Smell and taste are chemical senses: their receptors respond to molecules dissolved in mucus or saliva. Much of what we call flavor is actually smell.
Smell
Olfactory receptor cells sit in the roof of the nasal cavity and send fibers through the cribriform plate to the olfactory bulb. This is cranial nerve I, and its signals reach the memory and emotion areas of the brain directly.
Taste
Taste buds on the tongue and palate detect sweet, salty, sour, bitter and umami. The front two thirds of the tongue is served by the facial nerve (VII) and the back third by the glossopharyngeal nerve (IX).
Flavor
When the nose is blocked, food tastes bland because aroma from the mouth cannot reach the smell receptors. Many patients who report loss of taste actually have loss of smell.
Losing smell
Viral infections such as COVID-19, nasal polyps, head injury and aging can reduce smell. Early loss of smell can also appear in Parkinson disease and Alzheimer disease.
At the bedside
Test smell one nostril at a time with a familiar, non-irritating scent such as coffee. A head injury that breaks the cribriform plate can cause both loss of smell and a leak of cerebrospinal fluid from the nose.
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Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).