NEUS 609 - The Gustatory System

detect, avoid

taste evolved to ___ nutrients and ___ toxins

taste classes

• sweet

• sour

• salty

• bitter

• umami

papillae

taste buds are found mostly in the ___ of the tongue and the soft palate, and have different morphology

fungiform

papillae morphology which is a protruding mushroom-shaped with 2 taste buds on top

• more concentrated in the front of the tongue

foliate

papillae morphology which sits in between tongue invagination and has 100s of taste buds on the sides

• usually concentrated on the sides of the back of the tongue

circumvallate

papillae morphology which is large and V-shaped and sits in between 2 tongue invaginations

• concentrated on the very back of the tongue

receptor types

while specific ___ are found in certain morphologies that happen to be concentrated in an area on the togue, they can actually be found all over it

all

1 taste buds can respond to ___ tastes are they are made of multiple taste cells

• made up of 50-150 taste cells

apical region

region of the taste bud where all taste cells converge and is closer to tongue surface

basal region

bottom of taste buds where the stem cells differentiate as taste buds have a short lifespan

nucleus of the solitary tract

all taste sensory neurons synapse in the ____

type 1 cells

thin cells wrapping around other cells acting as supporting glia

• also mediate low salt sensation with excitatory sodium channels (ENaC)

type 2 cells

taste cells using G-protein coupled receptors for transduction of sweet, umami, and bitter taste

type 3 cells

cells that detect sour taste

• usually 2-20% more on the back of the tongue

• usually found in circumvallate papillae

sweet receptors

• made of 2 G-protein coupled receptors, each with 7 transmembrane domains

• T1R2 and T1R3

• type 2 cells 

• found mostly on foliate and circumvallate papillae

• respond to carbohydrates

umami receptors

• 2 G-protein coupled receptors, each with 7 transmembrane domains

• T1R1 & T1R3

• type 2 cells

• found mostly on fungiform papillae

• respond to L-amino acids, especially glutamate in humans

• also responds to inosine monophosphate

bitter receptors

• made of 1 G-protein coupled receptor

• T2Rs family of receptors (~30 subtypes)

• located in foliate or circumvallate papillae

• individual taste cell can express several types of T2Rs receptors

• type 2 cells

T1R, T2R

taste cells can express __ OR __ receptors, but NOT BOTH

T1R and T2R receptors mechanism

1. ligand binds to GPCR transmembrane domain

2. alpha G-protein subunit binds to GTP and activates PLCB2

3. PIP2 is then split into DAG and IP3

4. IP3 binds to intracellular IP3R which release Ca2+ into the cytosol

5. Ca2+ binds to TRPM5, which lets Na+ inside the cell and depolarize it

6. Depolarization open CALHM1 channels which let ATP out of the cell

7. ATP then binds to and open ligand-gated channels P2X on sensory neurons

Sour receptors

• ionotropic receptors

• type 3 cells

• made of Otopetrin-1 (Otop1) protein

• proton channel

sour receptors mechanism

1. allows entry of weak acids into the cells, therefore increasing intracellular H+ concentration

2. high proton concentration inhibits leaky KIR2.1 channels causing depolarization of cell and action potential

3. Ca2+ causes vesicular release of 5-HT (serotonin) which binds to 5-HT3 receptors on sensory neurons

low

___ sodium concentration go through ionotropic receptor ENAC aka excitatory sodium channels and elicit a positive response