Fig. 1 The lobes of the cerebral cortex.

The parietal lobe (Fig.3)

Behind the central sulcus, the primary somatic sensory cortex (SI), extending onto the medial surface to occupy the posterior part of the paracentral lobule, where the sacral spinal segments are represented, occupies the postcentral gyrus. The postcentral sulcus limits the postcentral gyrus posteriorly. Behind this, the sulcal pattern is variable, but one or more horizontal intraparietal sulci divide the lobe into superior and inferior parts. The second somatic sensory cortex (SII) is located in the parietal operculum, close behind the inferolateral tip of the central sulcus. Specific sulcal patterns in the transition region between the parietal lobe and the occipital and temporal lobes (the supramarginal and angular gyri) are important landmarks for the detailed localization of language functions.

Fig. 3 (a) Areas of the parietal lobe; (b) somatic sensory association pathways. TP, temporal pole; PHG, parahippocampal gyrus; MTG, middle temporal gyrus.

The occipital lobe (Fig.4;)

The occipital lobe is predominantly involved in vision and visual perception. The medial surface is grooved by the deep horizontal calcarine sulcus, which typically reaches the posterior pole of the hemisphere. Within its walls is the primary visual cortex. This area (area 17 of Brodmann) is often called the striate cortex; in the freshly sliced brain, a thin band of white matter, the stria of Gennari, is clearly visible running in the centre of the cortical grey ribbon. The extent of this stria precisely demarcates the primary visual cortex. Surrounding areas of the medial and lateral surface, the prestriate and peristriate cortex, contain some of the numerous separate visual association areas.

Fig. 4 The occipital lobe: (a) visual areas; (b) visual association pathways. TP, temporal pole; IT, inferotemporal cortex; PHG, parahippocampal gyrus; STG, superior temporal gyrus; MTG, middle temporal gyrus; Ant, anterior; Post, posterior.

Two horizontal sulci, the superior and inferior temporal sulci, divide the lateral surface of the temporal lobe into the superior, middle, and inferior gyri. The latter, extending onto the inferior surface, is also known as the inferotemporal cortex. On the medial surface, the collateral sulcus runs from close to the temporal pole to the calcarine sulcus posteriorly. Medial to this is the parahippocampal gyrus. Anteriorly, this curves dorsally and caudally to form the hook-shaped uncus. The entorhinal cortex occupies approximately the anterior third of the parahippocampal gyrus. Lateral to this, in the walls of the rhinal sulcus, lies the perirhinal cortex. The uncus closely overlies the amygdala; the primary olfactory cortex, the piriform cortex, lies immediately in front. The choroid fissure limits the parahippocampal gyrus medially. Passing into the floor of the lateral ventricle, the subicular areas of cortex lead to the hippocampus proper. A detailed consideration of the anatomy of the hippocampus is given below.

Fig. 5 Auditory association connections. TP, temporal pole; PHG, parahippocampal gyrus.

Fig. 6 Olfactory and limbic structures: (a) ventral surface of the brain; (b) medial temporal lobe. 1, Association cortex to parahippocampal gyrus; 2, parahippocampal gyrus to entorhinal cortex; 3, entorhinal cortex to hippocampus (perforant path).

The cortex of the temporal operculum contains Heschl's gyri, within which lies the primary auditory cortex. Diverse auditory association areas surround this, extending into the superior temporal gyrus. The functions of the cortex of the middle temporal gyrus are uncertain, but include complex visual, auditory, and somatic sensory association areas. The inferotemporal cortex is largely concerned with visual perception and cognition. The anatomical pathways that underlie these functions are considered below.

The insula

The anterior margin of the insula, where the cortex becomes continuous with the anterior perforated substance, is known as the limen insulae. Above and below, where the insular cortex rolls round onto the opercula, lies the circular sulcus, the superior and inferior rami of which fuse posterosuperiorly to form the apex of the insula. Several variable sulci mark the insula, but little is known of the functional subdivisions of this cortex; gustatory, somatic sensory, and auditory areas have been described.

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