Gustation: The Sense of Taste

Taste, or gustation, is a special sense that allows us to evaluate what we eat and drink.
Food molecules dissolved in saliva activate taste receptor cells in the mouth.
These chemical stimuli are converted into electrical signals.
Signals are sent via nerves to the brain, where they are interpreted as tastes.

Taste receptor cells are organized in groups called taste buds.
Taste buds are most abundant on the tongue but also found in other parts of the mouth.
On the tongue, taste buds reside on small visible bumps called papillae.
Different types of papillae exist on different parts of the tongue.
Taste buds all share a similar structure.

Each taste bud contains about 50 taste cells.
Microvilli project from the top of taste cells into a pit called the taste pore.
Taste molecules bind to receptors located on the microvilli within the taste pore.
Taste cells synapse with sensory nerve fibers at the base of taste buds.

There are five main types of taste receptors, each corresponding to a major taste sensation:
- Salty: Detects sodium and salts.
- Sweet: Binds to various sugars and sugar substitutes.
- Sour: Activated by acids.
- Umami: Meaty taste elicited by amino acids, particularly glutamates.
- Bitter: Associated with spoiled foods, natural toxins, and substances like quinine and caffeine.
Each taste cell has receptors for only one type of taste.
A taste bud typically comprises a variety of cells detecting different tastes.
All primary tastes can be perceived throughout the tongue, but some regions are more sensitive to certain tastes.

Each primary taste can be elicited by multiple chemicals.
Detection thresholds differ among chemicals that taste the same.
Sweet and salty substances generally have high thresholds, requiring larger amounts to be detected.
Bitter compounds typically have very low thresholds and can be tasted at very low concentrations.
Taste buds at the back of the tongue are especially sensitive to bitter substances, triggering rejection responses like gagging and vomiting to avoid toxin ingestion.
Supertasters have more taste buds, allowing them to detect subtle tastes at very low concentrations.

Binding of taste molecules to receptors results in either:
- Depolarization of taste cells.
- Activation of G protein and second messenger signaling.
Activated taste cells release neurotransmitters.
Neurotransmitters generate action potentials in sensory nerve fibers.
Nerve fibers carrying taste signals travel along cranial nerves VII, IX, or X to the solitary nucleus of the brainstem, depending on the taste bud location.

From the solitary nucleus, second-order neurons project to two destinations:
- Thalamus: Synapse with third-order neurons, which continue to higher cortical taste centers.
- Hypothalamus and Amygdala: Trigger autonomic reflexes (salivation, gagging, vomiting) and provide input for regulating eating behaviors.