In the mammalian central nervous system, the somatosensory cortex is crucial to senses such as pain, touch, temperature, and spatial orientation—a sensation termed proprioception. The somatosensory cortex consists of a network of neurons that work together to sense and then to process this information.
The somatosensory cortex is the most anterior portion of the parietal lobe and is demarked as the gyrus that is just posterior to the central sulcus, which separates the parietal lobe from the frontal lobe. The central sulcus is a prominent landmark in the mammalian brain as it is the longest, uninterrupted, “straight” groove on the lateral aspect of the cerebral hemisphere. The postcentral gyrus is the somatosensory cortex with the function of integrating and processing all sensory information from the body’s surface and the underlying viscera. This is where the neuronal cell bodies are found, making the cortex look gray, and why the cerebral cortex is also called “gray matter.” It is also important for the perception of the senses, particularly of touch, pain, and temperature. The somatosensory cortex is often referred to as Brodmann areas 3, 1, and 2. The number order for the somatosensory cortex may seem strange to the reader. This is because when German anatomist Korbinian Brodmann (1868—1918) first sectioned the brain, he did so at an oblique angle. The first area he studied was named Brodmann area 1. As he continued his research, he continued to number the regions based on the order he studied them. Today, when looking at the somatosensory cortex from the midline to lateral direction, the somatosensory cortex is numbered 3, 1, and 2.
Along the surface of the postcentral gyrus, the body is mapped from the midline to the temporal lobe. This means that a specific body region is located in a specific region of the postcentral gyrus. This map is called a homunculus, meaning “little man.” The purpose of the thalamus is to act as the relay center from the spinal cord or brainstem to the homunculus. This is the same as the reticular formation, but the reticular formation acts as a relay for the spinal cord or brainstem to the thalamus. The homunculus is the main processing center of the somatosensory system. It processes the sensory impulses from the body that were sent to the thalamus for modulation, which in turn are sent to the somatosensory cortex. If a reaction is necessary, such as to move away from a heat source, this sensory information in the cerebral cortex will be transferred to the motor cortex to respond.
For the sensory homunculus, the most medial and deep portion of the postcentral gyrus maps to the genitals. Just superior to that are the toes and foot. At the bend of the gyrus, the leg is represented. From the medial superior surface moving laterally the following are mapped: hip, trunk, neck, head, shoulder, arm, elbow, forearm, and wrist. The hand and fingers come next, but these are connected to a much larger area of the sensory cortex than any other previously described body part. This is because humans have significantly large numbers of sensory receptors in their hands and fingers, which helps produce discriminative touch. The specific mapping continues as: hand, little finger, ring finger, middle finger, index finger, and thumb. Now on the lateral surface superior to the temporal lobe, the next body regions are recorded: eye, nose, and face. As with the hands and fingers having a largely mapped region of the sensory cortex, so do the lips. This is because humans have many sensory receptors around the mouth that are mainly used for sensing taste, temperature, and proprioception. Thus, the following are represented: upper lip, lips, and lower lips. Nearing the final downward mapping are the teeth, gums, tongue, and pharynx. The very last body region that is demarcated, where the postcentral gyrus meets the temporal lobe, is the intra-abdominal. It is important to note that the viscera are not mapped to the postcentral gyrus.
Jennifer L. Hellier
See also: Discriminative Touch; Homunculus; Nociception; Sensory Receptors; Somatosensory System; Thalamus; Thermal Sense; Touch
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