Smell and Taste
Two chemical senses that work as a team. How the nose and tongue detect molecules and why food seems flavourless when you have a cold.
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Two Senses That Work as One
Smell and taste are often treated as separate senses, but in everyday experience they are inseparable partners. The rich flavour of a ripe strawberry, the comforting warmth of freshly baked bread, the warning signal of spoiled milk — all of these depend on both senses working together.
Both are chemical senses: rather than detecting energy (like light or sound), they detect molecules. The nose identifies airborne molecules; the tongue detects molecules dissolved in saliva. Together they give the brain a detailed chemical picture of what is entering the body — supporting not just pleasure but survival.
Smell and taste also connect directly to digestion: detecting food triggers saliva and gastric juice production. Our digestive system guide explains how this preparation begins even before food reaches the stomach.
How the Nose Detects Smell
Smell (the scientific term is olfaction) begins in the nasal cavity. When you breathe in, airborne scent molecules travel up into the nose and reach a small patch of specialised tissue high in the nasal cavity called the olfactory epithelium. This tissue is only about 5 square centimetres in area but densely packed with olfactory receptor neurons — roughly 6 to 10 million of them.
Each olfactory neuron has tiny hair-like projections called cilia that extend into a thin layer of mucus. Scent molecules dissolve in this mucus and bind to receptor proteins on the cilia. Humans have around 400 different types of olfactory receptor, each tuned to particular molecular shapes. When a receptor is activated, the neuron fires an electrical signal.
The brain combines signals from hundreds of different receptor types to identify a specific smell — a system similar to mixing primary colours to create any shade. This combination coding is why humans can theoretically distinguish more than a trillion different odours, though practically the number we can reliably name is far smaller.
Smell and the Brain
Signals from olfactory receptor neurons travel along the olfactory nerve (cranial nerve I) directly to the olfactory bulb at the front of the brain — skipping the thalamus relay station that most other senses use. From the olfactory bulb, signals spread to the piriform cortex (primary smell area), the amygdala (emotions) and the hippocampus (memory).
This direct anatomical link explains the famous phenomenon of smell-triggered memory — often called the Proustian memory effect. A scent encountered during a significant emotional experience can later, many years on, trigger both the memory and the emotion almost instantaneously.
Information also flows to the orbitofrontal cortex, where smell combines with taste and visual information to create the complex experience of flavour.
How the Tongue Detects Taste
Taste (the scientific term is gustation) is detected mainly by taste buds — small sensory organs clustered on small bumps called papillae on the tongue's surface. The tongue contains about 2,000 to 10,000 taste buds, though the number declines somewhat with age.
Each taste bud contains 50 to 100 taste receptor cells. When food is chewed, molecules dissolve in saliva and enter tiny pores in the taste buds, where they interact with receptor cells. These cells generate electrical signals that travel along cranial nerves (mainly the facial nerve and glossopharyngeal nerve) to the gustatory cortex in the brain.
Taste buds are not just on the tongue — they are also found on the soft palate, epiglottis and upper oesophagus, giving the body a broader chemical sampling system than most people realise.
| Taste | Detected by | Likely purpose |
|---|---|---|
| Sweet | Sugars and some other compounds | Identifies energy-rich foods |
| Salty | Sodium and other salt ions | Guides electrolyte intake |
| Sour | Acids (hydrogen ions) | Warns against spoiled or unripe food |
| Bitter | Wide range of chemicals; many plant toxins | Defence against poisonous substances |
| Umami | Glutamate (amino acid in protein-rich foods) | Signals presence of protein |
The Five Basic Tastes
For much of history, Western science recognised only four basic tastes: sweet, salty, sour and bitter. In the early 20th century, Japanese scientist Kikunae Ikeda identified a fifth taste — umami — from the Japanese word meaning "pleasant savoury taste." Umami is the taste of glutamate, an amino acid naturally present in foods like meat, fish, mushrooms, tomatoes and soy sauce.
Some researchers suggest there may be additional basic tastes — including responses to fat, starch or even water — but these are still under investigation. What is clear is that the "sweet, salty, sour, bitter and umami" framework represents the taste system's primary language, and infinite variety of flavour comes from combining these five signals with the thousands of detected aromas.
Flavour: Smell and Taste Combined
What we call "flavour" is largely constructed by the brain from smell and taste signals together. Taste alone gives a relatively coarse picture — just those five basic qualities. The extraordinary richness of flavour — the difference between a Granny Smith apple and a Golden Delicious, between mozzarella and cheddar — comes almost entirely from volatile molecules detected by the nose.
This happens partly through retronasal olfaction: when you chew and swallow, volatile molecules from food travel up through the back of the throat and into the nasal cavity from the inside. Your olfactory system detects them just as it does inhaled scents, but the brain integrates this signal with the taste signal and perceives it as "flavour" rather than "smell."
This is why blocking your nose almost completely eliminates the ability to tell different foods apart by flavour — the taste signal alone provides far less information than the combined taste-plus-smell experience.
Smell, Taste and Wellbeing
The chemical senses play an important role in appetite, nutrition and safety. Appetite is strongly driven by smell — the enticing aroma of food triggers salivation, stomach acid production and the release of digestive hormones that prepare the body for eating.
The sense of smell also serves as a warning system, detecting smoke, gas leaks, spoiled food and other environmental dangers. Bitter taste, similarly, evolved partly as a defence against ingesting plant toxins — many poisonous plant compounds are intensely bitter, prompting spitting or rejection.
Loss of smell (anosmia) or taste (ageusia) — which can follow viral infections, head injuries or other causes — can significantly affect quality of life, appetite, safety and emotional wellbeing. If you experience sudden unexplained loss of smell or taste, speaking to a healthcare professional is advisable.
To explore the other senses, visit our guides on eyes and vision and ear anatomy. Use the anatomy quiz to test your knowledge, and look up unfamiliar terms in the anatomy glossary.