Taste and Smell (Chapter 6b)

Components of the Tongue:

• Tip of tongue::fungiform papilla; do not contain very many taste buds::neurons in high density that code for the perception of taste

• Vallate papilla::at the back of the tongue, responds to bitter taste and works with the gag reflex (since bitterness is tied to poison aversion)

• Different proportions of cells associated w/individual taste categories - salty, sweet, sour, bitter, umami

The 5 Tastes:

• Salty::opening sodium and chloride channels results in net depolarization

• Sour::driven by hydrogen ions in sour foods, results in net depolarization

• Sweet::T1R receptor family, T1R2 and T1R3 pair together in a heterodimer and fuse and cause a metabotropic effect of perception of sweet taste

• Bitter::T2R receptor family - 30 members that are all unique

• Umami::heterodimer of T1R1 and T1R3 paired w/glutamate - think foods with MSG in them

Supertasters:

• We all have a different genetic array for taste

• Non-tasters have far fewer papillae (therefore taste buds) and supertasters have more papillae

• Supertasters avoid bitter/spicy foods unless culture has trained them that spice is integral to diet

• Taste cells are susceptible to damage/death via temp → we lose around 50% of our taste cells by age 20

Pathway of Taste:

• Taste is a chemical sense - food dissolved into molecules w/saliva; the tongue perceives touch as well - 3 cranial nerves are involved; info sent to somatosensory cortex about texture; info about temp; pain pathway; insula::tied to disgust

• 2 main pathways

• Ispsilateral connections

• Info sent to orbital prefrontal cortex::determines whether the taste is good or bad

Taste and Behavior:

• Taste perceptions and financial cost - things that are more expensive taste better

• Volkow research

• Carnivores (lots of bitter receptors) vs herbivores (almost no bitter receptors) vs omnivores (blend)

Olfaction:

• Olfactory receptor cells are just exposed to air

  • Cluster batches of neurons in the olfactory bulb (glomeruli) and synapse and go out to the mitral cells in the brain

    • Mitral cells::enter the brain and go directly to the amygdala or take a secondary path and goes into the primary olfactory cortex (skips the thalamus)

• Perception of around 10,000 smells in humans

• Orbitofrontal cortex::determines pleasant or aversive smells

• Amygdala::determines intensity of smell

• VNO (Vomeronasal Organ)::vestigial structure in humans; key driver of hypothalamic activity, organ that responds to the smell of pheromones - very few receptors to perceive hormones because the VNO is virtually empty

  • All other animals, sex is determined by pheromones - not for bonobos or humans

• Entorhinal cortex::shuttles info directly into hippocampus (coding memories in conjunction with the amygdala)

• Avg lifespan of an olfactory epithelial cell = 30 days; immature daughter cells fill in the gap → neurogenesis (only happens in this area of the brain)

Odor Perception:

• Females detect odor better than males - not seen prior to puberty/after menopause; heightened during ovulation

• Anosmia::absence of smell

• Hyposmia::decreased smell

• Dyosmia::distortion of smell

• Cacosmia::sense bad/foul smell

• Parosmia::smell in the absence of a stimulus

• Alzheimer’s - early stage prior to diagnosis - frequently report a decreased sense of smell or total lack of smell