What Is the Primary Somatosensory Cortex?

Andrew Kirmayer

The primary somatosensory cortex is a region of the brain where nerve signals from the sense of touch are normally received. It is generally located in a part of the brain called the parietal lobe, in a structure called the post central gyrus. A gyrus is a ridge along the surface of the brain; there are typically several of them on the cerebral cortex, or the outer part of the brain, that increase the surface area. The cortex is also in back of the central sulcus, a groove on the surface of the brain. Nerve stimuli from all parts of the body are received in specific places in the primary somatosensory cortex, while the body is represented by a type of map called a homunculus.

The primary somatosensory cortex is in back of the central sulcus, in the parietal lobe.
The primary somatosensory cortex is in back of the central sulcus, in the parietal lobe.

Body parts are mapped out in this area, but areas that are most sensitive have the most receptors, such as the face and hands. Nerve cells called neurons are more abundant in areas representing sensitive parts of the body. There are usually fewer neurons associated with the torso and legs than the face and hands in this part of the brain.

Nerve impuses from touching are processed by the primary somatosensory cortex.
Nerve impuses from touching are processed by the primary somatosensory cortex.

Three subsections called Brodmann areas generally make up the primary somatosensory cortex. This area also typically contains four sub maps, with columns of nerve cells representing particular areas of the body. These cells can reorganize if a body part is amputated, for example, and the neurons usually then respond when other areas are touched. Nerve input to the somatosensory area generally comes from a structure deeper inside the brain called the thalamus.

During brain surgery, surgeons may stimulate the brain and record the neurological responses.
During brain surgery, surgeons may stimulate the brain and record the neurological responses.

The primary somatosensory cortex isn’t the only area that can receive input from the senses. Another area called the secondary somatosensory cortex typically is where nerve signals travel to from the primary area. Nerve processing here is generally not as accurate because the cells are not as specific to the parts of the body. Information from both sections goes to a different place called the somatosensory association cortex; associations between different senses are often processed here. If this area is damaged, a person can touch something, feel it, but not be able to figure out what it is.

The nerve cells in the primary somatosensory cortex typically reorganize after an amputation.
The nerve cells in the primary somatosensory cortex typically reorganize after an amputation.

Scientists first mapped the primary somatosensory cortex in the 1950s. During brain surgery, surgeons could stimulate parts of the brain and see where a patient felt something. The locations of different body parts, and the proportion to which they are represented, can be seen by directly stimulating the brain and recording the neurological response.

Nerve cells called neurons are more abundant in areas representing sensitive parts of the body.
Nerve cells called neurons are more abundant in areas representing sensitive parts of the body.

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Discussion Comments

indigomoth

@Mor - I wonder if elephants have a more developed somatosensory cortex than humans do, considering how sensitive their feet and their trunk is supposed to be. I've heard that their feet are so sensitive they can basically shape them around objects like stones so that they won't injure themselves.

Although I've noticed that if I do step on something like a pin, my feet shoot away from it before I even register what's going on, which I think is a function of the central nervous system, not the somatosensory cortex.

Mor

@MrsPramm - Well, there's a reason that babies try to put everything in their mouths and it's not because they think it will be something to eat. I think they often swallow the thing by accident, when the real purpose is to explore it with their lips and tongue which, as you say, have a lot of receptors.

Also, I can definitely see why the feet would need a lot of receptors, even if we don't think of them as being particularly sensitive. Even with shoes, it helps to know what you're stepping on and without them it must have been extra important.

MrsPramm

There's a really interesting diagram you can find online if you search for "homunculus" and "touch". A homunculus is when someone makes a picture of what a human would look like if the senses were drawn in proportion to how they are used. So, a homunculus for touch has massive hands and lips, because they have the most touch receptors.

The tongue and the feet are also surprisingly large in these pictures.

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