Chemodetection: taste and olfaction as threshold switches

Taste and olfaction are threshold switches. A taste GPCR (T1R2/T1R3 sweet, T1R1/T1R3 umami) drives PLCβ2 and opens the TRPM5 cation channel past a concentration threshold — verified bistable, slope 3.12. Olfaction uses an OR GPCR → cAMP → CNG channel of the same superfamily as the rod, making vision and smell share one transducer primitive.

Chemodetection converts ligand concentration into an all-or-none channel opening. The taste pathway (T1R receptors → PLCβ2 → TRPM5) is simulated as an R19 cooperative gate, bistable with maximum slope 3.12 and hysteresis ≈1.49; EC50 is a cited input [L]. Olfaction's CNGA2 channel belongs to the same CNG superfamily as the rod's CNGA1 — a literal common switch across two senses; its master γ is owned by the DNA pipeline.

Taste is a receptor that opens a downstream channel. Sweet and umami are read by T1R-family GPCRs (bitter by T2Rs), which activate PLCβ2 and open the TRPM5 cation channel; the receptor sets specificity while TRPM5 carries the all-or-none current. Detection is therefore a threshold event, not a smooth ramp.

Taste detection is a cooperative threshold crossing

Cast as an R19 gate, the taste switch is bistable with maximum slope 3.12 and hysteresis width about 1.49, and the concentration–response is sigmoidal. The half-maximal concentration (EC50, in the millimolar range for sweet and umami, lower for bitter) is a cited physiological input [L], while the verified result is the switch structure.

Smell runs on the same channel family as vision

Odorant binding to an OR GPCR raises cAMP, which opens the olfactory CNG channel CNGA2 — the same cyclic-nucleotide-gated superfamily as the rod's CNGA1 (see phototransduction). Combinatorial coding across roughly 400 human odorant receptors then yields odour identity from a shared switch primitive.