The reasoning: two chemical senses, very different designs
Olfaction: ~350–400 functional odorant receptor genes (largest human GPCR family); each olfactory sensory neuron expresses one receptor; all neurons with the same receptor converge on ~2 glomeruli in the bulb (combinatorial code). Cascade: Golf → adenylyl cyclase III → cAMP → CNG channel (Na⁺, Ca²⁺) → Ca²⁺‑activated Cl⁻ efflux (ANO2) amplifies depolarisation. Adaptation via Ca²⁺–calmodulin. Bulb (mitral/tufted cells, periglomerular and granule inhibition) → lateral olfactory tract → piriform cortex, amygdala, entorhinal cortex (no obligatory thalamic relay) → orbitofrontal cortex (via MD thalamus and directly) for identification. Olfactory neurons regenerate from basal stem cells (~30–60 days). Trigeminal chemesthesis (ammonia, menthol, capsaicin) is separate.