Taste

Study Guide on Taste (Gustatory System)

1. Gustatory Cells and Taste Buds

• Gustatory cells are the specialized cells responsible for detecting tastants (chemicals) in food.

• Taste buds are made up of gustatory cells and supporting cells. Gustatory cells are similar to neurons.

• Taste pore is where tastants enter to interact with gustatory cells.

2. Types of Taste Sensations (Receptors)

There are 6 types of taste receptors that can trigger a taste sensation:

1. Sour: Triggered by acids, especially hydrogen ions (H⁺).

2. Salty: Triggered by salts, primarily sodium ions (Na⁺).

3. Sweet: Triggered by simple sugars like sucrose (table sugar). Complex sugars (e.g., starch) do not trigger sweet taste.

4. Bitter: Triggered by alkaloids, such as caffeine found in coffee.

5. Umami: A savory taste triggered by amino acids, especially glutamate (e.g., monosodium glutamate or MSG).

6. Fat: A receptor responds to fatty acids in food (not well understood yet).

3. Mechanisms of Taste Signal Transduction

There are two main mechanisms by which gustatory cells are activated:

1. Direct Depolarization (Ion Channels)

• Salt (sodium ions) and sour (hydrogen ions) taste receptors are activated directly by ions.

• When sodium (for salty) or hydrogen ions (for sour) enter the gustatory cell, they cause depolarization, making the inside of the cell more positive.

• This triggers voltage-gated calcium channels to open, causing the release of neurotransmitters from vesicles.

• The neurotransmitter activates the next neuron, sending the signal to the brain.

2. G-Protein-Coupled Receptors (GPCRs)

• Sweet, bitter, and umami tastes involve G-protein-coupled receptors (GPCRs).

• When a tastant (e.g., sucrose for sweet, bitter substances, or glutamate for umami) binds to the receptor, it activates a G-protein.

• The G-protein triggers a signaling cascade within the cell, which eventually causes calcium release and neurotransmitter release.

• The neurotransmitter then activates the next neuron, sending a signal to the brain.

4. Taste Pathways

• Cranial Nerves:

  • The facial nerve (VII) carries taste signals from the front of the tongue.

  • The glossopharyngeal nerve (IX) carries taste signals from the back of the tongue and the pharynx.

• Thalamus:

  • The taste information is processed in the thalamus, which then sends the signal to the primary gustatory cortex for conscious perception of taste.

5. Taste and Smell Connection

• Tastants need to dissolve in saliva to be detected by gustatory cells, similar to how odorants dissolve in mucus for olfactory detection.

• The sense of taste is strongly linked to the sense of smell. For example, vanilla is smelled (odor) rather than tasted, but it is often perceived as a flavor when we taste something sweet.

• When you plug your nose and eat something, it may taste bland or less flavorful, as most of the flavor we perceive comes from smell.

6. Important Concepts to Remember

• Depolarization: When a cell becomes more positive inside, causing activation and neurotransmitter release.

• G-Protein Coupled Receptors (GPCRs): Used in sweet, bitter, and umami tastes to cause signal cascades.

• Neurotransmitter Release: In both direct depolarization and GPCR pathways, neurotransmitters are released, which activate the next neuron and send signals to the brain.

• Thalamus: All taste information is routed through the thalamus to the primary gustatory cortex.

7. Review of Key Points

• Taste receptor types: Sour, salty, sweet, bitter, umami, and fat.

• Salt and sour are detected through direct ion channels (depolarization).

• Sweet, bitter, and umami are detected via G-protein coupled receptors (GPCRs).

• EPSPs are released from gustatory cell and a calcium influx happens from voltage gated channels, but bitter uses intracellular calcium stores from ER

• Action potential occurs from this depolarization and is sent to…

• Nerves involved: Facial nerve (front of the tongue) and glossopharyngeal nerve (back of the tongue and pharynx).

• Taste information is sent to the thalamus, then to the primary gustatory cortex.

• Smell and taste are closely linked; what you perceive as taste is often influenced by smell.

8. Summary Table for Taste Mechanisms