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3 types of papillae
- circumvallate
- fungiform
- foliate
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describe structure of taste bud
- taste pore at top
- receptor cell in middle
- surrounded by supporting cells ('glial like')
- basal cells at bottom dividing to create new receptor cells
- synapse at bottom
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CN output of taste
- glossopharyngeal
- chorda tympani
- superior laryngeal, from hypoglossal (on epitglottis)
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3 categories of taste cells
- receptor cells: narrow specificity, 2nd messenger: sweet, bitter, umami
- presynaptic cells: sour
- glial-like cells: K and transmitter homeostasis; salt
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where is first synapse in processing of taste?
Within taste bud
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Describe different types of receptor cells
- Umami: T1R1 and T1R3
- Sweet: T1R2 and T1R3
- Bitter: ~30 T2R
- all heterodimeric, GPCR, 2nd messenger coupled
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describe transduction within receptor cells
- GPCR
- IP3 activated by beta-gamma unit
- Ca release from intracellular stores
- and opening of TRP5M cation channels
- depolarisation
- transmitter release (ATP) via gap junction hemichannels
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describe sour taste processing. Which cells?
- Presynaptic cells
- protons cause K channels to close, leading to depolarisation
- Ca channel opening
- graded depending on strength of acid
- depolarisation causes vesicular serotonin release
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describe salt taste processing. Which cells?
- Glial cells
- ions enter directly through epithelial sodium leak channels
- protons can also enter through these channels
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Difference between transmission from receptor cells and presynaptic cells
- receptor: gap junction hemichannels (ATP)
- presynaptic: vesicular serotonin
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first synapse of taste CNs
nucleus of solitary tract of medulla in gustatory nucleus
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describe higher taste pathways
- gustatory NST ? VPM nu of thalamus ? gustatory neocortex (in anterior insula) and frontal operculum
- primary taste cortex ? orbitofrontal cortex (2ry taste area; pleasantness), amygdala (affective component), lateral hypothalamus (feeding behaviour)
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difference with higher taste pathways in lower mammals
thalamocortical pathway has a pontine relay in parabrachial nucleus
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describe sensory coding in taste
- 'across line'
- different nerves respond to all different tastes, but in varying proportions
- need to compare response
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2 inputs to afferent smell fibres
- ATP through gap junction hemichannels from receptor cells
- serotonin from presynaptic cells (these themselves receive purinergic input)
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