OCB02-2025

Question 1

What is taste?

Answer 1

What is detected by tastebuds

Question 2

What is smell?

Answer 2

What is detected by the olfactory epithelium

Question 3

What is flavour?

Answer 3

Combination of taste and smell

Question 4

What are the two routes of substances reaching the olfactory epithelium?

Answer 4

Orthonasal (environmental)

Retronasal (after swallowing)

Question 5

Describe the olfactory epithelium.

Answer 5

Neurons project through the epithelium and cilia lie on top, covered in a layer of “mucus” (made by Bowman’s glands)

Bipolar neurons with axons projecting to the olfactory bulb

Question 6

What can cause a loss of smell?

Answer 6

Inhaling noxious fumes like sulphuric acid gas

Concussion-type knocks to the head can damage neurons or cribriform plate

Question 7

What produces the “mucus” on top of the olfactory epithelium?

Answer 7

Bowman’s glands

Question 8

What part of the nervous system controls the Bowman’s glands?

Answer 8

Autonomic

Question 9

What does the secretions of the Bowman’s glands contain?

Answer 9

Mucins

Secretory antibodies (IgA) and other proteins

Odorant binding proteins

Question 10

What are odorant binding proteins?

Answer 10

Proteins that can specifically pick up different smells and transport them through the mucus to receptors on olfactory neurons

Question 11

What affects the receptors that are present on olfactory neurons?

Answer 11

Epigenetic regulatory mechanisms

Question 12

What is the ON olfactory transduction pathway?

Answer 12

Odorant binds odorant receptor which triggers the Golf protein

Increased adenylyl cyclase activity and increased cAMP

Opens Na+/Ca++ symport channel to depolarise cell to trigger an action potential

Calcium influx activates Ano2 Cl- channel to cause Cl- efflux to amplify the signal

Question 13

What is the OFF olfactory transduction pathway?

Answer 13

High calcium from ON mechanism helps to deactivate the Na+/Ca++ channel and phosphodiesterase removes cAMP

NCKX4 transporter pumps Ca++ out and 3Na+ in

Mitochondria take up Ca++

Extracellular proteases help to remove odorant from receptor

Question 14

What salivary glands are found at the bottom of the crypts of the circumvallate and foliate papillae?

Answer 14

Von Ebner glands (serous)

Question 15

What enzyme is present in the saliva from Von Ebner glands?

Answer 15

Lingual lipase

Question 16

What is the function of lingual lipase from Von Ebner glands?

Answer 16

Prevent blockage of channels/crypts by fat

May have a role in fat perception

Question 17

What vegetable does a tastebud look like?

Answer 17

Onion

Question 18

Describe a tastebud.

Answer 18

Onion-like, epithelial structures

Turn over fairly quickly ~8-10 days

Have a “taste pore” which opens onto the tongue surface and an associated nerve fibre

Question 19

Describe the type 1 cells in tastebuds.

Answer 19

Glial-like

Ensheath (cover and support) type 2 and 3 cells

Have some synapse-clearing properties - produce enzymes that remove K+ and ATP

Question 20

What are the type 2 cells in tastebuds?

Answer 20

Receptors to sweet, bitter and umami

Have no obvious synapse to nerves

Question 21

What are the type 3 cells in tastebuds?

Answer 21

Receptors to sour and salt

Obvious connections to nerves

Question 22

Which cells in tastebuds have obvious synaptic connections with nerves?

Answer 22

Type 3 cells

Question 23

What are the type 4 cells in tastebuds?

Answer 23

Basal cells

Question 24

What are the type 5 cells in tastebuds?

Answer 24

Marginal cells which may be stem cell-like

Question 25

What do type 2 cells respond to?

Answer 25

Bitter

Sweet

Umami

Question 26

What do type 3 cells respond to?

Answer 26

Salt

Sour

Question 27

What type of receptors do type 2 cells have?

Answer 27

7 transmembrane G-protein coupled receptors (7TMGPCR)

Question 28

Describe the receptor for bitter.

Answer 28

7TMGPCR

Homodimer made up of T2Rs

Question 29

Describe the receptor for sweet.

Answer 29

7TMGPCR

Heterodimer

T1R2 and T1R3

Question 30

Describe the receptor for umami.

Answer 30

7TMGPCR

Heterodimer

T1R1 and T1R3

Question 31

Describe the receptor for sour.

Answer 31

TRP receptor = PKD2L1

Responds to protons/H+

Question 32

Describe the receptor for salt.

Answer 32

ENaC = 2α, 1β, 1γ subunits

Responds to Na+

Question 33

Describe the signal transduction in type 2 cells.

Answer 33

Activation (7TMGPCR) activates PLCβ2 and formation of IP3

Release of Ca++ from ER

Increased intracellular calcium activates TRPM5 causing Na+ influx

Depolarisation opens voltage-gated Na+ channels so even more Na+ influx

Release of neurotransmitter ATP which bind to purinergic receptors P2X2/3 on coupled nerve fibre to send an action potential to the brain

Question 34

Describe the signal transduction for sour in type 3 cells.

Answer 34

Voltage-gated Na+ channel opens = Na+ influx and depolarisation

Release of ATP of 5-HT to stimulate coupled nerve fibre

Question 35

Describe the signal transduction for salt in type 3 cells.

Answer 35

Voltage-gated Na+ channel opens = Na+ influx and synaptic transmission

Question 36

What is high aversive salt detected by?

Answer 36

Bitter and sour receptors

Question 37

What is appetitive salt detected by?

Answer 37

Salt receptors (ENaC)

Question 38

Where does the central processing of taste occur?

Answer 38

Nucleus tractus solitarius (brainstem)

Question 39

Describe the central processing of taste.

Answer 39

Facial nerve, glossopharyngeal nerve and vagus nerve connect to NTS and synapse to travel into higher cortex

Modulated by descending input from amygdala, hypothalamus, gustatory cortex

Some input from gut

Signals sent to salivary glands to increase secretion of to increase licking (in animals), chewing and swallowing

Question 40

Where does central processing of smell occur?

Answer 40

Olfactory bulb and tracts (NOT brainstem)