Primary Somatosensory Cortex

The primary somatosensory cortex is found in the postcentral gyrus and is one of the sections of the somatosensory system(1). Also, the somatosensory system is a section of the sensory nervous system that responds to changes on the skin surface while postcentral gyrus is one major ridge in the human brain lateral parietal lobe.

According to research, it was originally explained in two different studies. The first study is called "Surface Stimulation" by Wilder Penfield and the second is called "Parallel Surface potential" by Bard, Woolsey, and Marshall.

Though these studies defined the primary somatosensory cortex to be similar to Brodmann areas 3, 1 and 2, the latest studies by Kaas have shown that for sameness with the other sensory areas only area 3 will be called the primary somatosensory cortex.

This part receives the majority of the thalamocortical signals from the sensory areas. For clarity, the thalamocortical projections/radiations are fibers between the thalamus and the cerebral cortex(2).

A tactile representation of the primary somatosensory cortex is ordered in an upturned style starting from the toe at the uppermost of the cerebral hemisphere to the mouth at the undermost(1).

Nonetheless, few body parts are governed by moderately imbricated regions of the cortex. The primary somatosensory of each cerebral hemisphere has a graphical display of the contralateral side of the body (3). Moreover, the amount of primary somatosensory cortex assigned to a body part is not equivalent to the size of the body part but to the bulkiness of the cutaneous tactile report of the body part. 

Generally, the degree of sensitivity of the tactile stimulation felt at a certain body part depends on the bulkiness of the cutaneous tactile receptor of a body section(3).  This is to mean; the human hands and lips have larger representation compared to the other body parts. 

Somatosensory system

The somatosensory system is made up of primary, secondary and tertiary neurons that enable touching and sensitive to temperature, position, and balance possible(3). They are distributed throughout the body and include sensory receptors neurons on the surface and deeper neurons within the central nervous system (CNS).

In
general, the somatosensory system works in the following body sections; the
periphery, spinal cord, and the brain.

1. Periphery-
   sensory receptors such as thermoreceptors
   and mechanoreceptors detect different stimuli.
2. Spinal
   cord- afferent somatic pathways in the spinal
   cord convey information from the periphery and other parts of the body to the
   brain.
3. Brain-
   this contains the Brodmann areas 3a, 3b, 1
   and 2 that form the somatosensory cortex. In summary, BA 3a is responsible for
   sensing the relative position of neighboring body parts and the amount of
   energy used during movement. On the other hand, BA 3b distributes somatosensory
   information to BA1 and shape and size information to BA2. 

Somatic pathways

Somatic
pathways also called somatosensory pathways, convey information between the
brain and nerve cells in the skin and other body organs (3). There are two main somatic
pathways namely anterolateral pathway and posterior column-medial lemniscal
pathway.

Anterolateral pathway- this
transmits pain, temperature, and crude touch sense.

Posterior Column-Medial Lemniscal pathway- transmits
proprioception, fine, discriminativetouch and vibration.

Structure of the
primary somatosensory cortex

The primary somatosensory cortex of the human brain is made up of Brodmann areas 3, 1 and 2. Brodmann area, one section of the cerebral cortex is familiar with its histological structure or cytoarchitecture and organization of cells (2).

Furthermore, the Brodmann area was discovered and named after a famous German neurologist called Korbinian Brodmann.

Brodmann area (BA) 3 is made up of two areas; 3a and 3b. Brodmann area 1 is hosted on the uppermost of the postcentral gyrus. The rostral fence of BA 3a is found in the bottom part of the central sulcus. It is posteriorly followed by BA 3b then BA 1 with BA 2 following and ending in the bottom of the postcentral sulcus(3).  The primary somatosensory cortex is commonly referred to as BA 3b because of the following scientific facts;

• It
  receives serried inputs from pulvinar nuclei (nuclei of the pulvinar or nuclei
  pulvinar) of the thalamus.
• The
  neurons are very sensitive to somatosensory stimuli and no other stimuli.
• The
  occurrence of lesions in this section affects somatic sensations.
• A
  somatic sensory feeling is as a result of electric stimulation.

Even though BA 3a collects bulk input from the thalamus, it is hugely responsible for proprioception which is the sense of self-movement and body position. This is sometimes called the “sixth sense” or kinesthesia(1). BA 3b sends signals to BA 1 and BA 2. Radiations/signals from BA 3b to BA 1 mainly sends texture information while radiations to area 2 convey shape and size.  Damages or infections to these areas result in infirmity in shape, texture, and size(3).

Similar to another neocortex, the somatosensory cortex is layered. This is to mean; the primary somatosensory neurons are joint together with common responses and inputs in the vertical column that elongates along with cortical layers(2).  Also, it is arranged somatotopically with patterns of the homunculus.

