The cerebral cortex is the outermost layer of the human brain. The somatosensory and somatomotor cortexes are two adjoining narrow bands of the topmost central area of the brain, stretching roughly from ear to ear. The motor cortex is partly responsible for the body’s voluntary muscle movements. The function of the somatosensory cortex is to receive and interpret most of the human sense of touch.
Skin is the largest organ of the human body. Among its main functions are to interact with the physical environment by sensing temperature, pressure and touch. Millions of nerve cells, called receptors, are imbedded in the skin to record this information. Some of these cells have direct connections to the brain, while others may converge with the body’s central bundle of nerves, called the spinal cord. Together, all of these parts comprise what is commonly referred as the somatosensory system.
The somatosensory cortex of the brain has several defined areas that are also sometimes numbered as “Brodmann areas,” named for a German neurologist’s early attempts to map the cerebral cortex. One area is called the primary somatosensory cortex. It is integrally connected to another part of the brain called the thalamus, which is believed to coordinate the sense of touch and sense of space with muscle movement.
There is also an area called the secondary somatosensory cortex, which extends deeper into the brain along the surface of its fissures, or crevasse-like folds. The function of this area is to receive and react to the sensation of pain, and also to keep the brain engaged if continuous attention to touch is deemed necessary. Other functions here include the evaluation of size, shape and texture of touch or pressure.
One of the most studied and well understood areas of the somatosensory cortex is the postcentral gyrus. It is, essentially, a one-to-one mapping of the human skin, the entire surface area of the body. The mapping was achieved by simply stimulating different locations of the cortex, and noting where a patient subjectively “felt” the sensation of touch. This somatic mapping of the brain is called a homonculus.
The main function of the somatosensory cortex is to simply identify the location of stimulated skin. It receives the signals from several different types of nerve cells. They include mechanoreceptors that detect pressure, thermoreceptors that register temperature, and nociceptors that signal physical trauma or pain.