Physiology of Olfaction and Gustation Flashcards
What type of cells are taste buds?
specialized epithelial cells
In what areas of the cell are the receptor proteins and signaling molecules concentrated?
on the microvilli emerging from the apical surface
What is the effect of a tastant binding a receptor?
voltage-gated ion channels open, and channels controlled by TRP (2nd messenger) open
-results in intracellular Calcium release
What does an influx of Calcium into the cell cause?
-synaptic vesicle fusion
-subsequent release of NTR’s
(serotonin or ATP, depending on the tastant)
-activation of primary afferent sensory neuron
What tastants (with their corresponding mechanisms of activation) are responsible for the release of serotonin?
- salty (Na+ ions)
- sour (H+ ions)
What tastants (with their corresponding mechanisms of activation) are responsible for the release of ATP?
- sweet (sugar binding GPCR’s)
- umami (glutamate binding mGluR4)
- bitter (various compounds binding GPCR’s)
What type of cells are olfactory cells?
bipolar neurons
What neurotransmitter is released by olfactory neurons?
glutamate
How does the olfactory pathway begin?
odorant in mucus binds to receptor molecule in cell membrane
The binding of an odorant to its receptor activates what?
the odorant-specific G protein activates adenylate cyclase, which generates a second messenger system to create cAMP
In the olfaction pathway, what does cAMP do?
opens sodium and calcium channels to begin depolarizing the neuron
In the olfaction pathway, the influx of calcium leads to what?
- opening of calcium-gated ion channels that allow influx of Cl-
- the influx of Cl- provides remainder of depolarization
What are the two ways of “getting used” to a smell?
1) phosphorylation of odorant receptor, decreasing sensitivity
2) cyclic nucleotide gated channels become less sensitive to cAMP, lead to less ion influx and less depolarization
What is the purpose of the high binding affinity of bitter tastants to their receptors?
- bitter tastants can be detected at very low concentrations
- bitter is innately aversive to guard against poisons (which usually taste bitter to humans)
What is the purpose of sour tastants being generally aversive?
- signals the presence of dietary acids
- avoid overloading acid/base balance
- spoiled foods are often acidic
What is the role of the umami tastebud?
-reflects a food’s protein content d/t presence of glutamate and other amino acids
What is the role of the salty tastebud?
-governs intake of sodium and other salts essential for maintaining body’s water balance and blood circulation
What is the purpose of the sweet tastebud?
-signals the presence of carbohydrates that serve as an energy source
What happens to gustatory and olfactory sense as humans age?
sensitivity decreases with age
What is the effect of sweet solution placed into the oral cavity of an infant?
- infant’s face relaxes
- decreased HR for several mins
- neural imaging shows patterns similar to a pleasurable stimulation
What is the most widely accepted mechanism for how a sweet solution acts physiologically to reduce pain in infants?
- sweet taste induces beta endorphins
- activates endogenous opioid system
Why do children have a preference for sweetness?
-children need more calories, which is usually linked to a food’s level of sweetness
Why might non-nutritive sweeteners cause problems?
-it’s physiologically unreliable to link the sweetness level of foods with non-nutritive sweeteners with the level of calories in the food
How do sodium salts such as MSG act on bitter tastes?
-sodium salts suppress the bitter taste at the level of the bitter receptor
How does sugar act on bitter tastes?
-sugar masks the bitter taste at the level of higher cognition
What is the role of the nucleus of the solitary tract in processing taste?
- sensory input from CN X relays info about viscera
- early site of gustatory/visceral info
-reflex circuit forms basis for salivary secretions, mimetic responses, and swallowing
What is the role of the Ventral Posterior Medial Nucleus (VPM) of the Thalamus in processing taste?
- relay station for taste perception
- processes the discriminative aspects of taste
What are the components of the gustatory cortex?
- insular taste cortex
- operculum of the frontal lobe
- postcentral gyrus
What is the role of the amygdala in processing taste?
- emotional context to eating
- memories of eating
What is the role of the hypothalamus in processing taste?
-hunger
What is the role of the orbitofrontal cortex in processing taste?
-integrating visual, somatosensory, olfaction, and gustatory stimuli to collectively appreciate the flavor of food
What is the role of the anterior olfactory nucleus in processing olfaction?
relay station to ipsi- and contralateral cortices
What is the role of the piriform cortex and medial orbitofrontal cortex in processing olfaction?
- integration of sight, smell, and taste of food
- appreciation of the flavor of food
What is the role of the anterior cortical amygdaloid nucleus in processing olfaction?
- emotional learning
- olfactory fear conditioning
What is the role of the periamygdaloid cortex in processing olfaction?
-integration of the emotional aspect of food as elicited by odor
What is the role of the entorhinal cortex and hippocampus in processing olfaction?
- memory formation
- how olfactory input facilitates memory/recall
