Week 3: Taste
How does PTC gene expression affect taste?
-/- PTC gene gives off non tasters (less papillae and fewer free nerve endings) +/- PTC gene gives off tasters +/+ PTC gene gives off super tasters (more papillae and more free nerve endings).
What is the process of taste?
1. tastants dissolve into saliva (released by chewing). 2. saliva (filled with tastants) bathes papillae on the tongue. 3. saliva (filled with tastants fills papillary trenches and bathes taste buds located on the papillae. 4. tastant receptors become activated (signal transduction occurs). 5. signals are passed to primary afferent neurons which travel to the brain via cranial nerves VII, IX and X). 6. the signal is passed to the neurons in the brain stem (the nuclear solitary tract). 7. the signal is passed to neurons in the thalamus. 8. the thalamus relays to the primary taste cortex (insula-fO) and secondary taste cortex (orbitofrontal cortex/OFC).
Roughly how many taste buds does the average human have?
5,000-10,000.
Where are the taste buds found?
90% of taste buds are found on the papillae, the rest are spread throughout the mouth.
What is being transduced in gustation?
A chemical stimulus (tastant) to a neural potential via neurotransmitter.
What is AMP?
A chemical that prevents bitter compounds from tasting bitter.
What do taste buds look like?
A garlic clove , they have multiple bulbs with cilia on one end.
What elicits the sensation of taste?
A sufficient concentration of tastants.
Where do the thalamic neurons project to?
All senses except for olfaction relay through the thalamus before going to the sensory cortices. Thus the thalamic neurons project to the primary gustatory cortex (insular fO) and the secondary taste area in the orbitofrontal cortex.
Where do the neurons of the brainstem project to?
Amydalar neurons, hypothalamic neurons and thalamic neurons.
What damage would cause complete loss of taste?
Bilateral insular damage (NB. you only need a cortex in order to perceive a sense).
What shaped neurons are found in the OFC?
Bimodal neurons.
What are super tasters more sensitive to?
Bitter things, eg. coffee, quinine, and grapefruit juice.
How does taste indicate safety of food?
Bitterness indicates toxicity and rancidness. Sourness indicates acidity and rancidness. Salt indicates salinity (you don't want too much salt in your diet).
From measuring neuronal activity at the level of the taste receptor neuron what leads us to understand that taste buds do not act via the pattern idea?
By knocking out a specific receptor it significantly alters neural firing (at the taste receptor neuron level) and an animals response to that tastant (this shows that one tastant specifically activates one type of taste neuron at the taste bud level).
How is taste intensity coded?
By the rate of firing.
Why does chemotherapy cause a decline in taste?
Cancer is a disorder of uncontrolled cell proliferation, so chemotherapy targets highly proliferative cells. Taste cells are highly proliferative so they are targeted by chemotherapy.
How does variety affect weight?
Causes weight gain, for you to eat more.
What are taste modifiers?
Certain chemicals that mask or alter the taste of tastants (blocking sweet receptors, or altering sour receptors so sweet tastants fit.
What pattern do bitter, salty and sour display?
Compression (a>1).
How does chemosensory information contribute to flavour?
Coolness of mint Carbonation of fizzy drinks Burn of alcohol Hotness of capsaicin (salivation, tearing, nasal mucus production, vasodilation, sweating, bronchoconstriction).
Which nerve is involved with chemosensory information often involved in flavour?
Cranial nerve V otherwise known as the trigeminal nerve.
What cortical processing goes on in the primary gustatory cortex (insular fO)?
Detection of tastant and intensity, and identification of the scent.
What hormone is released by gustation and what feeling does this create?
Dopamine is released which causes salience/pleasure.
What is hedonic eating?
Eating because you enjoy it.
What pattern do sweet and umami display?
Expansion (a<1).
What is satiety sensitivity?
Firing rate of satiety sensitive neurons decreases which may cause decreasing interest levels.
What is the function of stem cells in the taste bud?
For replacing the taste buds as they die every few days.
What creates umami?
Glutamate.
What creates sour taste?
HCl
WHat cortical processing goes on in the secondary taste cortex?
Hedonic (pleasure) assessment.
What would happen if you severed a cranial taste nerve (VII, IX, X).
If you severed a taste nerve you would not lose all sense of taste, because the others would compensate. You would have a weakened sense of taste (hypogeusia).
What are the three functions of taste?
Indication of safety. Nutritional value. Reinforcing eating behaviour.
How does genetics affect taste detection?
Individual variations in number of papillae, number of taste receptors and types of receptors.
What type of molecules can activate taste receptors?
Ions (eg H+), complex molecules and long chain molecules (eg glutamate) can all act as tastants.
Is the DA signalling relationship positive or negative in modern times?
It appears that the DA signalling relationship may be harmful as it causes a shift in hedonic feeding. We seek higher levels of DA and causes more eating and contribute to obesity.
What happens when there is damage to the OFC?
It causes impaired performance in choice preference tasks and time delay tasks.
How does taste reinforce eating behaviour?
It is an evolutionary function. You need food to live and if it tastes good you will want to eat more.
If all areas of the tongue respond to all 5 sub-modalities to some degree what does this suggest about taste bud receptors?
It suggests that taste buds contain mixed populations of taste receptors.
How many tastants does it take to activate taste receptors?
It takes 1 tastant to activate 1 tastant recpetor
How is the taste relay pathway unique?
It utilises multiple cranial nerves whereas all other senses only use on cranial nerve.
What type of sense is gustation?
Like olfaction, it is a chemical sense.
What chemical produces umami taste?
MSG
What are some products that AMP is used in?
Medicine for children. In fizzy drinks. In canned food to reduce the amount of salt needed.
What creates salty taste?
NaCl.
