taste

Question 1

what occurs when there is damage to taste buds?

Answer 1

• facial nerve SVA which innervates taste buds projects to lingual nerve
  
  • -> damage to lingual nerve
• fungiform papillae and taste buds disappear from the affected side
• 2-3 weeks later
  
  • tecture of tongue returns to normal
  • peripheral axons regenerate and regrow

Question 2

which tastes are mediated by ion channels?

Answer 2

salt

sour

Question 3

which tastes are mediated by GPCR?

Answer 3

sweet

bitter

umami

Question 4

how is salt tasted?

Answer 4

• taste cells have -ve resting potential inside plasma membrane

1. Na+ ions from salt enter cell through amiloride-sensitive sodium channels - depolarises cell slightly
2. voltage gated sodium channels open - increases depolarisation
3. opens voltage gated Ca2+ channels
4. influx of Ca2+ -> fusion of neurotransmitter (serotonin) filled vesicles with taste cell basal plasma membrane
5. exocytosis of serotonin
6. depolarisation of SVA sensory neurons that converge on taste cells basal membrane

Question 5

how is sweet tasted?

Answer 5

1. binding of sugar moleculars to GCPR - activates enzyme phospholipase-C-B2 (PLC-B2)
2. PLC-B2 catalyses conversion of PIP₂ (phosphatidylinositol 4,5-bisphosphate) to inositol triphosphate (IP₃)
3. IP₃ stimulates release of Ca2+ from stores in ER
4. increased conc of Ca2+ leads to extracellular influx of Na+ through TrpM5 Na+ channels - small depolarisation of taste cell
5. depolarisation opens voltage-gated Na+ channel -> greater depolarisation
6. depolarisation opens voltage-gated ATP channel CALHM1 - causes ATP release
7. ATP binds to P2X2/3 receptors on SVA neurons - depolarisation of neurons

Question 6

describe the levels of discrimination in the taste pathway

Answer 6

• taste cells only express 1 taste GCPR
• sour and salty found in same cell
• most cells respond to more than one
• At papilla level
  
  • Papillae selective to one preferred taste - at low conc
    
    • At high conc
    • Selectivity weakens
• At afferent level
  
  • One SVA neuron receive input from several taste cells
  • Each gustatory axons fires Aps to all 5 taste stimuli
    
    • But has preferred one

Question 7

describe SVA fibres

draw and label - where to they project

Answer 7

• carried by fibres in CN VII, IX, X
• project to NTS
  
  • taste vision

Question 8

how is taste information delivered to the primary gustatory cortex

Answer 8

three neuron relay delivers taste information to the primary gustatory cortex

• Some solitary tract neurons transmit taste information directly to the hypothalamus (ANS control), amygdala (limbic system) and reticular formation of the medulla

Question 9

where is the gustatory cortex

Answer 9

area of cortex concerned with taste

lies above and behind temporal lobe