Taste and Smell Physiology
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Direct answer
Taste begins with five qualities — sweet, sour, salty, bitter and umami — encoded by about 10,000 taste buds whose receptor cells release ATP onto afferent fibres of the facial (anterior two-thirds of tongue), glossopharyngeal (posterior one-third) and vagus (epiglottis) nerves, relaying in the nucleus tractus solitarius, then the VPM thalamus, then the gustatory cortex. Smell begins with olfactory receptor neurons in the cribriform plate epithelium, the only afferent pathway that reaches cortex without a prior thalamic relay, and it supplies most of what we call flavour through retronasal flow.
What you must remember
- Taste buds: fungiform papillae (anterior tongue, chorda tympani of CN VII), circumvallate and foliate (posterior, CN IX), plus a few epiglottic buds (CN X); taste cells turn over about every 10 days.
- Receptor mechanisms: sweet (T1R2/T1R3), umami (T1R1/T1R3), bitter (about 25 T2R types), salty largely via ENaC sodium entry, sour via hydrogen ions acting on proton channels and blocking potassium channels.
- Detection thresholds (Ganong): quinine about 0.000008 M, hydrochloric acid 0.0009 M, sucrose 0.01 M, sodium chloride 0.01 M — bitter sensitivity is exquisite, a poison-detection legacy.
- Taste pathway: afferents to the gustatory nucleus of the NTS, then (via the central tegmental tract) to the contralateral VPM, then to the gustatory cortex in the frontal operculum and insula.
- Olfactory receptor neurons are bipolar neurons with cilia, G-protein Golf, adenylyl cyclase, cAMP-gated channels; roughly 400 functional receptor gene types, each neuron expressing one.
- Olfactory receptor neurons regenerate from basal cells every 30–60 days — the only mammalian neurons routinely replaced.
- Olfaction reaches the piriform cortex, amygdala and entorhinal area directly, without a thalamic relay first — standard viva distinction.
- Anosmia plus hypogonadotrophic hypogonadism defines Kallmann syndrome, because GnRH neurons migrate from the same olfactory placode; cribriform plate shearing in head injury is the commonest acquired cause.
Why curry has no taste when you have coryza
Plug your nose, chew a piece of apple and an onion; most people cannot tell them apart. The demonstration proves that what we call taste while eating is mostly retronasal olfaction — odour molecules released by chewing and warming travel from the pharynx up behind the soft palate to the olfactory epithelium during deglutition and exhalation. In coryza, the swollen mucosa blocks this route even though the taste buds on the tongue work perfectly, which is why food goes flat. The same anatomy explains why flavour is lost early in COVID-19-associated anosmia and recovered last.
Walk an odour molecule the rest of the way. It dissolves in olfactory mucus, binds its specific receptor on a ciliary membrane, Golf activates adenylyl cyclase, the generated cAMP opens cation channels, calcium-activated chloride efflux amplifies the depolarisation, and the neuron fires. Glomeruli in the olfactory bulb collect inputs from neurons sharing a receptor type, mitral cells carry the pattern to piriform cortex — and, unusually, directly to limbic structures. That anatomical shortcut to the amygdala and entorhinal cortex is why a single odour can trigger a vivid emotional memory before it is consciously named.
High-yield traps in this chapter
First, the tongue map of school textbooks is wrong: all five qualities can be detected on all innervated regions, with regional thresholds differing only modestly. Second, chorda tympani injury (middle-ear surgery) loses taste from the anterior two-thirds only, not the whole tongue — a question asked as often by ENT vivas as physiology. Third, do not confuse taste cells with taste neurons; the receptor cell is an epithelial cell that signals via ATP, and only cranial nerve fibres carry the message centrally.
Frequently asked questions
Which cranial nerves carry taste, and from where?
The facial nerve (chorda tympani) from the anterior two-thirds of the tongue, glossopharyngeal from the posterior one-third, and vagus from the epiglottic area.
What is the taste pathway to the cortex?
Afferents synapse in the gustatory part of the nucleus tractus solitarius, project to the contralateral VPM nucleus of the thalamus, and end in the gustatory cortex of the insula and frontal operculum.
Why is smell considered unique among sensory systems?
Its afferents reach the cerebral cortex (piriform, amygdala, entorhinal) without first relaying in the thalamus, and its receptor neurons regenerate throughout life.
Which taste has the lowest detection threshold, and why?
Bitter — quinine is detectable near 8 nanomolar — because bitter signalling evolved to detect potentially toxic plant alkaloids at vanishing concentrations.
What explains anosmia in Kallmann syndrome?
GnRH-secreting neurons and olfactory neurons both arise from the olfactory placode; failed migration leaves both the sense of smell and puberty absent.