What does the hedonic treadmill lead us to seek out?
New tastes and more intense tastes.
DA signalling was an evolutionary function, is this necessary for modern day adults?
No because we have other cues to remind us to eat.
Do all animals have taste buds on their tongue?
No some animals have them on other parts of the body, eg. insects have them on their feet, fish have them on the exterior of their bodies.
Are the absolute thresholds the same for each sub-modality?
No umami has the highest around 20mM and bitter has the lowest around 0.002.
Does identity depend on what area of the tongue is activated?
No, although some areas are more sensitive to some tastants than others all areas of the tongue respond to all tastants.
Which neurons exhibit satiety sensitivity?
Orbitofrontal cortex neurons.
WHere is the secondary taste cortex?
Orbitofrontal cortex/OFC.
What is Steven's Power law?
Perceived intensity = k (I)^a.
What creates bitter taste?
Quinine.
How many sub-modalities do tastants fit into?
SOme tastants only fit into one sub-modality (eg sucrose, quinine and glutamate), while others elicit combinations of sub-modality activation (eg NaNO3= salty, sour and bitter).
What chemicals cause sweet taste?
Saccharin and sucralose.
What chemicals cause salt taste?
Salt substitutes.
What does activation of the taste receptor cause?
Signal transduction in that neuron.
How can expectation affect taste detection?
Simple knowledge of the identity of a tastant can improve detection performance.
What other sense activates the orbitofrontal cortex?
Smell/olfaction.
Which taste receptors are simple ion channels?
Sour and salty taste receptors.
How does temperature influence taste?
Sour is pretty constant but salt intensity decreases with increasing heat and sweet intensity increases with increasing temperature.
From measuring neuronal activity at the level of the primary sensory neuron (VII, IX, X) what can we determine?
Specificity lessens (firing occurs for all tastants), suggesting that these neurons are broadly tuned (knocking out the specificity taste bud theory).
What does a stand for?
Steven's constant.
What factors influence taste detection?
Sub-modality Temp Expectation Sex Age Genetics.
What creates sweet taste?
Sucrose.
Which PTC group are more sensitive to chemosensory information?
Super tasters (+/+).
What are the submodalities of taste?
Sweet Sour Salty Bitter Umami
How does taste indicate nutritional value?
Sweet indicates carbohydrates. Umami indicates protein. Salt indicates salinity (you need some salt in you diet).
What does the tip of the taste receptor neuron contain?
Tastant receptors.
What is the pathway of gustation?
Tastants are put on the tongue, dissolved in saliva so they reach the tastebuds to activate the taste receptors on taste neurons. The axons of these neurons make up VII, IX, X which relay to the brainstem and thalamus before reaching the primary taste cortex (insula) and the secondary taste cortex (occipitofrontal cortex).
How does age affect taste detection?
Taste detection declines with age.
Why is taste not the same as flavour?
Taste is just gustation, flavour involves taste, smell, texture, temperature and chemosensory information.
What types of cells are found in each taste bud?
Taste receptor neurons, support cells stem cells.
What does the pattern taste bud idea suggest?
That each cell expresses multiple receptor types and is broadly tuned to all tastants. This broad pattern of activity is sent through the relay.
What does the specificity taste bud idea suggest?
That each cell expresses only one type of receptor that is finely tuned to one specific tastant and specificity is maintained throughout the relay (neurons of the same tastant join up).
What does the combination idea suggest?
That each cell is finely tuned to one specific tastant however fibres pick up a broad pattern of activity from a group of cells.
What part of the tongue is related to X?
The back portion of the tongue.
From neuronal recording which taste bud theory seems most likely?
The combination theory which has specific neurons but broadly tuned fibres (specific at periphery and pattern encoding at more central cells).
What does I stand for?
The initial stimulus value.
Where is the primary taste cortex?
The insula-fO.
What part of the tongue is related to IX?
The middle.
What is the flavour cortex and why is it called that?
The orbitofrontal cortex. So called because it is activated by both gustation and olfaction, two key components of flavour.
What does Steven's power law allow us to determine?
The relationship between stimulus amount and perceived intensity.
What functions of taste are undermined by foodtech?
The safety and and nutritional functions.
What are the two theories that suggest that the taste-dopamine relationship influence obesity?
The theory that some people have innately fewer DA receptors causing them to eat more to get the same level of DA release/pleasure. The theory that suggests obesity itself leads to a blunted DA response to food, so obese people don't get the same pleasure form small amounts of food.
Which part of the tongue is related to VII?
The tip.
What is the physiological advantage of having a low absolute threshold for bitter and sour tastes?
These are the tastes that tend to indicate danger in foods so the low threshold allows us to easily determine poisonous food from good.
Are tastants lipophilic or lipophobic?
They are lipophobic/hydrophobic which means they are water soluble, and dissolve in water.
What are papillae?
They are the bumps on the tongue that contain the taste buds on either their tops or sides.
What happens to the number of taste buds we have over a lifetime?
They increase with age from birth until about 40 years and then decline with age.
Where must all information bound for the cortex go first?
Through the thalamus.
Which taste receptors are GPCRs?
Umami, bitter and sweet taste receptors.
What cranial nerves are involved in gustation?
VII, IX and X (7, 9, 10).
What happens to our ability to taste with age?
We become hypogeusic with age.
What is homeostatic feeding?
We feel hungry... so we seek out food.... so we eat.... satiety signal tells us to stop.... DA signal tells us to remember this process.
What does k stand for?
Weber's value.
Why does taste identification decline with age?
With age, semantic recall declines, salivation lessens and taste bud receptors.
Where are the taste receptor neurons located?
Within the taste buds.
How does sex affect taste detection?
Women are more sensitive than men.
