Taste and Smell

Question 1

What type of receptors detect taste and smell?

Answer 1

Chemoreceptors.

Question 2

Define ageusia.

Define anosmia.

Answer 2

• Ageusia is loss of taste.

- Anosmia is loss of smell.

Question 3

List 4 causes of anosmia.

Answer 3

1 - Epilepsy.

2 - Damage to the olfactory bulb.

3 - Parkinson’s.

4 - Alzheimer’s.

5 - Clinical depression.

Question 4

List the submodalities of the taste modality.

Where on the tongue are these submodalities detected?

Answer 4

1 - Sweet.

2 - Salt.

3 - Sour.

4 - Bitter.

5 - Umami (delicious! - meats).

• Although each taste cell has one type of chemoreceptor molecule to respond to one submodality, the different taste cells are not separated across the tongue (the tongue is not organised into discrete areas for different submodalities).

Question 5

List 3 locations of taste receptor cells.

Answer 5

1 - Tongue.

2 - Palate.

3 - Pharynx.

Question 6

Describe the structural organisation of taste buds.

Answer 6

• A por exists on the surface of the taste bud.
• This leads into a taste cell (not a neurone, but associated with a primary sensory afferent).
• The taste cell sits above a basal cell.
• Primary sensory afferents lead from the taste bud to the CNS.

Question 7

What is the lifespan of a taste cell?

Which cells replenish the taste cells?

Answer 7

• 2 weeks.

- Basal cells.

Question 8

Why does taste sensation decrease with age?

Answer 8

Because basal cells lose capacity to replenish taste cells.

*The same is true for olfactory cells.

Question 9

Describe the generic process by which a sensory receptor translates a sensory stimulus to their associated sensory afferent.

Answer 9

• A substance binds to the sensory receptor.
• This induces a receptor potential (transduction).
• The receptor potential causes voltage-gated Ca2+ channels to open, causing calcium influx.
• This causes neurotransmitter to be released into the synapse with the sensory afferent neurone.

Question 10

Describe the process of transduction in taste cells for salt.

Answer 10

• Sodium enters the taste cells via non-gated channels.

- This causes depolarisation.

Question 11

Describe the process of transduction in taste cells for sour.

Answer 11

• Sourness is basically acidity.
• H+ enters the taste cell via TRP channels.
• H+ also blocks K+ channels.
• These events cause depolarisation.

Question 12

Describe the process of transduction in taste cells for sweet, umami and bitter.

Answer 12

• Sensory stimuli for sweet, umami and bitter bind to different GPCRs such as dopamine receptors and adrenergic receptors.
• The families of receptor subunits of these GPCRs include T1R and T2R.
• Binding to these GPCRs causes depolarisation.

Question 13

How many neurones are required in chain for the primary pathway of taste sensation?

Describe the course of these neurones.

Answer 13

• 3:
• The afferent sensory neurone travels from the tongue / palate / pharynx to the gustatory nucleus in the fourth ventricle of the medulla, where it synapses with the second order neurone.
• The second order neurone travels to the ventral posteromedial nucleus of the thalamus, where it synapses with the third order neurone.
• The third order neurone travels to the relevant area of the gustatory cortex (according to the submodality).

Question 14

What is the insula?

Where is it located?

What is its function?

Answer 14

• A small region of the cerebral cortex located deep within the lateral sulcus.
• It is involved in bringing together different aspects of sensation.

Question 15

List 2 structures involved in the secondary pathways of taste sensation.

List the roles of the secondary pathways.

Answer 15

Components of secondary pathways:

1 - Medulla.

2 - Hypothalamus.

Roles of secondary pathways:

1 - Swallowing.

2 - Salivation.

3 - Satiety.

4 - Palatability.

Question 16

How does the sensitivity of taste compare to the sensitivity of smell?

Answer 16

Smell is more sensitive than taste.

Question 17

What is the name of the sensory receptors for smell?

Answer 17

Olfactory epithelium.

Question 18

Where are olfactory epithelia?

Describe their structure.

Answer 18

• At the roof of the nasal cavity.
• They emerge through the perforations of the cribriform plate and project cilia into the nasal cavity.
• The epithelia contain olfactory cells (specialised neurones) and supporting cells.
• The olfactory cells converge superiorly into the olfactory nerve.
• The olfactory nerves synapse at the olfactory bulb.

Question 19

List the locations in which neurogenesis persists in th brain.

Answer 19

1 - Subventricular zone.

2 - Hippocampal dentate gyrus.

Question 20

What is the lifespan of an olfactory cell?

Which cells replenish the olfactory cells?

Why might this explain anosmia in patients with clinical depression?

Answer 20

• 4-8 weeks.
• Cells of the subventricular zone.
• There is a suppression of the division of the cells in the subventricular zone in patients with clinical depression.

Question 21

How do olfactory cells differ from taste cells in the way that they respond to submodalities?

Answer 21

• Like taste cells, there is one type of chemoreceptor for each olfactory cell, however each chemoreceptor for smell binds a range of odorants, whereas each chemoreceptor for taste only binds to one / a few stimuli (smell chemoreceptors are more promiscuous than taste chemoreceptors).
• There are also many thousands of types of chemoreceptors for smell, whereas there is only one chemoreceptor for each submodality in taste (smell chemoreceptors are more diverse than taste chemoreceptors).

Question 22

What is population coding?

Answer 22

A way of identifying particular sensory stimuli for smell:

• For any one particular smell, there are some chemoreceptors that elicit a strong response, and some that elicit a weak response.
• Population coding involves coordinated firing of multiple neurones at different intensities, which allows the brain to determine the nature of the smell.

Question 23

Describe the common process of transduction in smell receptors.

Answer 23

• An odorant binds to the GPCR chemoreceptor.
• This induces the generic transduction cascade to cause depolarisation.
• This creates a receptor potential that, if sufficient, will produce an action potential.
• There is no excitation of a primary neurone here because olfactory cells are neurones (whereas taste cells aren’t).

Question 24

How many neurones are required in chain for the primary pathway of smell sensation?

Describe the course of these neurones.

Answer 24

• 3.
• The primary sensory afferents (olfactory nerve CN1) travel to the olfactory bulb, which is a very short distance from their chemoreceptors.
• Neurones that have similar chemoreceptors synapse close together, and form structures known as glomeruli, where they synapse with the second order neurone.
• The second order neurone sends its axon up the olfactory bulb to form the olfactory tract.
• It then travels to the olfactory cortex of the temporal lobe, where it synapses with neurones that can take 2 pathways:

1 - The first route sends its axons to the the medial dorsal nucleus of the thalamus, and then to the orbitofrontal cortex of the prefrontal cortex. This pathway is involved in recognition.

2 - The second route sends its axons to the limbic areas of the brain. This pathway is involved in association.