Hemispheric shape and the formation of gyri

The spherical shape of the early fetal hemisphere is transformed into the adult shape by differing rates of growth in the various regions of the telencephalon ( Fig 1).

The future insular lobe grows less than other telencephalic regions, so that by the eighth month it is covered by the frontal, parietal, and temporal lobes. In the adult brain the insula is completely buried in the depth of the lateral fissure.

The extensive growth of the parieto-occipito-temporal association cortex leads to a bend in the temporal lobe around the lateral fissure. At the same time the temporal pole is pushed rostrally. This direction of growth (Fig 1(d)) also affects the structures situated dorsomedially, i.e. the archicortex with the hippocampus, the corpus striatum, and the lateral ventricles. The corpus striatum is split by the ingrowing fibres of the internal capsule into the caudate nucleus and the putamen. The head of the caudate is situated ventrolaterally to the corpus callosum in the frontal lobe, and the tail of the caudate is located in the temporal lobe dorsal to the inferior horn of the lateral ventricle. The hippocampus forms its largest extension (the retrocommisssural part) in the temporal lobe, bends around the posterior end (splenium) of the corpus callosum, and reaches a position on top of the corpus callosum (the supracommissural part). The precommissural part of the hippocampus ends in front of the genu of the corpus callosum.

After the appearance of the lateral fissure, the neocortical surface develops many sulci and gyri. The central, collateral, cingulate, parieto-occipital, superior temporal, and calcarine sulci appear between weeks 16 and 21, followed by the pre- and postcentral, frontal, temporal, and intraparietal sulci. Highly variable secondary and tertiary sulci develop between week 29 and birth, when all sulci have been formed. (13,14)

The reasons for the formation of gyri in many mammalian brains, including the human brain, are not completely understood. Since the basic organization of the cerebral cortex is vertically oriented, with cell columns positioned side by side, growth of the cortex inevitably leads to a considerable enlargement of the cortical surface. A large unfolded cortical surface would have two disadvantages: the volume of the skull would increase to such a degree during fetal development that a normal delivery would be impossible; the distance between cortical regions interconnected by intrahemispheric projection fibres would increase and with it the information transmission time. Gyri allow the maximal cortical surface in the minimal volume, and they increase the speed of neural transmission between neighbouring cortical areas. Recent measurements show that gyrification is greatest in the association cortices. (14> Although all gyri and sulci are present at birth, the depth of the sulci increases until eventually two-thirds of the cortical surface is hidden in them. (1.3)

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Break Free From Passive Aggression

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