Taste and Smell

Smell (Olfaction)

• Chemoreceptors activated by chemicals dissolved in mucus.
• Olfactory epithelium on roof of nasal cavity & superior nasal conchae.

Olfactory Epithelium

• Contains olfactory sensory neurons, supporting cells, and stem cells (continual replacement).
  • Olfactory sensory neurons: Bipolar neurons with a dendrite extending to the epithelial surface, ending in olfactory cilia that house receptor proteins. Axons project through the cribriform plate to the olfactory bulb.
  • Supporting cells: columnar cells that surround and support the olfactory sensory neurons, producing mucus.
  • Stem cells (basal cells): located at the base of the epithelium, differentiate into new olfactory sensory neurons every $30 ext{–}60$ days.

Specificity

• Humans detect $\approx 10\,000$ odors.
• $\sim 400$ olfactory genes expressed only in nose.
• Nasal mucosa also hosts pain & temperature receptors.

Physiology of Smell

• Odorant must be gaseous & dissolve in mucus.

• Binding to cilia receptors

• depolarization of sensory neuron.

  • Odorant binding activates a G protein (G-olf)

• activates adenylyl cyclase

• forms cAMP

• opens $\text{Na}^+$ and $\text{Ca}^{2+}$ channels

• depolarization and generator potential. If threshold is reached, action potentials are generated.

Olfactory Pathway

• Receptor axons

• glomeruli in olfactory bulb (converge by receptor type).

• Mitral cells amplify/refine; inhibited by GABA from amacrine cells

• only strong signals proceed.

• Mitral axons (olfactory tracts) project to:

  • Primary olfactory cortex (perception)
  • Frontal lobe (conscious ID)
  • Hypothalamus, amygdala, limbic system (emotion/memory)

Taste (Gustation)

• Chemoreceptors in taste buds; mainly on fungiform, foliate, circumvallate papillae; few elsewhere (soft palate, cheeks, pharynx, epiglottis).
  • Fungiform papillae: Mushroom-shaped, scattered over tongue surface, contain $1 ext{–}5$ taste buds each.
  • Foliate papillae: Leaf-like folds on lateral edges of posterior tongue, contain taste buds primarily in children.
  • Circumvallate papillae: Large, round papillae in a V-shape at the back of the tongue, each surrounded by a trench, house many taste buds on their lateral walls.

Taste Bud Structure

• Each bud $\approx 50!–!100$ epithelial cells:
  • Gustatory cells (microvilli = taste hairs); $3$ types (one releases serotonin, one ATP).
  • Type I (Supportive cells): Elongated, slender cells that provide support, possibly detect salty taste.
  • Type II (Receptor cells): Detect sweet, bitter, and umami tastes; release ATP as a neurotransmitter to activate sensory neurons.
  • Type III (Presynaptic cells): Detect sour taste ($\text{H}^+$); release serotonin as a neurotransmitter at synapses with sensory neurons.
  • Basal cells (stem cells) renew every $7 ext{–}10$ days.

Basic Taste Sensations

• Sweet: sugars, saccharin, alcohol, some AAs, lead salts.
• Sour: $\text{H}^+$ (acids).
• Salty: metal ions.
• Bitter: alkaloids (quinine, nicotine), aspirin.
• Umami: glutamate, aspartate (meaty/cheesy).
• Possible sixth: long-chain fatty acids (lipids).

Physiology of Taste

• Tastant must dissolve in saliva

• diffuse into taste pore

• contact gustatory hairs.

• Reflex effects: $\uparrow$ saliva, $\uparrow$ gastric juice; protective gag/vomit.

  • Salivation helps dissolve tastants and cleanse the taste buds, while gastric juices prepare the digestive system. Protective reflexes prevent ingestion of harmful substances.

Taste Transduction

• Salty: $\text{Na}^+$ influx

• depolarization.

• Sour: $\text{H}^+$ opens cation channels.

• Sweet/Bitter/Umami: GPCR (gustducin)

• intracellular $\text{Ca}^{2+}$ release

• ATP neurotransmitter.

  • For sweet, bitter, and umami, tastant binding activates a G protein (gustducin) that triggers various intracellular signaling cascades (e.g., release of intracellular $\text{Ca}^{2+}$, inhibition of $\text{K}^+$ channels), ultimately leading to ATP release from the gustatory cell.

Gustatory Pathway

• Cranial nerves Facial (VII) & Glossopharyngeal (IX) from tongue; Vagus (X) from epiglottis/pharynx.

• Route: taste buds

• solitary nucleus (medulla)

• thalamus

• gustatory cortex (insula).

• Collaterals to hypothalamus & limbic system for taste appreciation & emotion.