Taste and smell

Chemoreceptors respond to particular chemicals. Mechanoreceptors respond to physical force such as touch and stretch, and thermoreceptors to temperature.

• Mechanoreceptor:
• Correct: Chemoreceptor:
• Thermoreceptor:

Cranial nerve I, the olfactory nerve, carries smell. The trigeminal nerve carries touch, pain and temperature from the face, and the facial nerve carries taste from the front of the tongue and moves the face.

• Correct: Olfactory nerve (I):
• Trigeminal nerve (V):
• Facial nerve (VII):

A G protein–coupled receptor protein passes the signal to a G protein, which switches on an enzyme that makes a second messenger such as cAMP. The second messenger then acts inside the cell.

• The messenger enters the nucleus and switches on a gene:
• The receptor protein opens as a channel and lets the messenger through:
• Correct: A G protein switches on an enzyme that makes a second messenger inside the cell:

Every region of the tongue that has taste buds detects all five taste qualities. The tip has taste buds with salt-sensing cells, so she tastes salt at once. The tongue map, which puts only sweet at the tip, is a myth.

• A sweet taste: Salt triggers the salty quality wherever it lands. Taste buds at the tip are not limited to sweet; that idea comes from the tongue map, which is wrong.
• No taste until the salt reaches the sides of the tongue: The sides of the tongue are not the only place salt is sensed. Taste buds at the tip detect it directly.
• Correct: A salty taste: Correct. Na+ enters salt-sensing cells at the tip, and she tastes salt immediately.
• A weak bitter taste: Sodium ions trigger the salty quality, not bitter. Bitter compounds act on their own receptor proteins.

A blow to the head can jolt the brain against the skull and tear the thin olfactory axons where they pass through the cribriform plate. His taste qualities are intact, so what he has lost is smell, which provides most of what people call flavor.

• The gustatory receptor cells in the taste buds of his tongue: His gustatory receptor cells work: he identifies all four solutions correctly.
• Correct: Olfactory axons passing through the cribriform plate: Correct. Anosmia after a head injury usually comes from torn olfactory axons at the cribriform plate.
• The facial nerve on one side: Damage to one facial nerve would reduce taste on the front two thirds of that side of the tongue, but it would not stop him identifying odors by sniffing.
• The gustatory cortex in the insula: Damage to the gustatory cortex would impair his perception of taste qualities, which is normal. His problem is with odors.

An odorant must dissolve in mucus before it can bind a receptor protein. Binding activates a G protein, which switches on the enzyme that makes the second messenger cAMP. cAMP opens cation channels; the calcium that enters opens chloride channels, and Cl− leaving depolarizes the cell further. Once the axon reaches threshold, action potentials pass through the cribriform plate to the olfactory bulb, and the olfactory tract carries the signal on.

• Correct order: 1. The odorant dissolves in the mucus over the olfactory epithelium 2. It binds an odorant receptor protein on a cilium of an olfactory sensory neuron 3. A G protein switches on an enzyme that makes cAMP 4. cAMP opens cation channels, and chloride leaving the cell adds to the depolarization 5. Action potentials travel along the axon through the cribriform plate 6. The axon synapses in the olfactory bulb, and the olfactory tract carries the signal backward

Sweet, umami and bitter tastants bind G protein–coupled receptor proteins, which set off a second messenger cascade. Salty and sour work through ion channels: Na+ and H+ flow straight into the cell.

• Salty: Not this one. Na+ enters directly through sodium channels.
• Correct: Sweet: Yes. Sugars bind a G protein–coupled receptor protein.
• Sour: Not this one. H+ enters directly through a proton channel.
• Correct: Umami: Yes. Glutamate binds a G protein–coupled receptor protein.
• Correct: Bitter: Yes. Bitter compounds bind one of about 25 G protein–coupled receptor proteins.

The glossopharyngeal nerve (IX) carries taste from the back third of the tongue. The facial nerve carries the front two thirds, and the vagus carries the throat and epiglottis.

• Facial nerve: The facial nerve carries taste from the front two thirds of the tongue, not the back third.
• Trigeminal nerve: The trigeminal nerve carries touch, temperature and pain from the tongue, not taste.
• Hypoglossal nerve: The hypoglossal nerve is a motor nerve that moves the tongue. It carries no taste.
• Correct: Glossopharyngeal nerve: Correct. Cranial nerve IX serves taste on the back third of the tongue.

Each odorant activates several kinds of receptor protein, some strongly and some weakly, and each receptor protein responds to several odorants. The pattern across all the kinds identifies the odor, the way a chord is identified by its combination of notes.

• Each olfactory sensory neuron makes all 400 kinds, so every neuron responds to every odor: Each olfactory sensory neuron makes just one kind of receptor protein. That is what lets the bulb sort signals by receptor type.
• Each odor has its own dedicated receptor protein, and new kinds are made as new odors are met: There are far fewer receptor protein kinds than odors, and the set is fixed by your genes. Odors share receptor proteins.
• Correct: Each odorant binds several kinds of receptor protein, and the brain reads the pattern: Correct. Odor identity is a pattern across many kinds of receptor protein, not a single kind.
• The olfactory bulb makes up for the small number by adding extra kinds of synapse: The bulb sorts and pools signals by receptor type; it does not create new ways of detecting odorants.

Olfactory sensory neurons keep chloride unusually high inside. When the channels open, Cl− moves down its gradient out of the cell. Negative charge leaving makes the inside less negative, so the cell depolarizes.

• Chloride in olfactory neurons carries a positive charge: Chloride is always a negative ion. What differs is the direction it moves.
• Correct: These neurons hold so much chloride inside that Cl− flows out: Correct. Negative ions leaving make the inside less negative.
• Chloride channels in olfactory neurons let Na+ through as well: These channels pass chloride. Na+ enters through the separate cAMP-gated cation channels.
• cAMP reverses the direction of every ion gradient in the cilia: cAMP opens cation channels; it does not change ion gradients. The chloride gradient is set by how much chloride these neurons accumulate.