The chemical sense

Question 1

Chemoreceptors in the neck

Answer 1

Measure CO2/O2 levels in our blood

Question 2

Chemical irritants

Answer 2

Nerve endings in skin/mucous membranes warn us of chemical irritants

Question 3

Acidity

Answer 3

Sensory nerve endings in muscle respond to acidity –burning feeling that comes with exercise and O2 debt

Question 4

Learned taste

Answer 4

Experience can strongly modify our innate preferences.

Humans can learn to tolerate or enjoy the bitterness of some substances (e.g. coffee)

Question 5

Innate taste

Answer 5

• Some of our taste preferences are inborn (or “innate”)
• Humans innately enjoy sweet flavours and avoid bitter flavours – this is evolutionarily ancient (e.g. distinguish food sources, avoidance of toxins)

Question 6

Palate

Answer 6

• Roof of mouth separating oral and nasal cavities – taste buds present in palate

Question 7

Epiglottis

Answer 7

Leaf shaped cartilage covering laryngeal inlet – taste buds present in epiglottis

Question 8

Pharynx and nasal cavity

Answer 8

Odours can pass, via the pharynx, to the nasal cavity to be detected by olfactory receptors

Question 9

Papillae

Answer 9

I. Ridge-shaped (foliate)
II. Pimple-shaped (vallate)
III. Mushroom-shaped (fungiform)

Question 10

Tatse bud

Answer 10

The papillae contain taste buds:
• Taste buds contains taste receptor cells
• Taste buds are surrounded by basal cells (precursors of taste cells) and
• gustatory afferent axons

Question 11

Taste receptor cells

Answer 11

• Three taste receptor cells sequentially exposed to salt, bitter, sour and sweet stimuli – membrane potential recorded
• Taste receptor cells display different sensitivities
• Taste receptor cells form synapses with gustatory afferent axons to transmit this gustatory information

Question 12

Saltiness and sourness

Answer 12

Ion channel mechanisms

Question 13

Saltiness taste transduction mechanism

Answer 13

• Na+ passes through Na+ selective channels, down its concentration gradient
• This depolarises the taste cell, activating voltage-gated Ca2+ channels (VGCCs)
• Vesicular release of neurotransmitter is elicited, and gustatory afferents activated
Special Na+ selective channel (amiloride sensitive) used to detect low concentrations of salt – insensitive to voltage and generally stays open

Question 14

Sourness - taste transduction mechanism

Answer 14

Protons (H+) are the determinants of acidity and sourness.
• H+ may affective sensitive taste receptors in several ways – although these processes are not well understood
• However, it is likely that H+ can pass through proton channels and bind to and block K+ selective channels
• This leads to depolarisation of the taste cell, activating VGSC and VGCCs
• Vesicular release of neurotransmitter is elicited, and gustatory afferents activated

Question 15

Bitterness, Sweetness, umami

Answer 15

GPCR mechanisms via T1 and T2 taste receptors

Question 16

T1 Rs and T2Rs are

Answer 16

G-protein coupled receptors (GPCRs) and are Gq coupled

Question 17

Bitter

Answer 17

Detected by approximately 25 T2Rs
• Bitter tastants binds to T2R, which is coupled to the G-protein Gq
• This stimulates the enzyme phospholipase C (PLC), leading to the production of inositol triphosphate (IP3)
• IP3 intracellularly activates a special type of Na+ ion channel and releases Ca2+ from intracellular storage sites
• Both these actions depolarise the taste cell – release of ATP is elicited, and gustatory afferents are activated

Question 18

Sweet

Answer 18

Detected by one receptor – T1R2 and T1R3 proteins

• Sweet tastants binds to dimer receptor formed from T1R2 and T1R3, which is coupled to the G-protein Gq

Question 19

Umami

Answer 19

Detected by one receptor – T1R1 and T1R3 proteins
• Umami tastants bind to dimer receptor formed from T1R1and T1R3, which is coupled to the G-protein Gq
• The same signal transduction mechanism as bitterness and sweetness occurs
• Shares T1R3 protein with sweetness – T1R subunit determines specificity to umami

Question 20

Why do we not confuse bitter and sweet tastes?

Answer 20

• Taste cells express either bitter or sweet receptors – not both
• In turn, bitter and sweet taste cells connect to different gustatory axons

Question 21

Central gustatory pathways

Answer 21

• The main flow of taste information is from taste cells to gustatory axons, into the gustatory nucleus (medulla), up to the ventral posterior medial nucleus (thalamus) and to the gustatory cortex
• Three cranial nerves carry gustatory axons and bring taste information to the brain:
CN 7
CN 9
CN 10

Question 22

Smell as a mode of communication

Answer 22

Pheromones are olfactory stimuli used for chemical communication between individuals.
In some animals, pheromones are important signals for reproductive behaviours, marking territories and indicating aggression or submission
However, the importance in pheromones in humans is unclear

Question 23

Olfactory receptor cells

Answer 23

• Site of transduction – genuine neurons unlike taste receptor cells

Question 24

Suppoorting cells

Answer 24

• Function to produce mucus – odorants dissolve in mucus layer before contacting cilia of olfactory receptor cells

Question 25

Basal cells

Answer 25

• Immature olfactory receptor cells able to differentiate into mature olfactory receptor cells – olfactory receptor cells continuously grow, degenerate and regenerate

Question 26

Olfactory transduction mechanisms

Answer 26

• Odorant molecules bind to odorant receptor proteins on the cilia
• Olfactory-specific G-protein (Golf) is activated
• Adenylyl cyclase activation increases cAMP formation
• cAMP-activated channels open, allowing Na+ and Ca2+ influx
• Ca2+ activated chloride channels open enabling Cl- efflux
• Causes membrane depolarisation of the olfactory neuron

Question 27

Central olfactory pathways

Answer 27

• Olfactory receptor cells send axons into the olfactory bulb
• Olfactory receptor cells expressing the same receptor proteins project to the same glomeruli in the olfactory bulb
• Signals are relayed in the glomeruli and transmitted to higher regions of the brain

Question 28

Population coding for gustation and olfaction

Answer 28

• Gustatory and olfactory receptor cells may express only one specific receptor protein
• However, gustatory and olfactory axons and the neurons they activate in the brain respond more broadly
• Only with a large population of neurons, with different response patterns, can the brain distinguish between specific tastes and smells

Question 29

An example of population coding for olfaction

Answer 29

• Olfactory receptor cells express a single olfactory receptor protein – each can respond to different odours with differing preferences
• When presented with a citrus smell, none of the three receptor cells can individually distinguish it from the other odours
• However, the brain can distinguish the citrus smell through the combination of responses from all three cells
• It is estimated that humans can discriminate at least one trillion different combinations of odour stimuli