The Limbic Zone Corticoid and Allocortical Formations

The basal forebrain is usually considered a subcortical structure. However, some of its constituents are

Encyclopedia of the Human Brain Volume 1

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Figure 1 Lateral (top) and medial (bottom) views of the cerebral hemispheres. The numbers refer to the Brodmann cytoarchitectonic designations. Area 17 corresponds to primary visual cortex, 41 and 42 to primary auditory cortex, 1-3 to primary somatosensory cortex, and 4 to primary motor cortex. The rest of the cerebral cortex contains association areas. AG, angular gyrus; B, Broca's area; CC, corpus callosum; CG, cingulate cortex; DLPFC, dorsolateral prefrontal cortex; FEF, frontal eye fields (premotor cortex); FG, fusiform gyrus; IPL, inferior parietal lobule; ITG, inferior temporal gyrus; LG, lingual gyrus; MPFC, medial prefrontal cortex; MTG, middle temporal gyrus; OFC, orbitofrontal cortex; PHG, parahippocampal gyrus; PPC, posterior parietal cortex; PSC, peristriate cortex; SC, striate cortex; SMG, supramarginal gyrus; SPL, superior parietal lobule; STG, superior temporal gyrus; STS, superior temporal sulcus; TP, temporopolar cortex; W, Wernicke's area. The insula is hidden from view.

Figure 1 Lateral (top) and medial (bottom) views of the cerebral hemispheres. The numbers refer to the Brodmann cytoarchitectonic designations. Area 17 corresponds to primary visual cortex, 41 and 42 to primary auditory cortex, 1-3 to primary somatosensory cortex, and 4 to primary motor cortex. The rest of the cerebral cortex contains association areas. AG, angular gyrus; B, Broca's area; CC, corpus callosum; CG, cingulate cortex; DLPFC, dorsolateral prefrontal cortex; FEF, frontal eye fields (premotor cortex); FG, fusiform gyrus; IPL, inferior parietal lobule; ITG, inferior temporal gyrus; LG, lingual gyrus; MPFC, medial prefrontal cortex; MTG, middle temporal gyrus; OFC, orbitofrontal cortex; PHG, parahippocampal gyrus; PPC, posterior parietal cortex; PSC, peristriate cortex; SC, striate cortex; SMG, supramarginal gyrus; SPL, superior parietal lobule; STG, superior temporal gyrus; STS, superior temporal sulcus; TP, temporopolar cortex; W, Wernicke's area. The insula is hidden from view.

situated directly on the ventral and medial surfaces of the cerebral hemispheres and are therefore part of the cerebral cortex. These basal forebrain structures include the septal region, the substantia innominata, the amygdaloid complex, and the anterior olfactory nucleus. Because of their simplified cytoarchitecture, these structures can be designated "corticoid," or cortex-like. In some corticoid areas, such as the septal region and the substantia innominata, the organization of neurons is so rudimentary that no consistent lamination can be discerned and the orientation of dendrites is haphazard. All corticoid areas have architectonic features that are in part cortical and in part nuclear. This duality is particularly conspicuous in the amygdala.

The next stage of cortical organization carries the designation of allocortex. This type of cortex contains one or two bands of neurons arranged into moderately well-differentiated layers. The apical dendrites of the constituent neurons are well developed and display orderly patterns of orientation. There are two allocor-tical formations in the mammalian brain: (i) the hippocampal complex (i.e., the dentate gyrus, the CA1-4 fields, and the subicular areas), which also carries the designation of archicortex, and (ii) the piriform or primary olfactory cortex which is also known as paleocortex. The corticoid and allocortical formations collectively make up the limbic zone of the cerebral cortex.

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