Olfaction, taste, touch and pain

Question 1

What are the five major classes of taste?

Answer 1

Sweet
Sour
Salty
Umami
Bitter

Question 2

How do different tastants transduce their signals into taste bud cells?

Answer 2

Salty - Na+ diffuses through an ion channel causing depolarisation
Sour - H+ diffuses through Na+ ion channels or blocks K+ channels causing depolaristion
Sucrose activates Galphas linked GPCRs leading to the phosphorylation of K+channels, inactivating them
All lead to depolarisation which activates VOCCs and the influx of calcium leads to the exocytosis of glutamate

Question 3

How do taste signals get from the tongue to the cortex?

Answer 3

Three cranial nerves carry the signal from the taste buds to synapse in the medulla
Secondary afferents carry the signal and synapse in the thalamus
Tertiary afferents carry the signal to synapse in the primary gustatory cortex

Question 4

How do we detect airborne odorants?

Answer 4

Olfactory epithelium GPCRS activate 2nd order neurons in the olfactory bulb

Question 5

Describe the central olfactory pathway

Answer 5

The largest bundle of 2nd order afferents exit the olfactory bulb as the lateral olfactory tract and synapses in the temporal lobe
Afferents from the temporal lobe project directly to the prefrontal cortex unlike other senses which usually relay via the thalamus

Question 6

What makes olfactory neurons unique compared to others?

Answer 6

Exposed to the external environment

Replaced every 4-8 weeks by differentiation of olffactory epithelium basal cells

Question 7

How were specific odour receptors identified for different odours?

Answer 7

Individual neurons cultured from olfactory epithelium
Neurons loaded with a dye that detects changes in calcium levels
Neurons exposed to odourants and the cells that lit up underwent single-cell reverse transcriptase PCR which identified which receptor was being expressed
Different combinations of receptors lead to different smells

Question 8

What is the major transduction pathway for olfaction?

Answer 8

Odarant binding to olfactory GPCRs -> cAMP -> Cyclic Nucleotide Gated Channels activated -> Depolarisation

Question 9

Where is sensory information perceived in the brain?

Answer 9

The homunculus