Somatosensory Cortex

The somatosensory cortex is a part of your brain that receives and processes sensory information from the entire body. Other names of somatosensory cortex include somesthetic area and somatic sensory area.

This part of the brain is essential for receiving sensory information from the body and processing it to initiate important movements that are required to deal a particular situation. It receives sensations of touch, pain, and vibration from the entire body.

In this article, we will study the somatosensory cortex in detail. We will go through its anatomical features, its location, and blood supply. We will discuss in detail how the somatosensory cortex plays an important role in processing the sensory information. In the end, we will discuss some disorders arising from the lesions of the somatosensory cortex.

Anatomy

In order to completely understand the anatomy of somatosensory cortex, we should first know that it is divided into two functional parts;

  • Primary somatosensory area or cortex (S1)
  • Secondary somatosensory area or cortex (S2)

Location

The somatosensory cortex is a part of the forebrain. It is present in the parietal lobe.

The primary somatosensory area (S1) occupies the postcentral gyrus on the lateral surface and the posterior part of the paracentral gyrus on the medial surface of the cerebral hemisphere.

The secondary somatosensory area (S2) is present in the superior limb of the posterior part of the lateral fissure (a fissure in the cerebral hemispheres that separates the frontal and parietal lobes from the temporal lobe).

Projection of fibers

Primary Somatosensory Cortex (S1)

The primary somatosensory area receives projection fibers from the ventral posterior lateral and ventral posterior medial nuclei of thalamus. These nuclei receive fibers from the contralateral half of the body in the form of medial, trigeminal and spinal lemnisci.

The efferent fibers from the primary somatosensory cortex terminate at the somesthetic association area.

Secondary Somatosensory Cortex (S2)

The secondary somatosensory area receives bilateral fibers from the entire body. However, the neuronal connections of this part are not well understood. It is believed that most of the fibers in the secondary area come from the primary somatosensory area.

Representation of Body

As the somatosensory areas receive sensations from the entire body, the representation of all body parts is present. However, the proportion of representation is different.

Representation in primary area (S1)

 In the primary somatosensory cortex, the contralateral half of the body is represented as an inverted homunculus. The pharyngeal region, tongue, and lips are represented in the most inferior part; followed by face, fingers, hands, arms, trunk, and thigh.

The leg and foot areas are represented in the posterior part of paracentral lobule, present on the medial surface of cerebral hemisphere. The anal and genital regions are present here.

The proportion of the cortex representation for a particular part of the body depends on its functional importance rather than its size. In fact, the area occupied by a particular body part is proportional to the number of sensory receptors present in it.

Representation in secondary area (S2)

The secondary cortex is much smaller and less important than the primary somatosensory cortex. Here, the body is bilaterally represented with the contralateral side dominant. The leg area is present most posterior, and the face area lies most anterior.

Blood supply of Somatosensory Cortex

The arterial supply to most of the primary somatosensory area (S1) and the secondary somatosensory area (S2) is derived from the medial cerebral artery.  This artery supplies the lateral surfaces of the cerebral hemispheres.

The part of the primary somatosensory area present on the medial surface of the cerebral hemisphere is supplied by the anterior cerebral artery.

Both these arteries are the branches of the internal carotid artery.

Blood from the somatosensory cortex finally drains into the superior sagittal sinus.

Physiology

As we said earlier, the somatosensory area is divided into two parts; the primary somatosensory area and the secondary somatosensory area. The reason for this division is that a distinct and separate spatial orientation of different parts of the body is found in the two areas. This orientation has been discussed in the section of Representation of Body.

Primary Somatosensory area (S1) has a high degree of localization whereas the secondary area (S2) has very poor localization. The functions of the secondary area (S2) are not well known. We will focus on the functions of the primary somatosensory area (S1).

Localization of Sensations

This is probably the most important function of somatosensory cortex. It is responsible for the discrete localization different sensations that arise in different parts of the body. Pinpoint the location of pain, tingling, touch, temperature, and other sensations is the function of somatosensory cortex, specifically area S1.

