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Flashcards in lecture 10 Deck (17):
1

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

- 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

2

What is flavour?

- 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)

3

What are the basic structures that contribute to flavour?

- 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

4

What is the taste map of the tongue?

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

5

What is the structure of taste buds?

- 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

6

How to taste buds have selectivity?

- 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

7

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

- 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

8

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

- 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?

9

What are TRP channels?

- 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.

10

How do we get from taste cell to gustatory afferent?

- 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

11

What are the central taste pathways?

- 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

12

What is the olfactory epithelium?

- 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

13

How does olfactory transduction occur?

- 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

14

How do we get tuning of olfactory receptor cells?

- 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

15

What is the structure of the olfactory bulb?

- 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

16

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

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

17

What are the central olfactory pathways?

- 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