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Flashcards in Chemotransduction Deck (14)
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1

Olfactory stimuli

-odorants: airborne, volatile, chemical stimuli for the olfactory system
-odorants pass over the olfactory epithelium during breathing, eating, and associated movements
-retronasal route: additional sensation of odours from mouth through nasal passage while eating

2

Start of olfaction

-olfactory epithelium:
-olfactory receptor neurons (ORNs)
-stem cells
-supporting + basal cells
-Bowman’s glands

-mucus layer:
-Secreted by Bowman’s glands
-provides ionic milieu for ORNs
-immunoglobulins and catabolic enzymes protect against chemicals, viruses, and bacteria

3

Olfactory receptor neurons

-bipolar structure
-long apical neck ends in olfactory knob, which is covered in olfactory cilia
-olfactory cilia studded with odorant receptor proteins that allow cells to detect odorants
-ORNs project through por our bone called cribriform plate
-head trauma can shear these fine nerve fibres leading to anosmia (lack of smell)
-other causes of anosmia: infections, mutations in specific proteins etc
-humans have relatively poor sense of small compared to other animals
-partially due to relatively small population of ORNs (less than dogs and rats)

4

Neurogenesis

-ORNs are continuously exposed to damaging agents
-bacteria + viruses
-pollutants
-allergens
-dirt/sand
-ORNs are renewed with continuous neurogenesis
-life cycle: 6-8 weeks in rodents
-relies on population of stem cells

5

Odorant receptor proteins

-transduction starts when odorant binds odorant receptor protein on ORNs
-huge family of G-protein coupled receptors (7TM domains)
-individual ORNs express one type of OR protein
-specificity of odorant coding probably relies on variety of odorant receptor proteins
-in all mammals odorant receptors are the largest known single gene family (3-5% of genome)
-many are pseudogenes and cant be transcribed

6

ORN signalling cascade

-Odorant molecule binds to receptor which activates the g-protein
-the a subunit (Golf) binds GTP and dissociates from BY then activates adenylyl cyclase
-adenylyl cyclase produces cAMP which binds to cAMP gated channels causing them to open
-open channels allow influx of Na+ and Ca2+
-Ca2+ binds to Ca2+ gated Cl- channel, and causes efflux of Cl-

-recovery:
-Ca2+ bound calmodulin kinase II (Ca2+-CAM) activated PDE to decrease cAMP levels
-binds and desensitizes cAMP-gated channels

7

Odorant code

-no clear relationship between odorant chemical structure, ORN activation, odor perception

8

Taste

-tastant: chemical that provide aesthetic, nutritive, and safety qualities of food
-carried by saliva
-contact taste papillae
-reach taste buds on lateral surfaces of papillae and trench walls
-fewer on palate, epiglottis and esophagus
-stimulate taste cells
-transduction site
-receptors on apical end near taste pore
-different taste cells generally express 1 class of receptor

9

Taste categories

-5 different taste categories can be detected across entire tongue but some regions more sensitive
-thresholds:
-sour: 2mM
-sweet: 20mM
-salty: 10mM
-bitter: 0.0001mM

10

Taste transduction

-graded depolarization from:
-direct ion influx (salty or acidic)
-GPCRs (sweet, bitter, umami)
-activated voltage gated channels
-Na+ and K+ channels signal propagation down basal domain
-Ca2+ channels - NT release
-activate primary sensory neurons of facial, glossoparhyngeal, and vagus nerves

11

Salty transduction

-amiloride-sensitive Na+ channel
-direct Na+ influx
-depolarizers cell
-activated VG channels
-Na+ and K+ channels signal propagation down basal domain
-Ca2+ cause NT release

12

Sour transduction

-TRP channel
-non selective cation channel
-direct H+ influx

13

Sweet and Umami

-2 GPCRs form functional heterodimer receptor
-T1R2-T1R3 for sweet
-T1R1-T1R3 for umami (amino acids)
-a subunit activates PLCB2 to make IP3 to activate TRPM5 channel allowing Ca2+ influx

14

Bitter transduction

-T2R receptor family has 30 members
-single taste cells expresses multiple T2R subtypes
-taste cells do not co-express T2Rs with T1R1, T1R2, or T1R3 receptors
-distinct taste cells for bitter
-bitter taste cells also have distinct Ga called Gastducin