Chapter 13 Tongue and Taste

What begins the process of gustation, or taste? Where are they located?

receptor cells mostly on the tongue, upper palate, and pharynx

Pore and Microvilli

Exposed to saliva, receptor proteins on microvilli bind dissolved tastants

Microvilli

increase the surface area exposed to tasty substances

Taste Receptor cells

release glutamate onto gustatory nerve fibers

Gustatory Stem Cells

renew TRCs every 10 days, this is because the mouth environment is harsh

What are the five types of taste receptor cells?

salt, sour, bitter, sweet, and umami

Salt

• Salt is mostly sodium chloride (NaCI)

• High Na+ in saliva opens the channel —> Na+ flows into sodium ion channels in the salt taste receptor cells —> depolarization —> neurotransmitter release

Sour

• The taste of acids

• Low pH opens a proton-sensitive ion channel

• High levels of H+ (proton) cause a positive charge to flow through channels into the sour taste receptor cells —> depolarization —> neural signal to the brain

Why do bitter, sweet, and umami use GPCRs?

Because of signal amplification, once a GPCR is bound, it activates a signaling cascade that opens an ion channel. Each step of the cascade allows room for amplification

Bitter

A ligand in the interior of the mouth contacts a microvilli of a bitter taste receptor cell —> binds to a GPCR —> intracellular signalling cascade —> releases neurotransmitter and signals to the brain

What allows different molecular shapes to be associated with bitterness?

The diversity of GPCRs:

• One receptor binds to many different G-proteins, leading to an enzyme cascade and a huge response from tiny amounts

What does bitterness help us detect? How does it help us survive?

it helps us to detect poisons, bitter-tasting things are common in nature

Why are there less molecular shapes associated with sweetness?

Because there are two distinct GCPRs involved in detecting sweetness, so a smaller diversity of molecular shapes is associated with sweetness

Sweet

• Taste of sugar or sucrose

• The molecular shape of sugar molecules binds as ligands to the sweet GPCRs —> proteins change shape —> initiate signal

What is the functional form of the sweet taste receptor protein?

two GPCRs that are linked to form the functional sweet receptor

Nonnutritive or Synthetic Sweeteners

• Molecules that are not sugars but taste sweet

• More potent than sucrose

• Used to sweeten foods without adding calories, gives rise to diet culture

Saccharin

• An artificial sweetener extremely sweeter than sucrose

• Binds to the sweet receptor protein and activates it more than sugar does

• Its molecular shape also interacts with one or more of bitter GPCRs

Aspartame

• Most widely used sweetener

• Has the molecular form of a dipeptide

Umami

• Tastes savory or meaty

• A fifth type of taste receptor cell was discovered that responds to glutamate

• Helps us detect protein-containing foods

What kind of experience is taste?

Taste is a mental experience, and exists within the subjective world of our perception

Are spiciness, hotness, or pungency considered a taste? Why or why not?

No! They are considered perceptual qualities and are associated with molecular components that activate receptors in the mouth. This is because signals enter the brain via pathways different from the primary tastes (5th cranial nerve)

Capsaicin

• A molecular constituent that we perceive as hot

• Binds to a receptor protein —> shape change —> ion channel opens —> calcium ions flow from outside the cell to inside to depolarize the cell —> increased neural excitability

How does thermal heat activate capsaicin-sensitive proteins?

• Increasing the temperature of the protein causes the same kind of shape change as the binding of capsaicin

• Chili is experienced as hot because of actual heat

Where are capsaicin/thermal proteins located? What do they help us do?

In the mouth and all over the body to help us sense temperature of our skin and enviornment

What do the TRPV1 ion channels open by?

• Capsaicin and heat

• Binding of pieperine

Menthol

• A molecule from mint plants that has a perception of coolness

• Menthol binds to an ionotropic receptor protein —> calcium channel opens

What family is the menthol/cool receptor part of? What name has it been given?

part of the TRP receptor family, given the name TRPM8

Flavor is a multisensory experience. What are some other qualities of food that contribute to this experience?

Texture, smell, pungency, and memory

How does taste reach the brain through the gustatory pathway?

1. Tongue - TRCs synapse onto cranial nerves

2. Brainstem - first CNS relay

3. Thalamus - taste routes through VPM nucleus of the thalamus

4. Cortex - gustatory cortex process taste and somatosensory cortex

What does the primary somatosensory cortex pick up on?

texture of food

When the CNS relays from the brainstem to the thalamus, what the qualities of taste that we process?

• Fast, reflexive responses to taste

• Regulates emotional responses

• Regulates physiological responses

When the thalamus sends the signal to the cortex, what qualities of taste do we process?

• Conscious perception and recognition of what we are eating

• Integrates taste with smell and texture

• Decision-making about food choices

What do sweet and fatty foods activate in our brain?

The reward system, called the mesolimbic pathway —> consists of neurons that are project from the VTA and release dopamine into the NAc region —> pleasurable feeling

Flavor is multisensory. What detects texture?

somatosensory cortex

Flavor is multisensory. What detects pungency?

TRP channels

Flavor is multisensory. What role does memory play?

learned associations, culture, context