lecture 10 Flashcards

gustation and olfaction: taste and smell sensors that detect chemicals - difference between olfaction and taste: contribution to flavour - the 5 different tastes: role of taste buds and taste cells; sensory transduction - central gustatory pathways - role of olfactory receptor cells (neurons): receptive fields, sensory transduction, pathway - role of olfactory bulb: mitral cells and granule cells - central olfactory pathways

1
Q

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

A
  • 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
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2
Q

What is flavour?

A
  • 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)
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3
Q

What are the basic structures that contribute to flavour?

A
  • 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

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4
Q

What is the taste map of the tongue?

A

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
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5
Q

What is the structure of taste buds?

A
  • 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
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6
Q

How to taste buds have selectivity?

A
  • 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
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7
Q

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

A
  • 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
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8
Q

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

A
  • 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?
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9
Q

What are TRP channels?

A
  • 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.
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10
Q

How do we get from taste cell to gustatory afferent?

A
  • 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
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11
Q

What are the central taste pathways?

A
  • 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
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12
Q

What is the olfactory epithelium?

A
  • 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
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13
Q

How does olfactory transduction occur?

A
  • 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
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14
Q

How do we get tuning of olfactory receptor cells?

A
  • 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
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15
Q

What is the structure of the olfactory bulb?

A
  • 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
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16
Q

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

A
  • receptor cells synapsing within a particular glomerulus all have the same receptive field (express the same odorant receptor)
17
Q

What are the central olfactory pathways?

A
  • 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