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synapse with third-order neurons in the thalamus, which in turn project to the postcentral gyrus. Notice that somatesthetic information is always carried to the postcentral gyrus in third-order neurons. Also, because of crossing-over, somatesthetic information from each side of the body is projected to the postcentral gyrus of the contralateral cerebral hemisphere.

Since all somatesthetic information from the same area of the body projects to the same area of the postcentral gyrus, a "map" of the body can be drawn on the postcentral gyrus to represent sensory projection points (see fig. 8.7). This map is distorted, however, because it shows larger areas of cortex devoted to sensation in the face and hands than in other areas in the body. This disproportionately large area of the cortex devoted to the face and hands reflects the fact that the density of sensory receptors is higher in these regions.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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