CorpusHuman Anatomy
Special SensesCell

Taste receptor cells

Also known as: gustatory cells, type I cells, type II cells, type III cells, sweet receptor, umami receptor, bitter receptors, T1R, T2R, gustducin, ENaC, OTOP1, taste transduction

The specialised epithelial cells of taste buds that transduce sweet, umami and bitter (type II, via G-protein-coupled receptors), sour (type III, via the OTOP1 proton channel) and salty (via ENaC) stimuli into transmitter release onto gustatory nerve fibres.

Show on body map Smell and Taste — Chemoreceptors

Overview

Type I cells (≈50% of the bud) are glia-like: they wrap other cells, clear transmitters (ATPase NTPDase2, glutamate transporters) and buffer K⁺; some may take part in salt taste.

Type II (receptor) cells (≈20–30%) express G-protein-coupled taste receptors, each cell one quality: sweet — T1R2 + T1R3 heterodimer (sugars, artificial sweeteners, some proteins); umami — T1R1 + T1R3 (L-glutamate, potentiated by the nucleotides IMP and GMP); bitter — T2Rs (≈25 TAS2R genes; recognise thousands of structurally diverse, often toxic compounds). Activation releases the G protein gustducin; its βγ subunits activate phospholipase Cβ2 → IP₃ → IP₃R3-mediated Ca²⁺ release from ER → Ca²⁺ opens TRPM5 (and TRPM4) cation channels → depolarisation → action potentials → ATP is released not by vesicles but through the large-pore CALHM1/CALHM3 channel ("channel synapse") onto P2X2/P2X3 receptors of afferent fibres.

Type III (presynaptic) cells (≈2–20%) detect sour: H⁺ enters through the proton channel OTOP1 (and weak acids diffuse in), intracellular acidification blocks an inward-rectifier K⁺ channel (Kir2.1), the cell depolarises, fires action potentials and releases serotonin (and GABA, noradrenaline) at conventional vesicular synapses. Type III cells also respond to carbonation (carbonic anhydrase 4) and to high salt.

Salt: low (appetitive) NaCl concentrations are detected through the amiloride-sensitive epithelial sodium channel (ENaC) in a dedicated subset of taste cells (current evidence suggests type II-like cells signalling via CALHM1/3); Na⁺ influx depolarises directly. High (aversive) salt recruits bitter and sour cells. Candidate additional qualities include fat (fatty-acid receptors CD36, GPR120), kokumi and water.

Location

Within taste buds of the tongue papillae, soft palate, pharynx and epiglottis. Taste receptors (T1Rs, T2Rs) are also expressed outside taste buds — e.g. gut enteroendocrine cells and airway epithelium (bitter receptors stimulating ciliary beating).

Parts & subdivisions

  1. Type I (glial-like)
    Supporting cells; transmitter clearance and ion homeostasis.
  2. Type II — sweet
    T1R2/T1R3 → gustducin → PLCβ2 → IP₃ → Ca²⁺ → TRPM5 → ATP via CALHM1/3.
  3. Type II — umami
    T1R1/T1R3 for L-glutamate (+IMP/GMP synergy); same cascade.
  4. Type II — bitter
    ≈25 T2Rs; same cascade; genetic variation (e.g. TAS2R38 for PTC/PROP "supertasters").
  5. Type III — sour
    OTOP1 proton channel → depolarisation → serotonin release at conventional synapses.
  6. Salt cells
    ENaC (amiloride-sensitive) for low-concentration Na⁺.
  7. Type IV (basal)
    Progenitor cells that replace the others.

Function

  • Transduce chemical tastants into neurotransmitter release
  • Encode the five basic taste qualities (largely one quality per cell)
  • Initiate cephalic-phase digestive and metabolic responses
Physiology

Type II cells lack conventional synaptic vesicles; their ATP release through CALHM1/3 is essential for sweet, umami and bitter taste (knockout mice are "taste-blind" to these). Taste cells fire action potentials despite being epithelial. Gustatory afferents respond preferentially but not exclusively to one quality.

Clinical significance

Genetic variation in taste

Polymorphisms in TAS2R38 determine whether people taste phenylthiocarbamide (PTC) and propylthiouracil (PROP) as bitter; "supertasters" have more fungiform papillae and often dislike bitter vegetables.

Artificial sweeteners and miracle fruit

Non-caloric sweeteners activate T1R2/T1R3 at very low concentrations; miraculin (miracle fruit) makes acids taste sweet by activating the sweet receptor at low pH; gymnemic acid blocks sweet taste.

Test yourself

Sour taste is transduced primarily by:
  1. T1R2/T1R3 receptors on type II cells
  2. T2R receptors on type II cells
  3. ENaC on type I cells
  4. The OTOP1 proton channel on type III cells

Answer: The OTOP1 proton channel on type III cells

H⁺ enters type III (presynaptic) cells through OTOP1, depolarising them; they release serotonin at conventional synapses. Sweet (T1R2/T1R3), umami (T1R1/T1R3) and bitter (T2Rs) use type II cells and ATP release through CALHM1/3.

More special senses questions

Compiled from standard anatomy and physiology references — see the Special Senses sources. For education only; not medical advice.