Chemical Senses - Taste and Smell

Question 1

What are the diverse function of chemical senses?

Answer 1

• finding food
• finding a mate
• avoiding dangerous substances
• homeostasis

Question 2

What are the different human chemical senses?

Answer 2

taste (gustation)
smell (olfaction)
other chemoreceptors - pH, CO2, O2
- internal environment
- skin
- GIT

Question 3

What are the 5 sub modalities of taste?

Answer 3

salt
sour
bitter
sweet
umami

Question 4

Where are the taste cells located?

Answer 4

tongue, palate and pharynx

Question 5

Describe the distribution of taste cells

Answer 5

Relative NOT absolute
Areas of highest sensitivity for each taste:
- bitter posteriorly
- sour next anterior
- salty next anterior 
- sweet at the front

Question 6

Describe the structural organisation of taste

Answer 6

papilla – > taste buds –> taste cells

large SA

Question 7

What are the 2 major properties of taste cells?

Answer 7

1. Taste cells are NOT sensory neurones
   - no AP’s, no axons
   - they synapse with first order neurones
2. Constantly replaced
   - 2 weeks
   - basal cells generate new cells
   - mechanically sloughed off/damaged

Question 8

Describe the pathway by which taste cells detect taste

Answer 8

• Chemical binds
• Receptor potential (graded, no AP’s!)
• Depolarisation
• Voltage gated Ca2+ channes open
• Calcium entry
• NT released
• Excites sensory neurones
• Action potential

Question 9

What is the mechanism by which salt is detected?

Answer 9

Simple!

• Sodium entry (non-gated Na+ channels)
• Depolarisation
• Transmitter release

Question 10

What is the mechanism by which sour is detected?

Answer 10

H+ ions 
entry via TRP (not voltage gated)
OR
K+ channel blocked  (less K+ out)
--> depolarisation

Question 11

Describe receptor distribution?

Answer 11

Individual taste cells respond to individual sub modalities

Question 12

What are the receptors for sweet, umami and bitter?

Answer 12

T1R and T2R family - GPCRs
Sweet = T1R2 and T1R3
Umami = T1R1 and T1R3
Bitter = T2R and T2R

Question 13

Describe the central taste pathways

Answer 13

Gustatory sensory axon
Cranial nerves CNVII, IX, X
Brain stem = VPN nucleus
Primary gustatory cortex - bottom of post-central gyrus

Question 14

What is meant by neural coding of taste?

Answer 14

comparision of all inputs is interpreted by brain

Question 15

What can be the result of a central lesion?

Answer 15

aguesia (loss of taste)

Question 16

What are the secondary pathways involved?

Answer 16

medulla –> swallowing, salivation

hypothalamus –> satiety, palatability

Question 17

Where are the smell receptors?

Answer 17

olfactory epithelium which lines the top of the nasal cavity

Question 18

What are the properties of olfactory cells?

Answer 18

• they are neurones
• replaced every 4-8 weeks
• migrate from linings of lateral ventricles
• each olfactory cell has only one type of receptor molecule
• each receptor molecule can bind a range of odourants

Question 19

What is meant by population coding?

Answer 19

coordinated firing of multiple neurones
–> odorent detection
learnt or innate?

Question 20

What are the properties of olfaction?

Answer 20

rapid adaptation

signal transduction mechanism same for all receptors

Question 21

What is the signal transduction mechanism?

Answer 21

• odorant binds to receptor
• G protein mediated events
• intracellular cascade
• Na+/Ca2+ channels open
• depolarisation
• receptor potential
• AP if threshold reached

Question 22

Where does cranial nerve 1 end?

Answer 22

At the olfactory bulb

Question 23

What is the structure of the olfactory bulbs?

Answer 23

Glomerulus

• neurones detecting similar smells group
• -> secondary neurones synapse here
• olfactory neurones form olfactory tract

Question 24

What are the central olfactory pathways?

Answer 24

Olfactory neurones
Olfactory bulb
Olfactory cortex 
--> limbic areas ASSOCIATION 
--> mediodorsal thalamus --> orbitofrontal cortex RECOGNITION

Question 25

What are the clinical implications of smell and taste?

Answer 25

Altered taste and smell -
- associated with various drugs 
- age - old people therefore eat less
- epilepsy -unusual activity of neurones --> predict seizures 
Loss of smell
- damage to olfactory bulb
- Parkinson's - early diagnostic marker
- clinical depression (reversed by successful antidepressant therapy)