2: Olfaction and Taste Flashcards
Compare and contrast the chemical sensory functions of the trigeminal sensory system, the olfactory system, and the gustatory system.
Trigeminal system: protective function by detecting noxious and irritating stimuli
Olfactory system:
- –High SENSITIVITY (down to a few molecules per sniff)
- –Broad DISCRIMINATION (> a trillion molecules/combinations)
- –Role in hunger/feeding, social interaction, reproduction, memories
Gustatory system: detects “flavor” as combination of sensory information
—Sweet, sour, bitter, salt, umami
Describe the role of the trigeminal sensory system and TRP receptors as a chemical sense in protection.
Nerve endings in mouth, face, mucous membranes
TRP channels: activated by changes in temperature and compounds that TASTE cool and hot/spicy
–Includes capsaicin TRP receptor
Describe the molecular mechanisms involved in development of odorant specificity.
Odorants depolarize olfactory receptor cells and cause a change in the rate at which they generate action potentials
—Each receptor is activated to a different extent by a large number of molecules
Discuss the mechanisms involved in changing sensitivity in the olfactory system in response to fear and hunger.
Emotional state (fear/excitement) -> enhanced sensitivity via EPI fibers
Locus ceruleus (Motivational state/hunger) -> enhanced sensitivity via norEPI fibers
Identify the locations of taste buds and different types of papillae. (4)
Non-uniform distribution of papillae on the tongue
- Vallate papillae (CN IX): largest, contain half of taste buds; like storage tanks, taste buds on side
- –> On BACK of tongue - Foliate papillae (CN IX, VII): leaf-like/ridge-like papillae on SIDES of tongue, taste buds on side
- Filoform papillae: NO TASTE BUDS
- Fungiform papillae (CN VII): taste buds on top
- –> On anterior 2/3 of tongue
List mechanisms that are capable of altering taste sensitivity. (2)
Hunger -> 5HT, CCK -> enhance taste receptor cell sensitivity
Aldosterone, ADH -> regulate water balance by enhancing taste receptor cell sensitivity to SALT
What is an anosmia?
Selective olfactory deficits (can’t smell something specific)
Why could the age-related decline in olfaction be important?
Could be important in age-related malnutrition and lack of hunger
What problems arise when trying to study olfaction? (3)
Can’t predict smell based on structure
Combination of compounds may smell different from individual components
Molecule smell may depend on concentration
What is a vomeronasal organ? Is it present in humans?
Used to detect pheromones
Doubtful - genes not expressed; regresses during fetal development
List and describe the three types of cells in the olfactory epithelium.
- -Basal cells: precursors for additional receptor cells (renew every 60 days)
- -Receptor cells (neurons)
- -Supporting cells: provide structural support
List and describe the three types of cells in the olfactory bulb.
Granule cells: interneurons
Mitral cells: the output cells of the olfactory bulb
Periglomerular cells: form synapses between neighboring glomeruli
What makes up a glomerulus?
Dendrites of a single mitral cell and axons of ~20,000 presynaptic neurons
How do cortical neurons function in odorant detection?
Cortical neurons compare information from multiple receptor types
- –Compare amount of activity in multiple inputs
- –Appear to have more of an ACROSS FIBER pattern for each odorant rather than a labeled line (when we respond to a single odorant, we look at the COMBINATION of glomeruli that are active
What is a “supertaster”?
Increased sensitivity to bitter substances (~25% of people)
Have a higher density of fungiform papillae on their tongue
What three cell types are in a taste bud?
Support cells
Basal cells: the precursors to taste cells (turn over every 10-12 days)
Taste receptor cells
Outline the central projections of gustatory cells.
Release NT onto nerve endings (CN X, IX, VII), so they are NOT primary sensory neurons
Project to NUCLEUS OF SOLITARY TRACT
–Mediates salivation, coughing, gagging
-> VPM of thalamus
-> Gustatory cortex in insula (near somatosensory information from the tongue -> full sensation of taste)
NST also -> amygdala, hypothalamus
Describe the coding of gustatory information in the cortex.
