the medial surface, contains the primary motor cortex. The precentral sulcus usually defines the anterior boundary, and in front of this lies the premotor cortex. The inferior margin of the sulcus runs into the pars triangularis, which includes Broca's motor speech area. On the medial margin and surface, the cortex includes the supplementary motor area. The lateral prefrontal cortex, in front of these motor and associated areas, is usually grooved by two major horizontal sulci, defining the superior, middle, and inferior frontal gyri. The cortex on the medial surface of the frontal lobe anterior to the prefrontal gyrus forms the medial prefrontal cortex. The concave inferior surface of the frontal lobe, overlying the bony orbit in vivo, is the orbitofrontal cortex.

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