Chemical Senses

Question 1

What are the 5 basic tastes?

Answer 1

• Sweet
• Salty
• Bitter
• Sour
• Umami

Question 2

What is the oldest and most common sensory system?

Answer 2

Chemical sensation

Question 3

What are the types of chemical senses?

Answer 3

• gustation (taste)
• olfaction (smell)
• chemoreceptors

Question 4

Describe the basic structure of a taste bud

Answer 4

• apical end with microvilli, open to the environment - this is the taste pore
• Comprised of structural cells, basal cell, and different receptor cells (light, dark and intermediate) held tightly together.
• synapses at the base to cranial nerves VII, IX, X

Question 5

What is the general process of taste receptor action?

Answer 5

1. taste stimuli at apical membrane
2. binds with receptor (either ion channel or membrane receptor)
3. channels open & depolarisation (not true AP)
4. VG Sodium & Calcium channels open OR Ca++ release from intracellular stores
5. ↑[Ca++] = NT release

Question 6

How many different tastes does a single taste receptor cell respond to?

Answer 6

Usually will have a preference for 1, but will often respond to 2 or 3, and to varying degrees for different stimuli

Question 7

What are examples of correspondence between chemicals and taste?

Answer 7

• sucrose - sweet
• sodium/NaCl - salty
• HCl - sour
• quinine - bitter

Question 8

Are axons selective to a single tastant?

Answer 8

No

Question 9

what are the 3 actions of a taste stimulant at the taste receptor cell membrane?

Answer 9

• pass directly through an ion channel (ie sodium)
• Bind to and block ion channels
• Bind to gPCRs & activate second messenger systems

Question 10

Describe the transduction mechanism for salty tastants

Answer 10

1. Na+ comes straight through amiloride-sensitive sodium channels
2. Na+ in leads to depolarisation
3. opens VGCCs
4. ca++ influx
5. serotonin released
6. binds to 5-HT receptors on nerve axon

Question 11

Describe the transduction mechanism for sour tastants

Answer 11

1. H+ enters cell via proton channel & closes K+ channel
2. H+ in & K+ stuck in = depolarisation
3. opens VGCCs
4. Ca++ influx
5. serotonin released
6. binds to 5-HT receptors on nerve axon

Question 12

Describe the transduction mechanism for sweet, bitter & umami tastants

Answer 12

1. Tastant binds to gPCR
2. activates PLC to convert PIP2 into DAG & IP3
3. IP3 causes Ca++ influx via
   * releasing from intracellular stores
   * opening Na+ channels → depolarisation → VGCC open
4. Ca++ influx causes ATP release through ATP channel
5. ATP binds to purine receptor on nerve axon

Question 13

How many different types of bitter receptor proteins are there and why?

Answer 13

30 different types (although a single cell will only express one type). This variation is for survival and detecting different poisons

Question 14

What are the gustatory afferent neurons?

Answer 14

Cranial nerves VII, IX & X

Question 15

What is ageusia?

Answer 15

The loss of taste perception

Question 16

Describe the central taste pathways

Answer 16

tongue/epiglottis
↓
cranial nerves VII, IX, X
↓
Medulla - gustatory nucleus
↓
Thalamus - ventral posterior median nucleus
↓
gustatory cortex
(with projections to amygdala & hypothalamus)

Question 17

What is the broad tuning of neurons?

Answer 17

Taste sensory neurons are not that specific as each fibre connects to several receptor cells. There is no specific “labelled lines” for specific tastes.

Question 18

Describe population coding

Answer 18

Intepreting taste information from lots of neurons that respond slightly differently to different stimulus, plus taking into account olfaction & somatic sensation.

Question 19

What is the general structure of the olfactory epithelium?

Answer 19

• Basal cells (for replacing cells)
• supporting cells (structural support)
• olfactory receptor cells
  -axons leading up through cribriform plate (constituting olfactory nerve)
  -dendrites are cilia extending into mucous later

Question 20

What is an odorant?

Answer 20

An olfactory stimulant that activated the transduction processes in neurons

Question 21

What is anosmia?

Answer 21

inability to smell

Question 22

Describe the mechanism of transduction by olfactory receptor neurons

Answer 22

odorant binds to gPCR
↓
activates G_olf protein
↓
activates Adenylyl cyclase, causes formation of cAMP
↓
cAMP binds & opens cation channel
↓
influx of Na+ & Ca++
↓
Ca++ opens Cl- channels causing Cl- efflux
↓
causes depolarisation
↓
receptor potential in cilia travel to soma & trigger AP along the axon

Question 23

Describe the broad tubing of olfactory receptor cells

Answer 23

• 1 receptor cells expresses 1 type of receptor protein
• 1 receptor protein binds many odorants
• therefore receptor cells respond to a broad range of odours (and can respond to varying degrees)

Question 24

what are mitral cells?

Answer 24

second-order olfactory neurons within the olfactory bulb

Question 25

What is a glomerulus

Answer 25

The bundle where mitral cell dendrites and olfactory nerve axons synapse

Question 26

How are primary olfactory axons mapped to glomeruli?

Answer 26

All receptor cells with a particular receptor protein type project their axons to the same glomerulus

Question 27

what type of cortex is pyriform cortex?

Answer 27

paleocortex (3 layered)

Question 28

Describe the central olfactory pathways

Answer 28

olfactory receptors
↓
olfactory nerve
↓
olfactory bulb
↓
olfactory tract
↓
pyriform cortex → hippocampus/amygdala
AND/OR
olfactory tubercle → thalamus (medial dorsal nucleus) → prefrontal cortex

Question 29

Describe spatial and temporal representation of olfactory information

Answer 29

spacial representation - olfactory population coding and sensory maps - particular localised areas of the optic bulb respond to particular odours.
Temporal - patterns of spiking in olfactory neurons, also evident in spatial odour maps. we don’t fully understand how this represents information yet.