6. Gustation Flashcards
what are the two major roles of gustation?
- evaluating the nutritional content of food
- preventing the ingestion of toxic compounds
taste buds sit on top of:
papillae
how many different types of papillae are there?
four
type of papillae with large bumpds and taste buds
fungiform papillae
type of papillae that are dome shaped, have taste buds, and forms a V at the back of the tongue
circumvallate papillae
type of papillae found at the base of the tongue (anterior to the circumvallate papillae) and has taste buds
foliate papillae
type of papillae that has small bumps but no taste buds
filiform papillae
flavour associated with foods rich in glutamates including monosodium glutamate (MSG)
umami
how many cells can be found on a taste bud?
50-150 cells
how often do taste cell receptors turn over?
5-10 days
how many types of taste cell receptors are there?
4
- types I, II, and III are taste receptors
- type IV basal cells become new taste receptors
what type of cells detect salty taste?
type I cells
what type of cells detect sour taste?
type III cells
what type of cells detect different variants for sweet, umami, and bitter
type II cells
what type of taste receptors don’t have any afferents?
type I receptors
how much salt is needed for food to taste salty?
50mM (saliva normally has 10mM salt)
when the concentration of Na+ in the mouth increases, it passes through:
permeable Na+ channels (not voltage-gated)
list the three steps in the theorized mechanism of type I cell activation
1) Na+ from salt food enters through a Na+ channel
2) the resulting depolarization opens voltage-gated Ca++ channels
3) the influx of Ca++ causes neurotransmitter release
what are the issues with the theorized mechanism of type I cell activation?
- no identified neurotransmitters
- not associated with afferent fibers
what taste is associated with type III cells?
sour taste
what type of receptors are type III cell receptors?
ionotropic
what molecule is detected by type III cell receptors?
detects hydrogen ions (H+) –> detects acid
involves a transient receptor potential channel (TRPP3) conjoined with a polycystic kidney disease family protein (PKD1L3)
type III cells
is the passage of ions through TRPP3 channels enough to depolarize the type III taste cells?
no (weak acids also pass through the lipid membrane)
how do H+ ions effect K+ channels in type III cells?
H+ ions close the K+ channels and cell depolarizes
what happens to type III cells when K+ channels close?
voltage-gated Na+ channels open –> action potentials –> voltage gated Ca++ open
what neurotransmitter is released from type III cells?
serotonin (5HT)
what taste is associated with type II cells?
sweet, umami, and bitter
what type of receptors are found on type II taste cells?
metabotropic receptors
a heterodimer of the two taste receptor proteins, T1R2 and T1R3, is responsive to:
sweet tastes (sugars)
a heterodimer of the two taste receptor proteins, T1R1 and T1R3, is responsive to:
L-glutamate and other L-amino acids (umami taste)
a homodimer of the two taste receptor proteins from the TR2 family is responsive to:
bitter tastes
type II cell receptors are _____ coupled, and lead to the opening of _____
Gq, TRPM5 channels
what happens when TRPM5 channels open?
an Na+ influx depolarizes the cell and action potentials are generated
in type II cells, the second messenger Ca++ opens ____ in addition to TRPM5 channels
Panx1 channels
what is the purpose of Panx1 channels?
releases ATP into the extracellular space –> acts as a neurotransmitter
ATP can bind to _____ on type II and type III cells and _____ on afferents
P2Y receptors (metabotropic), P2X receptors (ionotropic)
the opening of P2X channels allows for:
influx of cations (Na+ and Ca++)
are there any ‘spicy’ taste receptors?
no
where do ‘spicy’ chemicals bind?
free nerve endings = thermoreceptors in the tongue, mouth, and airway
have 6 transmembrane units and are found in many sensory systems
TRP channels
TRP channels activated by ‘warm’ sensations, capsacin, and spicy foods
TRPV1 channels
TRP channels activated by ‘cool’ sensations, and menthol
TRPM8 channels
TRP channels activated by very cool temperatures and wasabi
TRPA1 (Anktm1)
channels often co-expressed on sensory nerve fibers
TRPA1 and TRPV1
which three cranial nerves convey taste from the tongue to the brainstem?
- facial nerve (fungiform papillae on the anterior 2/3 of the tongue)
- glossopharangeal nerve (circumvallate and folitate papillae on the posterior 1/3 of the tongue)
- vagus nerve (associated with free nerve endings)
the afferents of CN VII, IX, and X project to the _____ in the brainstem, which projects to the _____ of the thalamus, which prjects to the _____
nucleus tractus solitarus (nTS), ventral posterior medial nucleus (VPM), insular cortex
- involved in perception arising from interoceptors
- contains the gustatory cortex
- activated during disgust and pain
- reciprocal connections with secondary somatosensory cortex and amygdala
- associated with orbitofrontal cortex and lateral hypothalamus
these are all characteristics of the:
insular cortex