Pressure Perception

It is also responsible for the perception of different degrees of pressure.  This area of the brain is essential to judge critical degrees of pressure against the body.

Weight Perception

When you carry any object, you are able to approximately judge the weight of the object. This function is also performed by the somatosensory cortex. It allows a person to judge the weights of objects.

Perception of Shape

When you hold a coin in your hand with your eyes closed, you are able to judge that its shape is round. Similarly, you can judge the cylindrical shape of a pen by mere holding it in your hand. This is called stereognosis.

The somatosensory area, along with other areas of brain plays an important role in stereognosis. It allows a person to judge the shapes of different objects, even when the eyes are closed.

Feeling the Texture

The somatosensory area (S1) also helps you judge the texture of the materials. This type of judgment depends on the critical sensations caused by the movement of fingers over the surface of the objects. This critical judgment is performed by the somatosensory cortex.

Clinical Significance

The sensory information is relayed from the lower centers of the brain, principally the thalamus to the somatosensory cortex for analysis.

It is necessary for appreciation of spatial recognition, recognition of intensity and recognition of similarities and differences.  Any lesion of somatosensory cortex can produce decline in the above-mentioned symptoms.

Widespread bilateral lesion of the somatosensory cortex causes following clinically important signs and symptoms:

Defective Localization      

The person is unable to discretely localize different sensations arising in different parts of the body. However, he or she can localize these sensations crudely, such as to a particular foot, to a major level of chest or to one of the arms.

This crude localization is possible because other parts of the brain such as the brain stem, thalamus, or other parts of cerebral cortex can do localization to some extent.

Defective Pressure Judgement

The person is unable to judge the critical degrees of pressure against the body. Although he is able to judge that pressure is applied, but he is unable to judge the degree and severity of the pressure applied.

Defective Weight Judgement

The person with a lesion in the somatosensory cortex is unable to judge the weight of objects. He cannot judge the weights of different objects after carrying them in hand.

Astereognosis

The person is unable to judge the shape or forms of different objects, a phenomenon called astereognosis.

Unilateral Lesion

Unilateral lesion of the somatosensory cortex causes sensory disturbances on the contralateral side of the body. The person remains unable to judge degrees of pressure, warmth, unable to localize pain and tactile stimuli accurately, and unable to judge the weights and shapes of the objects. Loss of muscle tone may also be a symptom of lesions of somatosensory cortex.

Conclusion/Summary

The somatosensory cortex is a part of cerebral cortex that receives and analyzes sensory information from the entire body. It is divided into two parts:

  • Primary somatosensory area (S1)
  • Secondary somatosensory area (S2)

The somatosensory cortex is present in the parietal lobe.

The primary somatosensory area (S1) is present in the postcentral gyrus and the paracentral gyrus.

The secondary somatosensory area (S2) is present in the vicinity of the lateral fissure.

The primary somatosensory area (S1) receives sensation from the contralateral half of the body. The entire body parts are represented in this area as an inverted homunculus with lips and head region present in the lowermost part and foot, legs and anal region in the uppermost medial part.

The secondary somatosensory area (S2) receives information from both sides of the body with the legs represented most posteriorly and face area represented in the most anterior part.

Area (S1) receives projection fibers from the thalamus whereas the neuronal connections of area (S2) are not well understood.

The arterial blood is supplied to the somatosensory cortex by the medial and anterior cerebral arteries.

Different functions performed by the somatosensory cortex include the following:

  • Crude localization of senses
  • Perception of degree of pressure and other sensations
  • Judgment of weight of objects
  • Perception of shapes of different objects
  • Feeling the texture of objects

Bilateral lesions of the somatosensory cortex cause decline in above-mentioned functions. Unilateral lesions cause defective localization of sensations on the contralateral side of the body.

References

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