Primary Somatosensory Cortex

Introduction

The human brain is primarily divided into three large processing centers, comprising a large number of structures: forebrain (the prosencephalon), midbrain (the mesencephalon), and the hindbrain (the rhombencephalon). While certain regions of the cerebral cortex perform specific and more complex functions of the brain, some of those regions have more generalized functions. Such is the primary somatosensory cortex (S1) of the brain.

The somatosensory association part of the cerebral cortex coordinates the general information from the sensory organs for taste, smell, sound, touch, and sight. This is the place where these processes first begin.

What Is the Function of the Primary Somatosensory Cortex?

A somatosensory function is the ability of the brain to interpret bodily sensations. Hence, this sensory area of the cerebral cortex is involved in the processing of the sensory stimuli for touch and other skin and muscle sensations from the opposite side of the body. More specifically put, the localization of pain, tingling, touch, temperature, and other sensations are the main functions of the primary somatosensory cortex of the brain.

Namely, the regions of the body that support a finer distinction of touch, such as the mouth and the palm, are represented on a wider area than the areas dedicated to the back or the feet, for instance. For this reason, the electrical neural impulses that aim for these specific regions in the cerebral cortex relay the information that touch has been applied to a particular part of the body.

The somatosensory cortex recognizes which part of the body is the source of sensation through the sensory homunculus.

What Is a Sensory Homunculus?

The cortical homunculus is how your brain views the body from the inside. More specifically, the sensory homunculus is a kind of a neurological ‘map’ of the human body in the brain. This mapping results from the way neural pathways and connections in the brain are ordered and arranged.

The sensory areas are sub-grouped into a more comprehensive map. Namely, our visual field is represented in the visual cortex. The songs are turned into maps in the auditory cortex, while the sensory pathways of the skin that register pain, touch, and temperature are registered in the somatosensory cortex.

These cortical representations and the importance of different body parts as seen by the brain are mapped onto the precentral and the postcentral gyri in the parietal cortex of humans. For instance, there is little need for the brain to know what’s happening with the arms or legs. On the other hand, the hands, the tongue, the genitals, and the facial features are of great importance since they provide us with plenty of sensory information.

Types of Cortical Homunculus

The postcentral gyrus houses the primary sensory cortex. Its main function is to process the signals received from the thalamus. On the other hand, the sensory homunculus is a kind of brain chart that maps the cortical areas involved in the translation of the sensory stimuli that are relayed via electrical impulses to different parts of the body.

There are two types of sensory homunculi:

Motor homunculus – located in the precentral gyrus.
Sensory homunculus – located in the postcentral gyrus of the brain.

The homunculus (the little man) has large hands, feet, and lips that correspond to the parts of the body with more sensory neurons.

Anatomy of the Primary Somatosensory Cortex

The primary somatosensory cortex consists of neuron cells that register the stimuli received via the sense of touch, and the Brodmann areas 1, 2, and 3. Similar to the primary motor cortex, this area of the cerebral cortex is highly organized, with specific regions representing each part of the body.

The somatosensory system of the primary cortex encompasses all parts of our body. The postcentral gyrus of the parietal lobe is the brain area where the primary somatosensory cortex is located.

The S1 area is involved in the translation of the stimuli received via the sense of touch. Also, it has a role in encoding the temperature of the body. This cortical area houses the peripheral sensory receptors.

The somatosensory neuron pathway consists of three types of neuron cells:

Primary neurons – In the periphery, these neurons serve as sensory receptors that detect the sensory stimuli, like touch or temperature.
Secondary neurons – The function of these neurons is to relay the sensory stimuli. They are located in both the spinal cord and the brain stem
Tertiary neurons – This type of neuron forms a sensory homunculus of the touch.

Brodmann Areas 1, 2, and 3

Defined by the number and organization of neuronal cells, the cellular composition of the primary somatosensory cortex results in the Brodmann areas 1, 2, and 3. These Brodmann areas are located in the parietal lobe and are associated with:

Localization of touch, temperature, vibration, and pain;
Sensory perception (proprioception, nociception, and crude touch);
Coordination of the orofacial movements;
Motor learning and control.

