olfaction and taste objectives - michael Flashcards
1) Be able to explain the functional capabilities of the trigeminal system with respect to chemical detection
a) Trigemial system consists of nerve endings in the mucus membranes of the mouth
b) These endings protect the mouth by detecting noxious or irritating compounds such as capsaicin
c) Capsaicin and heat open the TRP channel, depolarizing the cell and allowing you to sense pain and heat.
d) Another TRP channel detects menthol and cold
2) Be able to explain the transduction mechanisms in olfactory receptor cell cilia that are used to detect odorant molecules.
a) The trouble with olfaction: it’s hard to describe “what something smells like”
b) Receptors are GPCRs with seven transmembrane spanning regions
c) Activated G proteins modulate the activity of ion channels in the cilia
i) Some cause changes in adenylate cyclase activity and cAMP
ii) Others cause breakdown of phosphatidylinositol by phospholipase C and production of IP3
iii) Appears that each receptor cell has one or the other pathway
d) Nearly 1000 olfactory GPCR genes in humans
3) Be able to describe the pathways by which olfactory information is projected to specific sites in the brain and the functional role(s) of these sites in the sense of smell.
a) Cell types in olfactory epithelium:
i) Basal cells: precursors of receptor cells undergoing continual turnover (~60 day life-cycle)
ii) Support cells: support receptor cells
iii) Receptor cells: dendrite on epithelial surface, axons aggregate with others to form CN1 and synapse on olfactory bulb
b) Olfactory bulb: contains glomeruli, which contain endings of receptor cell axons and mitral cell dendrites
c) Mitral cells: projection neurons of the olfactory bulb, they go out to other regions of the brain
d) Anterior piriform cortex (olfactory cortex): receives input from multiple glomeruli (does not necessarily pass through the thamalus!)
e) Next, information goes to the amygdala and then to the hypothalamus. This gives rise to emotional and motivational aspects of certain odors.
f) Then, information goes to the thalamus and then to the frontal lobe (conscious perception). Finally, information goes to the entorhinal cortex and then to the hippocampus, where we store odors and our specific responses to them.
g) Quick note on olfactory modulation:
i) Adrenergic fibers provide heightened sense of smell during fear and excitement
ii) Locus ceruleus is the route emotional state influences odor: increased sensitivity to smell of food when hungry!
4) Be able to explain how individual odorants are “encoded” (identified) in the olfactory system.
a) Odorants are encoded by patterns of activity in the olfactory bulb correlated with odorant identity and concentration
i) “Across fiber” pattern: patterns include not only which cells are active but when those cells are active
b) In the anterior piriform cortex, some neurons only respond to mixtures of two individual odorants (does not respond to either odorant individually)
5) Be able to explain how the o lfactory system has a high degree of sensitivity yet broad discriminatory powers
a) Sensitivity: 1000’s of GPCRs
b) Broad discrimination: olfactory bulb processes odors are holistic “across fiber” patterns
6) Be able to describe the structural features of the tongue in which taste receptor cells are located
a) Four types of papillae:
i) Filiform: covers surface of tongue (no taste buds!)
ii) Vallate: 7-12 of these across the back of the tongue; ~250 taste buds on each, that’s 50% of the taste buds
iii) Foliate: packed into folds along back edge/side of tongue
iv) Fungiform: scattered across entire surface of tongue
7) Be able to describe the functional role of the different cell types found in a taste bud
a) Support cells: support…
b) Basal cells: give rise to receptor cells (~10 day life-cycle)
c) Receptor cells: secondary sensory cells (no axons to central processing)
8) Be able to explain the transduction mechanisms in taste receptor cells that are used to detect specific types of tastants.
a) Salt: voltage insensitive sodium channels are open at rest and can be blocked by amiloride. Salty foods cause leakage of Na ions into the cell.
b) Sour: sour means acidic; protons open up a cation-selective ion channel. Depolarizes the cell.
c) Sweet: GPCRs for sweet molecules, leading to phospholipase C activation, generation of IP3, release of Ca2+ and transmitter release.
d) Bitter: another second messenger system (different than sweet)
e) Umami: another bitter-like second messenger system involving a truncated metobotropic glutamate receptor
9) Be able to describe the pathways by which gustatory information from the tongue is projected to specific sites in the brain and the functional role(s) of these sites in the sense of taste.
a) CNVII, CNIX, CNX form synpases near sensory cells
i) Vallate and foliate papillae go with glossopharyngeal
ii) Fungiform go with facial
b) Projections go: nucleus of the solitary tract (NST) in the brainstem → ventral posterior medial (VPM) nucleus of the thalamus → gustatory cortex in the temporal lobe, near the insula (also to hypothalamus and amygdala)
c) Quick note on gustatory modulation:
i) Taste receptors have receptors for serotonin and cholecystokinin, both of which enhance taste cell response
ii) Aldosterone and vasopressin enhance taste response to salt
10) Be able to explain how individual tastants are encoded (identified) in the gustatory system
a) Each taste modality (sweet, bitter, salty, sour) is represented in distinct but overlapping patterns in the cortex
b) Specific patterns are used to discriminate between individual tastants
11) Be able to describe similarities and differences between the gustatory and olfactory systems
a) Similarities:
i) Both can be modulated
ii) Both have the same system of support, basal, and receptor cells
iii) Both odorants and tastants are encoded in the cortex by patterns of activity rather than by one specific location
b) Differences:
i) Olfactory sensory cells project centrally; gustatory sensory cells are secondary sensory cells
ii) Olfactory receptors use GPCRs exclusively, gustatory receptors can use GPCRs or ion channels
c) I’m sure there are others…
12) Be able to describe the alterations in olfactory and gustatory abilities that occur during specific diseases and disorders
a) Atrophic rhinitis: reduced olfactory abilities
b) Diabetes: reduced olfactory abilities
c) Chronic renal failure: reduced olfactory and gustatory abilities
d) Alzheimer’s disease: reduced olfactory abilities (loss of receptor cells)
e) Cystic fibrosis: enhanced olfactory and gustatory abilities
f) Epilepsy: enhanced olfactory and gustatory abilities
