Physiology of Smell and Taste

Physiology of Smell and Taste - Scent Structures & Signals

• Olfactory Epithelium: Located in nasal cavity roof. 📌 Mnemonic: "Old Sexy Basal Bows" for cell types (Olfactory, Supporting, Basal, Bowman's). Contains:
  • Olfactory Receptor Neurons (ORNs): Bipolar neurons; cilia detect odorants. Axons form CN I. Regenerate every 30-60 days.
  • Supporting (Sustentacular) Cells: Provide structural & metabolic support.
  • Basal Cells: Stem cells for ORNs.
  • Bowman's Glands: Secrete mucus to dissolve odorants.

• Odorant Receptors: Approximately 400 types of G-protein coupled receptors (GPCRs) on ORN cilia. Each ORN typically expresses one receptor type.

• Olfactory Transduction Pathway:

• Olfactory Bulb: First processing site. ORN axons synapse in glomeruli with mitral and tufted cells. Each glomerulus receives input from ORNs expressing the same receptor type.

⭐ The olfactory pathway is unique as many of its fibers bypass the thalamus to directly reach the primary olfactory cortex (e.g., piriform cortex, amygdala).

Olfactory Pathways & Clinicals - Nose Notes & Neuro Nods

• ORNs (epithelium) → cribriform plate → olfactory bulb (glomeruli). 1st order.
• Mitral/Tufted cells (2nd order) → olfactory tract → primary olfactory cortex (piriform, amygdala, entorhinal).
  • Unique: NO direct thalamic relay to primary cortex for conscious smell.
• Secondary: Orbitofrontal cortex (via MDN thalamus for discrimination), hippocampus.
• Clinicals:
  • Anosmia: Loss of smell. Causes: URI, trauma, polyps, Kallmann Syn. 📌 COVID-19 is a key cause.
  • Hyposmia (↓), Parosmia (distorted), Phantosmia (hallucination).
  • Olfactory groove meningioma: Can cause progressive anosmia.

⭐ Foster Kennedy Syndrome: Olfactory groove meningioma → ipsilateral anosmia & optic atrophy, contralateral papilledema.

Gustatory System & Transduction - Taste Bud Breakdown

• Taste Buds: Sensory organs on lingual papillae (fungiform, foliate, circumvallate), palate, epiglottis. Each 50-100 cells.

  • Cells: Type I (support, salt?), II (receptors: sweet, umami, bitter - GPCRs), III (presynaptic: sour - ion channels), IV (basal).

• Papillae Types:

  • Fungiform (anterior), Foliate (lateral), Circumvallate (posterior, most buds). Filiform (no buds, mechanical).

• Taste Transduction Table:

  Modality	Receptor(s)	Pathway Key Steps
  Sweet	T1R2+T1R3 (GPCR)	Gα-gustducin → ↑cAMP
  Umami	T1R1+T1R3 (GPCR)	Gα-gustducin → ↑IP₃/DAG → $Ca^{2+}$ release
  Bitter	T2Rs (GPCR)	Gα-gustducin → ↑IP₃/DAG → $Ca^{2+}$ release
  Salty	ENaC	$Na^{+}$ influx → Depolarization
  Sour	OTOP1/PKD2L1	$H^{+}$ influx / $K^{+}$ block → Depolarization

⭐ Fact: Circumvallate papillae (V-shape, posterior tongue) house the most taste buds.

Gustatory Pathways & Flavor - Tongue Trails & Taste Twists

• Taste Innervation & Central Route:
  • Anterior 2/3 tongue: CN VII (Chorda tympani).
  • Posterior 1/3 tongue: CN IX.
  • Epiglottis/Palate: CN X.
  • All synapse in Nucleus of Solitary Tract (NST), medulla.
  • NST → Thalamus (VPM) → Gustatory Cortex (Insula, Frontal Operculum).
• Flavor: More Than Taste:
  • Crucial interplay: Taste (gustation) + Smell (olfaction).
  • Other inputs: Texture, temperature, chemesthesis (e.g., capsaicin).
• Key Disorders:
  • Ageusia: Total taste loss.
  • Hypogeusia: Reduced taste.
  • Dysgeusia: Distorted taste.

⭐ The insula and frontal operculum constitute the primary gustatory cortex; lesions can cause significant taste impairment.

High‑Yield Points - ⚡ Biggest Takeaways

• Olfactory epithelium (superior nasal cavity) houses bipolar receptor neurons using GPCRs.
• Olfactory pathway largely bypasses thalamus, projecting to piriform cortex & amygdala.
• Taste buds on fungiform, foliate, circumvallate papillae contain gustatory cells.
• Five basic tastes: sweet, sour, salty, bitter, umami.
• Taste transduction: Salty/Sour via ion channels; Sweet/Bitter/Umami via GPCRs.
• Cranial nerves for taste: CN VII (anterior 2/3 tongue), CN IX (posterior 1/3 tongue), CN X (epiglottis).
• Gustatory pathway: Nerves → Nucleus Solitarius (medulla) → Thalamus (VPM) → Insular cortex (gustatory cortex).