Know Your Brain: Primary Somatosensory Cortex
Where is the primary somatosensory cortex?
Primary somatosensory cortex (in blue)
The primary somatosensory cortex is located in a ridge of cortex called the postcentral gyrus, which is found in the parietal lobe. It is situated just posterior to the central sulcus, a prominent fissure that runs down the side of the cerebral cortex. The primary somatosensory cortex consists of Brodmann's areas 3a, 3b, 1, and 2.
What is the primary somatosensory cortex and what does it do?
The primary somatosensory cortex is responsible for processing somatic sensations. These sensations arise from receptors positioned throughout the body that are responsible for detecting touch, proprioception (i.e. the position of the body in space), nociception (i.e. pain), and temperature. When such receptors detect one of these sensations, the information is sent to the thalamus and then to the primary somatosensory cortex.
The primary somatosensory cortex is divided into multiple areas based on the delineations of the German neuroscientist Korbinian Brodmann. Brodmann identified 52 distinct regions of the brain according to differences in cellular composition; these divisions are still widely used today and the regions they form are referred to as Brodmann's areas. Brodmann divided the primary somatosensory cortex into areas 3 (which is subdivided into 3a and 3b), 1, and 2.
The numbers Brodmann assigned to the somatosensory cortex are based on the order in which he examined the postcentral gyrus and thus are not indicative of any ranking of importance. Indeed, area 3 is generally considered the primary area of the somatosensory cortex. Area 3 receives the majority of somatosensory input directly from the thalamus, and the initial processing of this information occurs here. Area 3b specifically is concerned with basic processing of touch sensations, while area 3a responds to information from proprioceptors.
Area 3b is densely connected to areas 1 and 2. Thus, while area 3b acts as a primary area for touch information, that information is then also sent to areas 1 and 2 for more complex processing. Area 1, for example, seems to be important to sensing the texture of an object while area 2 appears to play a role in perceiving size and shape. Area 2 also is involved with proprioception. Specific lesions to any of these areas of the somatosensory cortex support the roles mentioned above; lesions to area 3b, for example, result in widespread deficits in tactile sensations while lesions to area 1 result in deficits in discriminating the texture of objects.
Each of the four areas of the primary somatosensory cortex are arranged such that a particular location in that area receives information from a particular part of the body. This arrangement is referred to as somatotopic, and the full body is represented in this way in each of the four divisions of the somatosensory cortex. Because some areas of the body (e.g. lips, hands) are more sensitive than others, they require more circuitry and cortex to be devoted to processing sensations from them. Thus, the somatotopic maps found in the somatosensory cortex are distorted such that the highly sensitive areas of the body take up a disproportionate amount of space in them (see image to the right).
Reference:
Purves D, Augustine GJ, Fitzpatrick D, Hall WC, Lamantia AS, McNamara JO, White LE. Neuroscience. 4th ed. Sunderland, MA. Sinauer Associates; 2008.
