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The cerebrum, which is the largest area of the human brain, contains the central sulcus, a fold that divides two large regions, the frontal and parietal lobes. On the parietal lobe, just behind this sulcus, is the postcentral gyrus, a large ridge that runs across the top and down the sides of the cerebrum, parallel to the central sulcus. This area of the brain is also called the primary somatosensory area, and it serves the purpose of detecting touch information from the entire body. For this reason, this gyrus can be considered part of the somatic sensory system, along with the spinal cord and the nerve cells spread throughout the body that detect touch.
The postcentral gyrus contains brain cells called neurons that integrate sensory information from distinct parts of the body. Nerve cells from the left half of the body send information to the right half of the brain, and vice versa. Each half also has a homonculus arrangement, where adjacent parts of the body have their sensory information integrated in adjacent parts of the postcentral gyrus in a topographical fashion. When these brain cells are mapped corresponding to the part of the body they represent, the result appears as a representation of the human body. Areas of the body with more nerve cells are served with a greater number of neurons devoted to them in this gyrus.
Three different cell types compose the postcentral gyrus which are referred to as Brodmann areas 3, 1, and 2. The term Brodmann area refers to a different way of mapping the brain by the variations of cells that comprise each region. Different types of touch information is processed by each area, with Brodmann areas 3 and 1 working together to determine texture, and Brodmann areas 3 and 2 determining sizes and shapes of items touched. Each Brodmann area projects to the next area immediately behind it, with the cells of Brodmann area 2 sending signals to the secondary somatosensory cortex, which further integrates the sense of touch with other sensory input.
A variety of mental deficits related to the sense of touch can result from damage to the postcentral gyrus. Damage to the hemisphere processing information from the side of the non-dominant hand can result in a complete neglect of information from that hemisphere. Other types of damage can result in astereognosia, or being unable to recognize objects while touching them.
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