Chemotransduction

Question 1

Olfactory stimuli

Answer 1

• 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

Question 2

Start of olfaction

Answer 2

• 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

Question 3

Olfactory receptor neurons

Answer 3

• 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)

Question 4

Neurogenesis

Answer 4

• 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

Question 5

Odorant receptor proteins

Answer 5

• 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

Question 6

ORN signalling cascade

Answer 6

• 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

Question 7

Odorant code

Answer 7

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

Question 8

Taste

Answer 8

• 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

Question 9

Taste categories

Answer 9

• 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

Question 10

Taste transduction

Answer 10

• 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

Question 11

Salty transduction

Answer 11

• 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

Question 12

Sour transduction

Answer 12

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

Question 13

Sweet and Umami

Answer 13

• 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

Question 14

Bitter transduction

Answer 14

• 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