Question: How do smells occur?

Nina Rzechorzek answered on 6 Jul 2020:

Great question! As the others have so elegantly covered smell, I will give you more info on taste and how it relates to smell:

Like other sensory systems, smell (olfactory) and taste systems gather information about the external environment. Sensory receptors of these systems respond to chemical molecules mixed in air or saliva, and the two systems complement each other for better interpretation of what we eat and smell. Taste and smell not only determine flavours, but also help maintain a consistent chemical balance in the body. The main function of taste is to evaluate the nutritious content of food and prevent the ingestion of toxic substances.

Taste is ultimately the sensory impression of food or other substances on the tongue, induced by binding of chemical molecules with taste receptors on taste receptor cells. These cells are located in taste buds on the tongue. Sensory cells of the taste buds are able to differentiate between different tastes through detecting interaction with different molecules or ions. To date, we recognise that the tongue can detect five basic flavours (salt, sweet, sour, bitter, and umami). Sweet taste helps us to identify energy-rich nutrients, umami allows the recognition of amino acids, salt taste ensures the proper dietary electrolyte balance, and sour and bitter warn against the intake of potentially noxious and/or poisonous chemicals.

Taste buds are composed of groups of 50 – 150 taste receptor cells, bundled together like a cluster of bananas, arranged to create a small ‘taste pore’ at the centre. Chemical molecules that trigger the sense of taste are dissolved by the saliva; they enter the taste bud through the pore. Taste receptor cells live for about 10 days and have to be replaced. There are a relatively small number of taste cells compared with a large number of molecules that trigger taste sensations. We think that each taste cell has receptors for only one type of flavour, so each cell in a taste bud detects either sweet, sour, bitter, salty, or umami.

You may have heard of a ‘tongue map’ which traditionally suggested that taste receptor cells for the five basic flavours were preferentially distributed in taste buds in different parts of the tongue. This myth has pretty much been debunked by recent molecular and functional data, revealing that responsiveness to the five basic modalities is present in all areas of the tongue.

The binding of molecules with taste receptors ‘depolarizes’ the membrane of the taste cells – this means that the difference in charge between the outside and inside of the plasma membrane of the cell is transiently lost (this is essential for nerve impulse transmission). The mechanisms that generate membrane depolarization depend on taste molecules that bind to their specific receptors; there are different receptors for each type of taste but the molecular mechanisms are only well understood for some of them. Taste receptors for umami respond to glutamates present in meat broths and fermented products and this is generally considered to give us the ‘savoury’ flavour.

Taste receptor cells on the tongue are connected to bipolar neurons (specialized sensory nerve cells with one axon and one dendrite). The dendrite is an extension of a nerve cell that conducts electrochemical information towards the nerve cell body (in this case from the taste receptor cells). The axon is the long extending process of the nerve cell that conducts action potentials (electrical impulses) away from the nerve cell body (in this case towards the central nervous system).

The bipolar neurons used for taste contribute to two cranial nerves: the facial nerve (cranial nerve 7) and glossopharyngeal (cranial nerve 9). FYI – we have 12 pairs of cranial nerves that emerge directly from the brain/brainstem. The facial nerve innervates taste buds at the front of the tongue, whilst the glossopharyngeal nerve innervates taste buds at the back of the tongue.

Cell bodies of the bipolar neurons used for taste are located in ganglia (nerve cell clusters) of the facial and glossopharyngeal nerves. Bipolar neurons in each of the ganglia send their axons to the brainstem (part of the central nervous system that connects the brain to the spinal cord). Neuronal fibres from the brainstem then send information to two places (1) a core processing/integration centre in the brain called the ‘thalamus’ and (2) a part of the brain called the ‘amygdala’, a key region for sensing emotions. Neurons from the thalamus then project to the cerebral cortex (the outer surface of the brain), which is responsible for analyzing all of that information so that we can interpret specific flavours!

You have probably noticed that your taste perception varies according to context. For example, the addition of sugar to lemon juice masks its sourness without affecting its acidity. Our perception of taste is massively influenced by other sensory inputs as well as prior experience, and how hungry we are. This suggests that taste qualities are combined and compared, and this information is integrated with a lot more information from other sensory systems to produce our final perception of flavor in the brain.

Some interesting species comparisons: taste receptors of dogs respond to the same chemicals that give the sense of taste in humans, but unlike humans and some other animals, dogs may not have very sensitive taste buds for salt. In all, dogs have about 1700 taste buds, while humans have about 9000! Although a dog’s ability to taste is only a fraction of a human’s, the canine sense of smell is up to one million times stronger so dogs predominantly use smell over taste to decide what to eat! Dogs still have more taste buds than cats, which average only about 470.