What Is the Function of the Postcentral Gyrus of the Parietal Lobe?

The postcentral gyrus is located in the touch processing area of the brain. It borders with the following cortical structures:

The lateral sulcus – located under the postcentral gyrus;
The medial longitudinal fissure – borders with the postcentral gyrus in the middle part of the parietal lobe;
The postcentral sulcus – borders with the postcentral gyrus in the back part of the parietal lobe;
The central sulcus – located in front of the postcentral gyrus.

The main function of the postcentral gyrus of the parietal lobe is to translate the sensory stimuli received through the skin.

This cortical structure is also the location of the secondary somatosensory cortex. In this way, the postcentral gyrus is also involved in the processing of information received from the changes these sensory stimuli trigger in other parts of the body.

What Is the Function of the Secondary Somatosensory Cortex?

The somatosensory cortex is the processing area of the brain dedicated to the translation, encoding, consolidation, and integration of sensory stimuli. It houses the Brodmann areas 40 and 43.

While the Brodmann area 40 is involved in phonology and the meaning of written words, the Brodmann area 43 regulates the pressure created on the middle ear and the eardrum during the processes of drinking and eating (swallowing).

What Is the Function of the Parietal Lobe in the Brain?

The parietal lobe regulates the following abilities:

Speech;
Reading;
Writing;
Cognition;
Spatial orientation;
Navigation;
Visual perception;
Sensation of pain;
Sensation of touch;
Sensation of smell;
Sensation of taste;
Sensation of hearing;
Coordination of movements;
Mathematical computation;
Regulation of temperature.

How Is Touch Recognized by the Brain?

A large number of receptors in our skin receive various stimuli from the world that surrounds us. The receptors of touch produce electrical or chemical signals for the received sensation. This information is received via the sensory organs that further relay the signal from the neuron cells to the thalamus.

The thalamus consolidates and encodes this sensory information and then transmits it to the other dedicated processing areas of the central nervous system. The middle portion of the parietal lobe is the particular area that supports the recognition of the relations of people versus objects in space, the identification of objects, as well as the translation of pain and touch.

What Happens When the Primary Somatosensory Cortex Is Dysfunctional?

The information gained from the sensory organ of touch is processed within the somatosensory system. When the primary somatosensory cortex becomes dysfunctional, it can lead to the following disorders:

The somatosensory disorder refers to a dysfunction of the somatosensory system. The affected individuals usually experience numbness, tingling, or a feeling of complete numbness in an arm or a leg due to compression of the nerves.

Any lesion of the somatosensory cortex can lead to defective localization of the sensations by the affected individual. The patient is not able to recognize sensations in different body parts. These sensations are localized only in a particular side of the body or one of the extremities.

The primary tactile disorders refer to an impaired ability to feel skin sensations and being unaware of the position or posture of the body.
Judgment of a defective pressure – The affected individual loses the ability to estimate the amount of pressure against any part of the body. This person is able to feel the pressure but is unable to estimate its force or degree.

Fun Facts

Did you know?

1. The parietal lobe regulates our logical and artistic skills.
2. Also, the parietal lobe is responsible for our good management of a multitude of tasks simultaneously.
3. Other names of the somatosensory cortex include somesthetic area and somatic sensory area.
4. When our brain detects that our shoes are too tight – that’s due to the proper functioning of the primary somatosensory cortex of the brain.
5. Dr. Wilder Penfield and his co-investigators Edwin Boldrey and Theodore Rasmussen are the scientists who discovered the sensory and motor homunculi.
6. The homunculus has a meaning of ‘little man’ or ‘cortex man’ in Latin. Its proportions are sized according to the amount of sensation carried out by each body part.

Orofacial movements are the involuntary repetitive movements of the mouth and face.

Motor learning refers to the complex processes in the brain that occur in response to the practice of a certain skill, i.e. behavioral, biomechanical, and neural bases of development, acquisition, and performance of functional movement skills.
The somatosensory area also helps us evaluate the texture of the materials. The stereognosis is the ability to evaluate the shapes of different objects, even when our eyes are closed.
The term haptic relates to the sensation of touch that helps us recognize the texture, size, or shape of external stimuli.