lecture 10

Question 1

Why do foods lose their flavour when you have a cold?

Answer 1

• because it is much harder to detect smells and so your sense of flavour is diminished
• flavour ≠ taste
• flavour is far more subtle while taste is fairly straightforward

Question 2

What is flavour?

Answer 2

• the sensory experience of food and drink
• dominated by smell and taste: but can include texture, appearance, temperature, pain (chilli), fat
• taste is confined to 5 well defined sensory stimuli and two less well defined: pungent or piquant, metallic
• smell – humans can detect more than 2000 different odours (only 5 odours in a sequence however)

Question 3

What are the basic structures that contribute to flavour?

Answer 3

• both olfaction and taste contribute to flavour
• food/drink in mouth can activate taste (gustatory) afferents and olfactory afferents – olfaction via diffusion of volatile odorants into nasal cavity

structures:
- nasal cavity
- palate
- tongue
- pharynx
- epiglottis

Question 4

What is the taste map of the tongue?

Answer 4

Different parts of the tongue sensitve to different tastes

• sweet, tip
• salt, forward edges
• sour, central edges
• bitter, back edges and back centre
• umami (glutamate), ???? - enhances all the other tastes, enhances olfactory senses
• this is a little old fashioned - the whole tongue can respond to any one of the tastes but there are certain regions that will pick out certain tastes more

Question 5

What is the structure of taste buds?

Answer 5

• papillae are taste sensitive structures
• taste buds are collections of taste receptive cells
• usually have 2000-5000 taste buds
• taste cells turn over in about 2 weeks
• apical surface of taste cell is exposed on the surface of the tongue and therefore can contact the chemical environment of the tongue
• basal end has on it nerve terminals

Question 6

How to taste buds have selectivity?

Answer 6

• taste cells in a single taste bud have different sensitivities
• final output from each taste bud must be integrate centrally to achieve tongue’s regional selectivity
• taste cells that are depolarised by a chemical interacting with its apical surface can release neurotransmitter, which in turn activates the nerve terminals

Question 7

What is the sensory transduction process for simple tastes in the taste system?

Answer 7

• salt detects sodium ions – depolarise taste cell via amiloride sensitive Na channels (different from a voltage dependent channel): inflow of calcium ions releasing granules that contain the equivalent of neurotransmitter onto the primary sensory neuron - activation of an action potential
• acid detects hydrogen – depolarise taste cell via H+ channel

Question 8

What is the sensory transduction process for more complex tastes in the taste system?

Answer 8

• sweet and umami may have more than one receptor mechanism
• each operates via a heterodimer receptor
• • T1R2/T1R3 = sweet
• • T1R1/T1R3 = umami
• receptors coupled via alpha-gustducin (G-protein) to phospholipase C and hence to depolarising mechanisms
• Note, T1R1/T1R3 receptors respond to glutamate and are enhanced by inosine. Also have a specialised metabotropic glutamate receptor with similar coupling to membrane potential
• bitter receptors said to be T2R
• however something missing from our knowledge in regards to how we have different effects of the same chemicals on different cells - why? how?

Question 9

What are TRP channels?

Answer 9

• Ion channels (e.g. TRPM5) that are activated by the receptors for things such as glutamate. - Transient receptor potential channels.
• The classic first one described is TRPC.
• Best known = TRPV1 = receptor for Capsaicin (i.e. chilli) a
• let a large amount of calcium in down the concentration gradient therefore actively depolarising the cell.
• TRPM tend to respond to menthol.
• High calcium permeability channels that can be activated by second messenger systems as well as small molecules.

Question 10

How do we get from taste cell to gustatory afferent?

Answer 10

• depolarisation of taste cell leads to release of excitatory transmitter that depolarises gustatory afferent terminals
• transmitter controversial
• • serotonin (5-HT)
• • glutamate
• • acetylcholine
• • noradrenaline (norepinephrine)
• • GABA
• all of these can be and have been implicated in the transmission from the taste cell to the primary afferent terminal
• likelihood is that all of them are likely to be involved in one way or another

Question 11

What are the central taste pathways?

Answer 11

• gustatory afferents synapse in gustatory nucleus part of solitary nucleus of medulla (ipsilateral projection)
• perception mediated via gustatory projection to ventral posterior medial nucleus of thalamus
• primary gustatory cortex – insula and frontal operculum
• note, interactions with olfactory centres lead to sensation of flavour

Question 12

What is the olfactory epithelium?

Answer 12

• lies on the roof of nasal cavity
• contains olfactory receptor cells (neurons) that are continually turning over
• olfactory receptor cells send axons through Cribiform plate to the olfactory bulb

Question 13

How does olfactory transduction occur?

Answer 13

• many different odorants can be detected
• selectivity depends on odorant receptor molecule
• pathway common after activation of the receptor
• odorant receptors located on the ends of cilia
• g-protein coupled receptor - g-olf that when dissociated activates adenyl cyclase
• leads to an increase in cAMP concentration which activates a class of receptor that are called cyclic nucleotide activated cation channel - lets calcium and sodium into the cytoplasm therefore depolarising the membrane, and activates a chloride channel that further depolarises the membrane

Question 14

How do we get tuning of olfactory receptor cells?

Answer 14

• different olfactory receptor neurons have different response profiles to arrays of odorants
• profile determines “receptive field”
• specificity a property of odorant receptor and central processing
• usually some sort of spectrum of odorants detected but central processing singles one out

Question 15

What is the structure of the olfactory bulb?

Answer 15

• very primitive part of brain
• mitral cells receive olfactory information
• second order olfactory neurons have branching dendritic trees that form glomeruli with terminals of olfactory receptor cells
• individual glomeruli encode only one odour
• granule cell neurons act as tuning interneurons

Question 16

How do we get specific mapping of olfactory neurons onto olfactory bulb?

Answer 16

• receptor cells synapsing within a particular glomerulus all have the same receptive field (express the same odorant receptor)

Question 17

What are the central olfactory pathways?

Answer 17

• projection neurons of olfactory bulb project directly to olfactory cortex and then to thalamus
• also have projection via olfactory tubercule to medial dorsal thalamus and then orbitofrontal cortex
• integration with mood and affect via amygdala