This means that the trunk and legs crumple in the midline; the hands and arms are in the middle whereas the face close to the undermost part. Hands and lips are expanded on a good homunculus. This is because several neurons in the cerebral cortex are assigned to process signals from these body parts(3).

Brodmann
areas 3, 1 and 2 also comprise of cells that run to the secondary somatosensory
cortex (S2) which is part of the cortex found in the parietal operculum on top
of the lateral sulcus.

Functions of the primary somatosensory cortex

The primary somatosensory cortex is mainly responsible for the processing of somatic sensations. According to research, somatic sensations are bodily sensations of touch, pain, temperature, vibration and proprioception(1).

The sensations arise from receptors located throughout the body and are responsible for detecting sensations. When a receptor detects any form of sensation, the information is conveyed to the thalamus and then to the primary somatosensory cortex which tells the body how to respond.

Examples
of somatic sensations include; 

1. Proprioception

Also known as kinesthesia, proprioception is the sense of self-movement, balance and body position. Proprioception is facilitated by proprioceptors which are mechanosensory neurons found within muscles, joints, and tendons(3). Different proprioceptors are activated by different behaviors and encode unique types of information such a limb velocity and limb limits.

2.  Nociception

Derived
from the Latin words "to harm or hurt", nociception is the somatic
sense of pain. In this case, the primary somatosensory cortex responds to
certain harmful or potentially harmful stimuli(1). For example, if you are cut or some
chili powder is poured to your eyes, nociceptors generate projections along the
spinal cord to the primary somatosensory cortex. These may result in a variety
of physiological and behavioral responses.

3. Thermoception

Also
known as chemoreception, thermoception is sensitive to changes in environmental
temperatures such as hot and cold(1). Thermoreceptors are found on the skin surface and
send signals to the primary somatosensory cortex which gives a response on what
you should do.

4. Touch

This
is a sensation produced by touching something. According to studies, every hair
in your body is sensitive to touch which allows you to sense even the slightest
displacement.

Clinical significance

According
to neurosurgeons, the primary somatosensory cortex is prone to the lesion.
Moreover, lesions affecting the primary somatosensory cortex produce symptoms
such as;

1.  Agraphesthesia

This is one kind of disorder of directional cutaneous kinesthesia (proprioception) or disorientation of sensation of the skin across the skin space (2). This means that the patient experiences difficulty in recognizing written numbers/letters traced upon the skin. Patients diagnosed with Schizophrenia and Alzheimer's are more likely to get Agraphesthesia.   

This
disease can be diagnosed using a variety of tests like the Palm Writing subset
which comprises a series of trials whereby patients are asked to identify
whether an X or O was written on their palm(3).

2. Hemihypesthesia

It is a reduction in sensitivity on one part of your body(2). Patients with this condition may be not felt if touched on one side of the body. Nonetheless, he/she has normal functions on the other side of the body. Hemihypesthesia may occur from the damage of the thalamocortical fibers found in the posterior limb of the internal capsule.

3. Astereognosis

Astereognosia is the inability to identify an item by the active touch of the hands without other sensory inputs like visual or sensory information(1). If only one hand is affected it is called tactile agnosia.

In
conclusion, lesions in this section of the brain result in hemineglect. Damage
of Brodmann areas 3, 1 and 2 produce contralateral hemihypesthesia and
astereognosis.

References

1. Borich, M. R., Brodie, S. M., Gray, W. A., Ionta, S., & Boyd, L. A. (2015). Understanding the role of the primary somatosensory cortex: Opportunities for rehabilitation. (Neuropsychologia.) The book can be found online on https://books.google.co.ke/books?id=euzZvQEACAAJ&dq=primary+somatosensory+cortex&hl=en&sa=X&ved=0ahUKEwj5k7-Mz8jlAhWmyIUKHa_yBBYQ6AEIJTAA
2. Clark, D. L., Boutros, N. N., & Mendez, M. F. (2010). The brain and behavior: An introduction to behavioral neuroanatomy. The book can be found online on https://books.google.co.ke/books?id=32Ucobqp97EC&pg=PA41&dq=primary+somatosensory+cortex&hl=en&sa=X&ved=0ahUKEwj5k7Mz8jlAhWmyIUKHa_yBBYQ6AEINjAD#v=onepage&q=primary%20somatosensory%20cortex&f=false.
3. Kalat, J. W. (2007). Biological psychology. Belmont, CA: Thomson/Wadsworth. The book can be found online on https://books.google.co.ke/books?id=vM7zwhSwwPsC&pg=PT235&dq=primary+somatosensory+cortex&hl=en&sa=X&ved=0ahUKEwiEn5bS0MjlAhVPA2MBHTSsDwsQ6AEISDAF