Olfactory Epithelium and Smell Perception

• Olfactory receptors, supporting cells, and basal cells make up the olfactory epithelium.

Cells of the epithelium:

• Olfactory receptors are the first-order neurons of the olfactory pathway
• Basal cells continually divide to make new olfactory receptors and supporting cells
• Supporting cells give support and insulation
• Every 30-60 days in adults, the olfactory epithelium is replaced
• Cranial nerve VII innervates the epithelium and glands.

Smell Perception:

• The ability to recognize 10,000 different odors depends on patterns of activity.
• Olfactory transduction starts with an odorant molecule binding to a transmembrane receptor protein.
• This is coupled to a G protein, which then activates adenylate cyclase.
• Adenylate cyclase produces cAMP, which opens sodium ion channels, and resulting sodium inflow causes a depolarizing generator potential.
• Only a few molecules are required for an action potential because the adaptation occurs quickly and the threshold of smell is low.
• Added to natural gas, methyl mercaptan provides warning of gas leaks due to its extremely low threshold

Olfactory Pathway:

• Olfactory receptors synapse in the olfactory bulb
• Olfactory nerves pass through the cribriform plate
• Through the olfactory tract, olfactory sensations reach the cerebral cortex without synapsing in the thalamus
• Projections to the limbic system and hypothalamus account for emotional and memory responses.
• The orbitofrontal area identifies and discriminates odors
• Noxious stimuli picked up by trigeminal nerve in the posterior nasal cavity

Olfactory Bulb Processing:

• Olfactory receptor neurons send input to the glomerular layer of the olfactory bulb, which is also connected by interneurons
• Mitral cells filter and boost the signal to noise ratio with granule and periglomerular cells.
• Mitral and granule cells release GABA.

Olfactory Dysfunction:

• Anosmia is absence of smell sensation
• Reduced ability to smell, distortion of smell sensation and the perception of smell in absence of odorant are types of olfactory issues
• Early Parkinson disease, Alzheimer disease and Huntington's can damage smell ability.
• Parosmia is distortion of smell sensation with Cacosmia characterized as perception of a bad or foul smell.

Gustation

• Considered a chemical sense, as molecules must be dissolved in saliva
• Taste sensations are evenly distributed on the tongue with overlap
• Many flavors identified attributed to smell rather than the tongue
• Activated by metal ions, hydrogen ions, alkaloids (caffeine, nicotine, etc.) and L-glutamate.
• Roughly10,000 taste buds found on tongue, soft palate, epiglottis & pharynx
• Taste sensitivity decreases with age.

Papillae

• Structures that increase surface area to contact potential tastes
• 3 types are Vallate, Fungiform, and Filiform

Taste Buds

• Oval bodies composed of gustatory receptor cells, supporting cells, and basal cells
• Gustatory and supporting cells are replaced every 10 days
• Gustatory hairs respond to tastants, and result in the release of neurotransmitters and the development of receptor potentials
• The most sensitive taste is bitter
• Salty food affect depolarization via Na+ channels in the plasma membrane
• Sour tastants affect depolarization via cells via H+ channels
• Sweet, bitter, and umami tastants bind to G-protein receptors, activate gustducin and increase certain intracellular molecules

Taste Thresholds:

• Individual preferences exist based on the distribution of taste buds
• The bitter taste has the lowest threshold, while salty and sweet have the highest

Gustatory Pathway:

• First-order fibers are found in the facial, glossopharyngeal, and vagus cranial nerves
• Insula, in temporal lobe contains the primary gustatory cortex
• Fibers extend to the thalamus, primary gustatory area, limbic system & hypothalamus
• The thalamus also projects to the insula, temporal lobe and the medulla regulates reflexes like salivation.

Taste and Smell Summary Comparison

• Olfactory threshold is fast, and gustatory is 10 days
• Olfactory uses methyl mercaptan, gustatory uses separate receptors
• Olfactory involves cranial nerve > bulb, gustatory uses CN VII, IX, X
• Olfactory goes from the olfactory tract to the limbic system, gustatory uses medulla, salivary reflex.
• The temporal lobe for olfaction, thalamus (VPN), limbic, and hypothalamus for gustatory
• Olfactory epithelium is replaced monthly, and separate cells are replaced every 10 days in gustatory
• 3 papillae types exist for gustatory