Taste and Smell

Chemosensation: Overview - Chemical Detectives

• Chemosensation: body's detection of environmental chemical stimuli.
• Relies on specialized sensory cells: chemoreceptors.
• Crucial for:
  • Identifying food (nutrition).
  • Detecting harmful substances (danger avoidance).
  • Influencing social behaviors.
• Main modalities:
  • Gustation (Taste): Senses dissolved tastants in the mouth.
  • Olfaction (Smell): Senses airborne odorants in the nasal cavity.
• Both systems convert chemical data into neural signals.

⭐ Primary chemosensory modalities are taste (gustation) and smell (olfaction), both critical for nutrition and danger avoidance.

Gustatory System: Taste - Tongue Twisters

• Basic Tastes: Salty, Sour, Sweet, Bitter, Umami.
• Taste Receptors: Taste buds in papillae.
  • Fungiform: Anterior 2/3 (CN VII).
  • Foliate: Posterolateral (CN IX).
  • Circumvallate: Posterior 1/3 (CN IX).
  • (Filiform: mechanical, no taste buds).
• Transduction:
  • Salty: $Na^+$ influx via ENaC.
  • Sour: $H^+$ influx (PKD2L1); $K^+$ channel block.
  • Sweet: T1R2+T1R3 (GPCRs).
  • Umami: T1R1+T1R3 (GPCRs).
  • Bitter: T2Rs family (GPCRs); Gustducin.
• Innervation:

  ⭐ Taste sensation from the anterior two-thirds of the tongue is carried by the chorda tympani branch of the Facial nerve (CN VII), posterior one-third by the Glossopharyngeal nerve (CN IX), and epiglottis/pharynx by the Vagus nerve (CN X).

• 📌 Pathway: "Solitary Man in Thalamus loves Insula(ted) Food" (Solitary Nuc. → VPM Thalamus → Gustatory Cortex).

Olfactory System: Smell - Nosey Neurons

• Epithelium (Sup. Nasal Cavity):
  • ORNs: Bipolar, ciliated (GPCRs), regenerate (basal cells).
  • Supporting cells: Support.
• Odorant Receptors: ~400 types of GPCRs; one type/ORN.
• Transduction: Odorant → GPCR (Golf) → ↑Adenylyl Cyclase → ↑$cAMP$ → Cation channel opening → Depolarization → AP.

⭐ Olfactory receptor neurons are unique bipolar neurons that are replaced throughout life (neurogenesis) and their axons form the olfactory nerve (CN I), synapsing in the olfactory bulb.

• Bulb & Pathway:
  • ORN axons (CN I) → Olfactory bulb glomeruli (synapse w/ mitral/tufted cells).
  • Bulb → Tract → Primary olfactory cortex (piriform, amygdala); then Thalamus (MDN) → OFC (perception).

• 📌 Kallmann's: Anosmia + hypogonadism.

Taste & Smell: Clinicals - When Senses Stray

• Olfactory Disorders (Smell):
  • Anosmia: Complete loss.
  • Hyposmia: Reduced ability.
  • Hyperosmia: Increased acuity.
  • Dysosmia (Parosmia/Cacosmia): Distorted perception.
  • Phantosmia: Olfactory hallucination (odor absent).
  • Olfactory Agnosia: Inability to classify/contrast odors.
  • Common Causes: URI (most frequent), head trauma, nasal/sinus polyps, neurodegenerative (Parkinson's, Alzheimer's), toxins, congenital.
• Gustatory Disorders (Taste):
  • Ageusia: Complete loss.
  • Hypogeusia: Reduced sensitivity.
  • Hypergeusia: Enhanced sensitivity.
  • Dysgeusia: Distorted perception (e.g., metallic taste).
  • Phantogeusia: Gustatory hallucination (taste absent).
  • Common Causes: Medications (ACE inhibitors, chemotherapy, metronidazole), infections (oral, COVID-19), nutritional deficiencies (Zinc, Vit B12), nerve damage (CN VII, IX), radiation, Sjögren's.

⭐ Kallmann syndrome is a genetic disorder characterized by anosmia (or hyposmia) and hypogonadotropic hypogonadism, due to failed embryonic migration of GnRH-releasing neurons.

High‑Yield Points - ⚡ Biggest Takeaways

• Taste buds are located on fungiform, foliate, and circumvallate papillae; filiform papillae lack taste buds.
• The five primary taste sensations are sweet, sour, salty, bitter, and umami.
• Taste innervation: CN VII (anterior 2/3 tongue), CN IX (posterior 1/3 tongue), CN X (epiglottis & pharynx).
• Olfactory receptor cells are bipolar neurons located in the olfactory epithelium.
• Odorants bind to G-protein coupled receptors (GPCRs) on the cilia of olfactory receptor cells.
• The olfactory pathway is unique as it projects to the piriform cortex before the thalamus for conscious perception.
• Anosmia refers to the loss of smell; ageusia refers to the loss of taste sensation.