Taste and Smell: Where Chemistry Meets Biology

  Hey! Happy New Year!! Its been a long time, isn't it. Well, the string theory blog really worked me up but I'm back and back with a banger, no less! If there is something we all love, it has to be food. We need it for our survival and its a really enjoyable part of life unless your mom made spinach, in which case, my condolences. But rather than talking about how it gets digested or made into poop, i wanna talk about the more interesting part, how you taste it and how smell helps in distinguishing flavours, so lets get into it

Let's start with a basic question:

How Important are these 2 senses?

And to answer this, let me tell you about a case study. 

Lallu was a fine 29 year old man, who was slightly above subsistence level, but one day, he met with an accident. You see, he was driving his bike when a bus driver hit him, aw 🥹. Then he had some head trauma, but he went to the hospital and got cured. 

But something strange which lallu and his family noticed was that he was not able to smell things properly and his sense of taste was gone, he became like Ranbir Kapoor from Animal. 

He drank expired milk once cause' he couldn't taste it, he spoiled his tea as he couldn't smell that it was burning, he had lost a sense of what was going on in the world. 

The condition he had is known as Anosmia, in which people can't smell or taste anything. This condition can be caused by head trauma or even just aging, thats crazy! 

So these 2 senses are very important as we need them to be aware of our surroundings and really just enjoy the world, why even eat when you can't taste it? 

Brief Neuroanatomical Overview

Before we get into the nitty gritty of how you smell or taste, I need to tell you about 3 types of receptors which help us sense different things. 

How we sense things comes down to how the sensory cells translate chemical or mechanical information into electrical action potential for the brain to understand. 

This process is known as Transduction. 

The 3 types of receptors involved in transduction are: 

1. Mechanoreceptors- The senses of touch and hearing use these receptors as they respond to the vibration of sound waves and pressure on the skin. If you've read my blog on ears, you would know about the ones involved in hearing.

2. Photoreceptors- These help us detect light waves, present in rod and cone cells.

3. Chemoreceptors- These are the ones we are going to be concerned with and they help us taste and smell things.

Olfaction (Smell)

The ability to smell is known as Olfaction. It's a rather fast and ambiguous process, so let's jump into it!

Olfaction happens because the molecules of any given substance are present in a gaseous form. They need to be diffused through the air to reach our nose. A lot of the molecules get picked up by nasal cilia right away as they enter the nose. 

Some of them reach the Olfactory Epithelium on the roof of the nose. This is a small yellowish patch of tissue. It contains millions of sensory olfactory neurons surrounded by supporting insulating columnar cells.

The molecules reach here and dissolve in the mucus made by the epithelia, then they bind to the chemoreceptors of the olfactory sensory neurons. 

Then this information travels through the ethmoid bone and reaches the Olfactory Bulb.

 It reaches a ball known as the Glomerulus. The glomerulus contains dendrites of another neuron called a Mitral Cell. 

Here's the fun part: Each olfactory sensory neuron can detect only one smell!

 One mitral cell receives signals from a lot of sensory neurons at once. Then it converts that signal to an electric one and sends it to the brain through the Olfactory Tract. 

Then the signal reaches 2 places:

One, it reaches the olfactory cortex in the temporal lobe where the brain identifies the smell.

Two, it reaches the limbic system of the brain, where the smell triggers any memories or emotional responses related to the smell.

 Ex- When you inhale smoke, and it reaches the limbic system, the brain identifies a threat and triggers the fight or flight response. 

Taste and Smell: How they come together

So, that is all about how smell works. And it's very closely related to taste as they use the same receptors, so if you have a blocked nose, you are gonna have an impaired sense of taste. Like you would be able to tell if its bitter or sweet, but you won't be able to differentiate between a lime and a lemon. 

Small Tit Bit: Our noses can smell upto 10,000 smells!!

What Flavour Means
Taste solely means what your tongue perceives but flavour is the combination of taste, smell and temperature of food. 

Gustation (Taste)

The ability to taste is known as Gustation.  Its similar to olfaction, let's get into it!

Each taste bud in our tongue is found under fungiform pappilae (projections of skin), which make the surface of the tongue rough.

 The millions of taste buds in our tongue are hidden by the stratified squamous epithelium. Each taste bud contains 50-100 special gustation epithelial cells. 

These are of 2 types-

1. Special Gustation Cell- This is the cell which helps in sensing taste, it contains the chemoreceptors.

2. Special Gustatory Basal Cell- These are stem cells which replace worn out or dead gustatory cells every 2 weeks or so. They can differentiate into gustation cells once they replace it.

Each gustatory cell has gustatory hairs, which are made up of actin and increase the surface area for gustation. These hairs open up into taste pores which are little holes between the stratified squamous epithelium. 

The food molecules get absorbed into the saliva and bind to chemoreceptors on gustatory cells. 

Then the information gets converted into action potentials and through cranial nerves 7 (Facial Nerve), 9 (Glossopharyngeal) and 10 (Vagus) goes to the brain. 

The signal reaches the gustatory cortex in the parietal lobe. Then the brain interprets the taste. 

Now this is where something captivating happens:

Different types of tastes are sensed based on how action potentials get triggered.

 Salty tastes contain positive sodium ions and opens up sodium channels which trigger action potentials.

 Bitter/sour taste contains hydrogen ions and opens up proton channels which trigger action potentials. 

It's a Myth!

Now, you know all about gustation and olfaction. With this information in mind, let me a drop a startling bomb on your head: Spicy is not a taste.

Yes! You heard that right, the only accepted tastes are Sweet, Salty, Bitter, Sour and Umami. Spicy is a sensation of pain, not a taste. Its a sensation of pain as it causes pain, it makes our tongue burn, makes our nose open up, makes us cry, makes our stomach blow up. All sensations of pain, not a taste. These sensations are carried by cranial nerve 5 (Trigeminal). If something is sweet, it can be spicy (Sweet and Sour sauce) if it causes the sensation of pain. 

Fun Fact: The taste map of the tongue which is taught to some in school is total nonsense! 

It does not exist and you can try it for yourself, just put something salty on the sweet side and you can still taste it.

Outro

That's it for this blog! It was a delightful and tasty journey through the abyss of the nose and tongue. I promise I will be back for another blog in no time! 

Refrence to other blogs: The foundation of human anatomy: Tissues Part 1 and 2, The nervous system and the Brain, Hear Me Out: The Fascinating World of the Ears, Anatomy of the Eye: It's disorders and it's Accomodation

You can visit all of these at deeparshbhanot.blogspot.com

By,

Deeparsh Bhanot