CorpusHuman Anatomy

Smell and Taste — Chemoreceptors

(a) Olfactory epithelium and bulb · (b) Taste bud in a papilla

Panel (a) shows the roof of the nasal cavity in section. Odorant molecules rise from the inspired (or retronasal) air and dissolve in the mucus secreted by Bowman glands, where they bind receptors on the non-motile cilia of olfactory receptor neurons. Each neuron expresses one receptor type (colour-coded); their axons pass through the cribriform plate of the ethmoid as the olfactory nerve fila and — although the neurons are scattered across the epithelium — all axons of one type converge on the same glomerulus in the olfactory bulb, where they synapse on mitral cells whose axons form the olfactory tract. Supporting (sustentacular) cells and basal stem cells, which replace neurons every 1–2 months, complete the epithelium.

Panel (b) shows a taste bud embedded in the stratified epithelium of a papilla (the locator shows a circumvallate papilla with taste buds in the walls of its trench). Tastants dissolved in saliva enter the taste pore and act on the microvilli of type II receptor cells (sweet, umami, bitter) and type III presynaptic cells (sour); type I cells support them and basal cells regenerate the bud every ≈8–12 days. Afferent fibres of cranial nerves VII, IX and X carry the signal towards the nucleus of the solitary tract.

Step-by-step

  1. Smell 1 · Odorants dissolve in mucus

    Odorants reach the olfactory region in the roof of the nasal cavity (sniffing creates eddies) and dissolve in the mucus secreted by Bowman glands, aided by odorant-binding proteins.

  2. Smell 2 · Binding to receptor cilia

    Odorants bind G-protein-coupled receptors on the cilia of olfactory receptor neurons. G_olf activates adenylyl cyclase III; cAMP opens cyclic-nucleotide-gated channels (Na⁺, Ca²⁺ in) and Ca²⁺-activated Cl⁻ efflux amplifies the depolarisation. Each neuron expresses just one of ≈350–400 receptor types.

  3. Smell 3 · Supporting and basal cells

    Supporting cells insulate and detoxify; basal stem cells replace receptor neurons every 1–2 months.

  4. Smell 4 · Through the cribriform plate

    Unmyelinated axons gather into ≈20 olfactory fila per side (together the olfactory nerve, CN I) and pass through the cribriform plate of the ethmoid. Shearing of these fila in head injury causes anosmia.

  5. Smell 5 · Convergence on glomeruli

    All neurons expressing the same receptor — scattered across the epithelium — converge on the same glomerulus, creating a spatial odour map in the olfactory bulb.

  6. Smell 6 · Mitral cells to the cortex

    Mitral and tufted cells, sharpened by granule-cell lateral inhibition, send axons in the olfactory tract directly to the piriform cortex, amygdala and entorhinal cortex — the only sensory pathway without a first thalamic relay.

  7. Taste 1 · Tastants enter the taste pore

    Tastants dissolved in saliva diffuse through the taste pore to the microvilli of taste cells. Taste buds sit mainly on fungiform, foliate and circumvallate papillae (filiform papillae have none).

  8. Taste 2 · Receptor cells transduce

    Type II cells detect sweet (T1R2/T1R3), umami (T1R1/T1R3) and bitter (T2Rs) via gustducin → PLCβ2 → IP₃ → Ca²⁺ → TRPM5, releasing ATP through CALHM1/3 channels. Type III cells detect sour through the OTOP1 proton channel and release serotonin at conventional synapses; low salt enters via ENaC.

  9. Taste 3 · Supporting and basal cells

    Type I glia-like cells support the bud and clear transmitter; basal cells renew the 50–100 cells of each bud every ≈8–12 days.

  10. Taste 4 · Afferent fibres to the brainstem

    Gustatory fibres of VII (anterior two-thirds of the tongue), IX (posterior third) and X (epiglottis, pharynx) carry signals to the nucleus of the solitary tract, then via the VPM thalamus to the insular–opercular gustatory cortex.

Labelled parts