Somatosensory Cortex – Location, Function & More

Somatosensory Cortex is responsible for processing somatic sensations. These sensations arise from receptors positioned throughout the body that are responsible for detecting touch, proprioception (the position of the body in space), nociception (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.

Somatosensory Cortex/Primary Somatosensory Cortex

Definition

Have you ever wondered if you feel things the same way other people do? How do you know ‘red’ is really the same red to everyone? Maybe the person next to you sees green as red. These thought-provoking questions can’t be answered precisely with science, but we can learn more about how external stimuli, like colors, are processed in the brain. This is where the somatosensory cortex comes in. This part of the brain processes sensations, or external stimuli, from our environment. Before we learn more about the somatosensory cortex, we need to learn a little bit about brain anatomy and where the somatosensory cortex is located.

The primary somatosensory cortex is located in the postcentral gyrus and is part of the somatosensory system. It was initially defined from surface stimulation studies of Wilder Penfield, and parallel surface potential studies of Bard, Woolsey, and Marshall. Although initially defined to be roughly the same as Brodmann areas 3, 1 and 2, more recent work by Kaas has suggested that for homogeny with other sensory fields only area 3 should be referred to as “primary somatosensory cortex”, as it receives the bulk of the thalamocortical projections from the sensory input fields.

At the primary somatosensory cortex, tactile representation is orderly arranged (in an inverted fashion) from the toe (at the top of the cerebral hemisphere) to mouth (at the bottom). However, somebody parts may be controlled by partially overlapping regions of cortex. Each cerebral hemisphere of the primary somatosensory cortex only contains a tactile representation of the opposite (contralateral) side of the body. The amount of primary somatosensory cortex devoted to a body part is not proportional to the absolute size of the body surface, but, instead, to the relative density of cutaneous tactile receptors on that body part. The density of cutaneous tactile receptors on a body part is generally indicative of the degree of sensitivity of tactile stimulation experienced at a said body part. For this reason, the human lips and hands have a larger representation than other body parts.

Structure

Brodmann areas 3, 1, and 2 make up the primary somatosensory cortex of the human brain (or S1). Because Brodmann sliced the brain somewhat obliquely, he encountered area 1 first; however, from anterior to posterior, the Brodmann designations are 3, 1, and 2, respectively.

Brodmann area (BA) 3 is subdivided into areas 3a and 3b. Where BA 1 occupies the apex of the postcentral gyrus, the rostral border of BA 3a is in the nadir of the Central sulcus and is caudally followed by BA 3b, then BA 1, with BA 2 following and ending in the nadir of the postcentral sulcus. BA 3b is now conceived as the primary somatosensory cortex because 1) it receives dense inputs from the NP nucleus of the thalamus; 2) its neurons are highly responsive to somatosensory stimuli, but not other stimuli; 3) lesions here impair somatic sensation; and 4) electrical stimulation evokes the somatic sensory experience. BA 3a also receives dense input from the thalamus; however, this area is concerned with proprioception.

Areas 1 and 2 receive dense inputs from BA 3b. The projection from 3b to 1 primarily relays texture information; the projection to area 2 emphasizes size and shape. Lesions confined to these areas produce predictable dysfunction in texture, size, and shape discrimination.

The somatosensory cortex, like another neocortex, is layered. Like other sensory cortex (i.e., visual and auditory) the thalamic inputs project into layer IV, which in turn projects into other layers. As in other sensory cortices, S1 neurons are grouped together with similar inputs and responses into vertical columns that extend across cortical layers (e.g., As shown by Vernon Mountcastle, into alternating layers of slowly adapting and rapidly adapting neurons; or spatial segmentation of the vibrissae on mouse/rat cerebral cortex).

This area of cortex, as shown by Wilder Penfield and others, is organized somatotopically, having the pattern of a homunculus. That is, the legs and trunk fold over the midline; the arms and hands are along the middle of the area shown here, and the face is near the bottom of the figure. While it is not well-shown here, the lips and hands are enlarged on a proper homunculus, since a larger number of neurons in the cerebral cortex are devoted to processing information from these areas.

The positions of Brodmann areas 3, 1, and 2 are – from the nadir of the central sulcus toward the apex of the postcentral gyrus – 3a, 3b, 1, and 2, respectively.

Somatosensory Cortex Function/Primary Somatosensory Cortex Function

Function

The somatosensory cortex receives all sensory input from the body. Cells that are part of the brain or nerves that extend into the body are called neurons. Neurons that sense feelings in our skin, pain, visual, or auditory stimuli, all send their information to the somatosensory cortex for processing. The following diagram shows how sensations in the skin are sent through neurons to the brain for processing.

The skin transmits signals through other neurons to the brain sensory to brain pathway. Each neuron takes its information to a specific place in the somatsensory cortex. Next, that part of the somatosensory cortex gets to work on figuring out what the information means. Think of it like scientists sending data to a data analyst. Each scientist, like the neuron, gathers information and sends it to a master analyzer or the somatosensory cortex.
Some neurons are very important and a big chunk of the somatosensory cortex is devoted to understanding their information. The senior scientist sends the most important information to our analyst, and he spends a lot of time understanding it. However, our junior scientists or volunteers gather less important information, so our analyst, or somatosensory cortex, spend less time on that data.

Location

The brain is the control center of the whole body. It is made up of a right and left side, or lobes, which are connected in the middle by the corpus colossum. Each lobe is devoted to a different function. The outer layer of the brain is called the cerebral cortex. Think of it like the skin on fruit, the skin is the cerebral cortex, and the fruit is the white insides of the apple. The cerebral cortex helps with processing and higher-order thinking skills, like reasoning, language, and interpreting the environment. This image shows a cross-section of the brain, with the cerebral cortex shown as the dark outline.

The somatosensory cortex is a part of the cerebral cortex and is located in the middle of the brain. This image shows the somatosensory cortex, highlighted in blue in the brain.