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New Chapt. 21 Functions of the Cortex(1)

New Chapt. 21 Functions of the Cortex(1) - CHAPTER 21 THE...

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222 CHAPTER 21 THE FUNCTIONS AND STRUCTURE OF THE CEREBRAL CORTEX The cerebral cortex (the layer of cells covering the cerebrum) is a highly folded plate of neural tissue about two millimeters thick. The name cortex derives from the Greek root meaning bark, like the bark on a tree, and like the bark on a tree, the cortex forms a thin "coat" of cells that covers and envelopes the cerebral hemispheres. Of all portions of the brain, the cerebral cortex has undergone the greatest development during phylogeny, reaching enormous size in primates, especially in humans where it dwarfs all other regions of the central nervous system (Fig. 1). It is the highest level of the nervous system, and the part the not only endows us with our sense of being human but even endows each of us with our own sense of identity; it is what makes "you", you and not someone else. It is not an understatement to say that it is the most complex piece of matter in the universe and achieving an understanding of how the cortex functions is one of the most important questions in science. Fig. 1. Drawings showing a representation selection of vertebrate brains, all drawn to the same scale. In vertebrates lower than mammals the cerebrum is small. In carnivores and particularly in primates, it increases dramatically in both size and complexity. s The human cortex is divided into 4 lobes A picture of the human cerebral cortex is shown in Figs. 1 & 2 where the enormous convolutions, the sulci (indentations) and gyri (hills between the indentations) are apparent and present the features that so characterize the human cortex. The cortex is divided into four major lobes: the frontal lobe, the parietal lobe, the occipital lobe and the temporal lobe. The lobes are separated by prominent sulci or deep fissures. Perhaps the most prominent sulcus is the central sulcus which runs down the center of the brain. The central sulcus separates the frontal lobe, in front, from the parietal lobe, which lies just in back of it. The gyrus just in
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223 front of the central sulcus is called the precentral gyrus, while the gyrus just in back of the central sulcus is called the postcentral gyrus. The precentral gyrus is the motor cortex: neurons in the precentral gyrus send their axons down the brain as the corticospinal tract to synapse on motor neurons in the spinal cord. Thus, the motor cortex occupies a small strip of cortex in the frontal lobe. The postcentral gyrus is the somatosensory cortex, the region that receives the projections of the somatosensory portion of the thalamus. The somatosensory cortex then occupies a small strip of cortex in the parietal lobe. The most caudal pole of the cortex is the occipital lobe (it is not distinctly separated from the parietal lobe by a prominent sulcus).
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