Sense and Sensation: The Little Humans Behind It

Sense and Sensation: The Little Humans Behind It

Let’s begin with a little experiment – lift your right index finger and place it on the screen you’re reading this on. Well, assuming you could do that – you somehow managed to use one particular finger, move it to a particular location and sense its texture. We perform such simple and very complex tasks throughout the day, but have you ever thought about how your brain can sense what part of you is sending it a message? Or how it can specifically instruct the one finger to move forward? How did it sense the screen?

Beginning from the very beginning of animal life, even primitive animals (comb jellies, anemones) have basic building blocks of the nervous system – excitable cells called neurons. The human nervous system too, begins with sensory neurons present throughout our bodies, due to their polarised membranes, they are very excitable and can transmit sensations in the form of impulses. When you touched the screen, a neural impulse was generated and transmitted in an instant as neurons pass it on from one to the other via junctions called – synapses. The impulse is transmitted to the brain through the spinal cord (part of the Central Nervous System), and the brain then integrates the information sensed by neurons in your eyes, ears, finger, and body, to create the sensation that you feel. After this, you decide to move your finger away, a message sent to the finger via motor neurons. Thus, while rather oversimplifying it – a loop is formed by the sensory and motor nerves in animals.

However, how and where does the brain process and react to all this information?

Sensory Homunculus: A representation of the surfaces devoted to different body parts in the somatosensory cortex

In our brain, these nerves that sensed the screen, connect to a part called the somatosensory cortex. This is not merely a cluster of tangled neurons though, the somatosensory cortex has separate clusters for separate body parts – with a surface proportional to how much we sense around that body part. This proportional representation of the detail in which body parts sense stimuli has led to the creation of a model describing how our brain perceives messages from different parts of our body – it is called the homunculus (Latin – the little man).

Quite a novel sight at first glance, understanding the concept of proportionate representation helps us see why he has such gigantic hands, massive lips and a mouth, whilst his elbows and back, where we don’t sense much, are comparatively tiny.

The Motor Homunculus

The motor neurons too, converge in the motor cortex, where there is a similar proportionate surface representation of body parts (motor homunculus) that move in a more advanced fashion and those that have limited movement. Here too, the stark contrast between the size of the tongue representation and that of the arm can be seen. The homunculus can be seen as a small map to our body, present in the forebrain.

Homunculus Sensory and Motor Cortex


And thus, these ‘little men’ in our brain help us feel sensations throughout the body, varying in complexity, and then react to them with detailed, precise movements.

This article doesn’t go into the detailed mechanisms behind how the homunculus was discovered and how it has advanced medical science in terms of treating tumours and localised seizures, but I do encourage you do read about its fascinating discovery in Dr. Wilder Penfield’s surgery.

Samved Ojha, 11C

References:
https://academic.oup.com/brain/article/140/11/3055/4566636
https://anatomyzone.com/articles/cortical-homunculus/
https://www.youtube.com/watch?v=kMKc8nfPATI&t=633s
https://www.sciencenewsforstudents.org/article/scientists-say-cortical-homunculus
https://www.ncbi.nlm.nih.gov/books/NBK549841/#:~:text=The%20sensory%20homunculus%20is%20a%20map%20along%20the%20cerebral%20cortex,nerve%20carries%20the%20face’s%20sensations.
https://www.nationalgeographic.com/science/article/simple-sponges-provide-clues-to-origin-of-nervous-system
http://www.pbs.org/wgbh/aso/databank/entries/bhpenf.html


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