Physiology of Taste

Question 1

What is another name for taste?

Answer 1

Gustation

Question 2

What’s the Gustatory pathway?

Answer 2

1. Facial/glossopharyngeal/vagus sensory afferent fibres carry signals and synapse in the medulla Oblongata
2. Axons of postsynaptic neurons enter the medial lemniscus and synapse in the thalamus
3. Information on taste is projected to the appropriate portions of the gustatory cortex of the INSULA
4. Conscious perception of taste as the brain correlates info received from the taste buds with other sensory data from other somatic sensory information (touch/pressure/pain)
5. Level of stimulation from olfactory receptors plays an overwhelming role in taste perception as well

Question 3

What are the 4 types of taste buds?

Answer 3

1. Salt - transduced directly by Na+ ions
2. Sour - transduced directly by H+ ions
3. Sweet - involves 2nd messenger (ligand binding to taste buds)
4. Biter - involves 2nd messenger (ligand binding to taste buds)

Question 4

What’s umami?

Answer 4

A taste associated with glutamate and other nucleotides; has receptors located at the back of the pharynx

Question 5

How is primary sensory input transmitted to the brain?

Answer 5

Facial nerve - anterior 2/3 of tongue

Glossopharyngeal - posterior 1/3 of tongue

Vagus nerve - Soft palate and mouth

Question 6

How are taste buds distributed on the tongue?

Answer 6

Random assortment of taste buds with no distinct area

Question 7

What’s the anatomy of taste buds?

Answer 7

1. Taste receptor cell

2. Support cell

Question 8

How do taste receptors initiate an action potential?

Answer 8

1.Signal transduction causes Ca2+ ions to cause exocytosis of vesicles containing neurotransmitter which create a action potential

Question 9

What is a T1 receptor?

Answer 9

T1/Orphan receptors because their precise ligands are unknown

• Possess large extracellular domain for ligand binding (sweet)

Question 10

What is a T2 receptor?

Answer 10

• Bitter receptors

Question 11

What is a T-mGluR4 receptor?

Answer 11

• The taste-metabotropic glutamate receptor is similar to a receptor found in the brain but its extracellular domain is much shorter in the tongue

Question 12

What are the MDEG/ENaC receptors?

Answer 12

• Belong to a superfamily of ion channels implicated in Na salt and acid sensation

Question 13

Outline taste transduction?

Answer 13

1. Bitter & sweet ligands use G-protein coupled membrane receptors
   1a. Bitter ligand, transducin releases Ca2+ from intracellular stores
   1b. Sweet ligand, gustducin, activates a cAMP 2nd messenger that closes K+ channels and depolarises the cell
   1c. Ionic ligands for sour and salt alter ion channels and depolarise the cell - triggering extracellular Ca2+ entry
2. All cases increase Ca2+ intracellularly = triggers neurotransmitter release