Sensory Cortex

Generally, the cortex is derived from the Latin word “bark” and it is the outmost layer of any structure. The cortex of the human brain is categorized into three functionally unique areas namely; associative, sensory and motor (2).

The motor cortex is responsible for planning, controlling and executing voluntary movements. Moreover, the associative cortex integrates generated visual, auditory, gustatory and other general sensory signals.  The sensory cortex is defined as all cortical areas linked with sensory functions(1).

In another definition, the sensory cortex is a section of the cerebral cortex which is responsible for receiving and interpreting sensory information from different parts of the body.

Stimuli received from different receptors such as nociceptors and thermoreceptors are transduced to an action potential which is conveyed along one or more afferent neuron to a specific section of the brain(3). According to studies, the sensory cortex comprises the visual cortex, auditory cortex, the primary olfactory cortex, the gustatory cortex and the primary somatosensory cortex(1).

Visual cortex

The visual cortex commonly known as cortex visualis in Latin is part of the sensory cortex found in the occipital lobe(2). Furthermore, the occipital lobe is one of the four primary lobes of the human brain and it acts as the visual processing center.

Therefore, the visual cortex is responsible for processing visual information. Visual nerves run from eye to primary visual cortex, then to the visual association cortex.

For the visual cortex to respond, visual information from the eyes passes through the lateral geniculate nucleus found in the thalamus. The section of the visual cortex that receives sensory input from the thalamus is called the primary visual cortex, also referred to as visual area 1 (V1) or striate cortex (1). 

Both the right and left hemispheres of the human brain contain the visual cortex.

The visual cortex found in the left hemisphere receives radiations from the right visual field whereas the visual cortex in the right hemisphere receives radiations from the left field of vision(1).

For newbies, a visual field or field of vision is the total area/field in which items can be seen in the side (peripheral) vision as you concentrate your eyes on a central point(3).

Auditory cortex

The auditory cortex is positioned on the upper side of the temporal lobe. Its main role is to process auditory information (1). In humans, the temporal lobe processes sensory input to clear meanings which facilitates language comprehension, visual memory and emotion association.

The auditory cortex is one part of the auditory system that does common and higher roles in hearing like language switching.

Also, the auditory cortex is comprised of sections belongs to the transverse temporal gyri, the superior temporal gyrus that is, planum temporal and the planum polare (2).

Transverse temporal gyri

Also known as Heschl’s gyri, these are gyri located in the primary auditory cortex. It is found in varying numbers in the right and left hemispheres (1). According to studies, transverse temporal gyri are the first cortical structures to process incoming auditory information.

Superior Temporal gyrus

Its main function is to process sounds. Furthermore, some sections of the superior temporal gyrus have been designed to process the combination of frequencies and others are designed to process changes in amplitude or frequency(1).

The superior temporal gyrus is also part of the Wernicke’s area, found in the left hemisphere. It is involved in language comprehension and social cognition.

Gustatory cortex 

It is part of the sensory cortex responsible for tasting. Neurons in the gustatory cortex respond to sourness, sweetness, saltiness, and bitterness. They also code the intenseness of the taste stimulus(1).

The gustatory cortex is made up of 2 substructures namely 1) The anterior insula and 2) The frontal operculum. Therefore, the gustatory cortex is a time called the AI/FO or (Anterior Insula/Frontal Operculum) (1).

  1. Frontal operculum (FO)- it is found in the frontal lobe which is located on the front part of the brain just directly behind the forehead.
  2. Anterior insula (AI)- it is found on the insular lobe which is located deep within the cerebral cortex, beneath the frontal-parietal and temporal lobes.   

The gustatory cortex functions with the help of taste buds to build the taste feeling(2). The tongue is filled with taste buds and taste buds in different sections of the tongue sense different types of flavors.

Olfactory cortex

It is part of the sensory cortex responsible for smelling. Humans have the main olfactory system that detects airborne substances and the accessory olfactory system for fluid-phase stimuli(1).

Gustatory and olfactory systems work hand-in-hand and are a time called the chemosensory system since they provide the brain with information on the chemical composition of items.

The olfactory cortex is housed in the uncus and it comprises of the piriform cortex, amygdala, olfactory tubercle and parahippocampal gyrus(2).

Piriform cortex

This is a section of the rhinencephalon.

Amygdala

This is found within the temporal lobe and performs unique roles such as processing of memory, decision-making and emotional response such as anxiety, fear, and anger(1).

Olfactory tubercle

Also called the tuberculum olfactorium, the olfactory tubercle is a multi-sensory processing center and plays an important role in reward cognition(1). Apart from that this part also plays a significant role in locomotor and attentional behaviors, mainly concerning social and sensory responsiveness(2).

Parahippocampal gyrus

This part plays a critical role in memory encoding and retrieval(1). Different tastes felt by the tongue are memorized in the parahippocampal gyrus which enables you to remember the taste of different items.

Structure of the Primary Somatosensory Cortex

The primary somatosensory cortex of the human brain consists of Brodmann areas 3, 1 and 2. Brodmann area, a section of the cerebral cortex, defined by 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 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). 

Besides, 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 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.

Conclusion

The sensory cortex of the human brain is very important because it enables users to carry out our daily activities with ease. Dysfunctions of the sensory cortex may result in losing some sensing abilities such as hearing, sight or balance.

References

  1. Carter, R., Aldridge, S., Page, M., & Parker, S. (2019). The human brain book. The book can be found online on: https://books.google.co.ke/books?id=q4SzDwAAQBAJ&dq=the+human+brain&hl=en&sa=X&ved=0ahUKEwiM5t3V8svlAhUh3uAKHcWcBx8Q6AEILTAB
  2. Petrides, M. (2017). Atlas of the morphology and cytoarchitecture of the human cerebral cortex on the average MNI brain. The book can be found online on: https://books.google.co.ke/books?id=XHN1oAEACAAJ&dq=cortex+2018&hl=en&sa=X&ved=0ahUKEwiZvO6l9MvlAhXQTN8KHfjaBD8Q6AEILTAB.
  3. Franzén, O., Johansson, R., &Terenius, L. (1996). Somesthesis and the Neurobiology of the Somatosensory Cortex. Basel: Birkhäuser Basel. The book canfound online on https://books.google.co.ke/books?id=q230BwAAQBAJ&pg=PA170&dq=somatosensory+cortex&hl=en&sa=X&ved=0ahUKEwj8qJLv9MvlAhXDTN8KHX8LBu4Q6AEIJTAA