Primary Somatosensory Cortex

Somatosensory Cortex: 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, some body 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 said body part. For this reason, the human lips and hands have a larger representation than other body parts.

Primary Somatosensory Cortex

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.

The somatosensory system is the part of the sensory system concerned with the conscious perception of touch, pressure, pain, temperature, position, movement, and vibration, which arise from the muscles, joints, skin, and fascia.

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.

Primary Somatosensory Cortex

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 a 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.

Somatosensory Cortex FunctionThe 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 red in the brain.

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).

Primary Somatosensory Cortex 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.

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, spends less time on that data.Each neuron takes its information to a specific place in the somatosensory 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.

The Somatosensory Cortex Is Responsible For Processing

The primary somatosensory cortex (areas 1, 2, and 3) is on the postcentral gyrus and is a primary receptor of general bodily sensation. Thalamic radiations relay sensory data from skin, muscles, tendons, and joints of the body to the primary somatosensory cortex. Lesions of this cortex produce partial sensory loss (paresthesia); rarely does complete sensory loss (anesthesia) occur. A lesion causes numbness and tingling in the opposite side of the body. Widespread destructive lesions produce gross sensory loss with an inability to localize sensation.

The primary somatosensory cortex is called S1. This area of the cerebral cortex receives sensory information from the somatic senses, plus proprioceptive senses and some visceral senses. It is located on the postcentral gyrus of the parietal lobe, as shown in Figure 4.3.6. The topological arrangement of the somatic senses is preserved as they enter the spinal cord, travel up the dorsal column tracts, to the nucleus gracilis or nucleus cuneatus, and is preserved through the thalamus to eventually map onto the cortex. Thus the surface of the body maps onto the surface of the brain.

Somatosensory Cortex Definition, Somatosensory Cortex Location

the level of decussation, the neurons in a somatosensory pathway represent the contralateral (i.e., opposite) side of the body or face. It is important to learn the decussation site, as it will aid in clinical diagnosis. When an afferent pathway is damaged somewhere below the site of decussation, the sensory loss will be on the side ipsilateral to the lesion (i.e., the loss is on the same side as the lesion or ipsilesional). When an afferent pathway is damaged somewhere above the site of decussation, the sensory loss will be on the side contralateral to the lesion (i.e., the loss is on the side opposite the lesion or contralesional).

In the medial lemniscal pathway, the axons of the gracile and cuneate nuclei decussate in the medulla. The decussation in the neospinothalamic pathway is in the spinal cord and involves the axons of the posterior marginal nucleus. The spinal trigeminal nucleus axons decussate upon leaving the nucleus in the medulla and lower pons, whereas the main sensory trigeminal nucleus axons decussate at mid pons levels immediately upon leaving the nucleus.