1
Q
Gustation
A
taste
- to identify food, avoid noxious/toxic food
2
Q
5 dimensions of taste
A
sweet,sour, bitter, salty, umami
3
Q
free protons
A
H+ - acidic
4
Q
umami
A
- sensation induced by glutamate
- ‘savory tast’ -‘meaty’‘brothy’
e.g. soy sauce, msg
5
Q
what does umami indicate
A
high protein
6
Q
anatomical units of gustation
A
- taste buds embedded in epithelium of tongue
7
Q
taste buds
A
~10 taste cells
- opening on top of taste buds allows cells to be activated
8
Q
cells in taste buds
A
- Type I cell (glial)
- Type II cell
- Type III cell
9
Q
Type II & III cells
A
- only signal within taste bud, not directly to brain
- neurons respond to stimulant in taste bud with graded changes in polarisation (depolarisation) and neurotransmitter release
10
Q
depolarisation
A
occurs when stimulant enters cell, cations move in, more neurotransmitter released
- sodium/ potassium exchanger
11
Q
resting potential sodium/potassium exchanger
A
- high intracellular potassium
- high extracellular sodium
12
Q
synaptic release
A
- voltage gated sodium and calcium channels for depolarisation and neurotransmitter release
13
Q
type III cells
A
sour
14
Q
type II cells
A
- bitter
- sweet
- umami
15
Q
type II cells: GPCRs
A
- G protein Gustducin: Gβγ
- beta/gamma subunit goes off and activates phospholipase C
- PLC catalyses PIP2->IP3+DAG
16
Q
IP3
A
inosotyl triphosphate
17
Q
DAG
A
Diacylglycerol
18
Q
IP3 as second messanger
A
- binds to and activates receptor on endoplasmic reticulum
- leads to calcium leaving ER and entering cytosol, depolarising neuron
19
Q
what is calcium detected by in taste buds
A
cation channel -TRPM5
20
Q
TRPM5
A
opens when Ca content high in cell, and allows more cations in e.g. Na, depolarisation
21
Q
Type II cells summary
A
- receptor activation
- activates G protein gustducin
- b/y interact with PLC
- PLC increases IP3 and DAG
- IP3 activates receptors on endoplasmic reticulum to allow calcium to leave ER to cytosol
- increased Ca, leads to TRPM5 channel opening, letting cations into neuron
- neurons depolarise
22
Q
Type II cells specificity to detect ‘sweet’
A
- sucurose, fructose, aa etc
- GPCR T1R2:T13
- hetero dimer to detect sweet
23
Q
Type II cells specificity to detect ‘umami’
A
- option 1: hetero dimer T1R1:T1R3
- option 2: ‘conventional’ metabotropic glutamate receptors
mGluR1/mGlur4
24
Q
Type II cells specificity to detect ‘bitter’
A
- variety of natural and synthetic compounds
- T2R
- ~30 genes
25
Type III cells: sour
- organic acids e.g. citric and acetic acids
- pH sensitive K+ leakage channel
26
potassium leakage channels
when acid enters cell, pH falls, K channels close, leakage of cations stops, cells become depolarized
27
free protons
from acid, can also depolarize cells
28
salty taste
NaCl
- high vs. low, we don't know if they have the same mechanisms
- ENaC: epithelial sodium channel, Na into cell and depolarizes it
29
type III cells: neurotransmitter released to cranial nerves
serotonin
30
type II cells: neurotransmitter released to cranial nerve
ATP
31
TRP channels on somatosensory receptors
- chilli, ginger, pepper etc.
- 'warmth'
- act directly on TRP
32
type III releasing GABA
inhibits type II , e.g. too sour so can't taste anything else
33
fatty acids
- innate preference for fats
- may activate pancreas?
- no clear taste for this
34
kokumi
'rich'
no clear evidence
35
Ca2+ detection
- might be a separate mechanism
- bitter/sour
- innate aversion
36
T2R proteins
GPCRs
- detect presence of chemicals in extracellular environment based on the shape and chemistry of ligand and receptor aa sequence
...
different genes encode receptors detecting different compounds
37
bitter taste cells contain...
more than one T2R
38
phenylthiocarbamide (PTC)
- isothiocynate
- act as iodine collators, more results in a deficiency
- tasters vs. non-tasters
- 'balancing natural selection'
39
receptor responsible for detecting PTC
TAS2R38
- amino acid composition determines whether it's bitter or not
40
non-PTC sensitive polymorphism
- convergent evolution
- do not lack the gene
- potentially detect other bitter compounds instead?
note: those that are heterozyote have an advantage
41
advantages of T2Rs summary - PTC
- bitterness is about avoiding toxins
- receptor affinity for different compounds under strong selection
- allows a wide variety of toxins to be detected
- as changes in aa sequence alter compound it detects
- helps us avoid toxins
- fine tuning of receptor specificity by natural selection
42
linking taste buds to hind brain
- sensory ganglia
43
3 synapses linking taste buds to brain
- olfactory sensory neuron
- gustatory area of the nucleus of the solitary tract
- target neurons in the brain
44
what is taste determined by
the identity of the cell activated, not the nature of activation
- local connections between taste cells mixes things up at the start
45
what two tastes can co-occur
umami and sweet
46
ganglionic neurons
- receive input from different types of sensory neurons of multiple dimensions
- combinatoric code; you have to look at population of neurons to see the taste of something
47
specialist ganglionic neurons
respond to one taste
48
generalist
respond to multiple tastes
49
how do we determine what to eat?
50
innate aversion
bitterness
51
innate cravings
sweet, umami, salt
52
what happens when NaCl balance impaired
- adrenal tumour
BIOL21341/ Sensory Systems
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