Individual taste receptor cells exhibit differential activation to tastants
–ACROSS-FIBER pattern, as in olfaction
What effect does atrophic rhinitis have on chemical sensation?
Reduced olfactory abilities?
What effect does diabetes have on chemical sensation?
Reduced olfactory abilities
What effect does chronic renal failure have on chemical sensation?
Reduced olfactory and gustatory abilities
What effect does Alzheimer’s disease have on chemical sensation?
Reduced olfactory abilities (loss of receptor cells)
What effect does cystic fibrosis have on chemical sensation
Enhanced olfactory and gustatory abilities
What effect does epilepsy have on chemical sensation?
Enhanced olfactory and gustatory abilities
Describe the structure and function of olfactory receptor cells and their axons.
Sensory neurons in mucus layer in olfactory epithelium
CILIA play a key role in olfactory transduction: odorant on cilia -> depolarization; odorant on soma -> very little response
Unmyelinated axons travel through cribiform plate -> synapse on olfactory bulb
Describe the molecular mechanisms involved in signaling in the olfactory receptor cell.
Primary mechanism: includes cAMP-gated and Ca-gated channels in the membranes of the cilia
–>Odorant molecule binds GPCR -> G(olf) protein -> AC -> cAMP -> **activate Na/Ca channel -> Na/Ca influx -> action potential
Secondary mechanism (for some odorants): involves IP3-gated ion channels (G-protein -> PLC -> IP3 -> activate Ca channel -> Ca influx -> action potential)
All olfactory receptors are GPCRs
—Have 950 olfactory receptor genes -> broad discriminatory powers
Discuss convergence and glomerular specificity in terms of odorants.
CONVERGENCE: 25,000 neurons (all of the same odorant receptor type) project to the same glomerulus -> contributes to olfactory sensitivity
Different combinations of glomeruli activation -> different odorants
Discuss the organization and termination of central processes of mitral cells.
Axons from mitral cells in olfactory bulb go to 3 places (NOT directly to the thalamus!
- PIRIFORM CORTEX (3-layered cortical region, primitive)
- Amygdala (emotions)
- Entorhinal cortex -> hippocampus (memory)
Then, -> thalamus, hypothalamus, orbitofrontal cortex
Describe the differential specificities of different regions of the tongue.
Bitter is in the back (vallate papillae, CN IX)
Sour - along sides of the tongue (foliate papillae, CN IX/VII)
Salty - U-shape at tip of tongue (fungiform papillae, CN VII)
Sweet, umami - at tip of tongue (fungiform papillae, CN VII)
Explain the mechanism involved in gustatory cells that sense sweet.
Saccharide compounds -> GPCR -> PLC -> IP3 -> Ca release from ER -> NT (ATP or 5-HT) released
Explain the mechanism involved in gustatory cells that sense salty.
Amiloride-sensitive Na channel: constitutively open, allows Na and H to flow in -> depolarization -> Ca flows in -> NT (ATP or 5-HT) released
Explain the mechanism involved in gustatory cells that sense sour.
H-activated channel -> allows Na and Ca to flow in -> depolarization -> more Ca flows in -> NT (ATP or 5-HT) released
Explain the mechanism involved in gustatory cells that sense bitter.
Bitter compounds -> GPCR -> PLC -> IP3 -> Ca release from ER -> NT (ATP or 5-HT) released
Explain the mechanism involved in gustatory cells that sense umami.
Glutamate -> GPCR -> PLC -> IP3 -> Ca release from ER -> NT (ATP or 5-HT) released
Compare and contrast olfactory receptor cells and taste receptor cells. (4)
Both turn over frequently (gustatory turn over faster)
Olfactory receptor cells have axons, while gustatory cells do not
Single cells of each type respond to more than one molecule
Both -> coding via an across-fiber pattern of identification
