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Quiz: Chemical Messengers and the Serotonin Myth

Test your understanding of Chapter 5. Choose the best answer, then click Show Answer to check.

Question 1

Which cells make most of the body's serotonin?

  1. Serotonin neurons in the brainstem
  2. Platelets in the blood
  3. Enterochromaffin cells in the gut lining
  4. Gut bacteria in the colon
Show Answer

The correct answer is C. More than 90% of the body's serotonin is made in the gut, mostly by enterochromaffin (EC) cells, even though they are less than 1% of the gut's lining cells. Platelets (option B) store serotonin from the gut but don't make it.

Concept Tested: Enterochromaffin Cell Bloom's Level: Remember

Question 2

What is tryptophan?

  1. An essential amino acid from food that the body uses to make serotonin
  2. An enzyme that breaks serotonin down in the brain
  3. A type of gut bacterium that produces serotonin
  4. A medicine that blocks the serotonin transporter
Show Answer

The correct answer is A. Tryptophan is an essential amino acid, so it must come from protein in food, and it is the raw material for serotonin. Option B mixes it up with enzymes; the enzyme that turns tryptophan into serotonin is tryptophan hydroxylase.

Concept Tested: Tryptophan Bloom's Level: Remember

Question 3

Which neurotransmitter is the brain's main inhibitory ("brake") signal?

  1. Acetylcholine
  2. Dopamine
  3. Serotonin
  4. GABA
Show Answer

The correct answer is D. GABA mostly makes receiving neurons less likely to fire. Acetylcholine (option A) is a main "go" signal for gut muscle and is released by enteric motor neurons and the vagus nerve's motor fibers.

Concept Tested: GABA Bloom's Level: Remember

Question 4

Why can't serotonin made in the gut raise serotonin levels in the brain?

  1. The liver turns all gut serotonin into tryptophan.
  2. Serotonin cannot cross the blood-brain barrier, so the brain makes its own from tryptophan.
  3. Gut serotonin is a different molecule from brain serotonin.
  4. The vagus nerve blocks serotonin from traveling upward.
Show Answer

The correct answer is B. The blood-brain barrier keeps gut serotonin out, and brain neurons make their own serotonin from tryptophan, which can cross. Option C is a common mix-up: it's the same molecule (5-HT) in both places; what differs is where it's made and where it can travel.

Concept Tested: Blood-Brain Barrier Bloom's Level: Understand

Question 5

Why do scientists say the body has two separate serotonin supplies?

  1. Gut EC cells use TPH1, brain neurons use TPH2, and the blood-brain barrier keeps the two pools apart.
  2. Men and women make different kinds of serotonin.
  3. Serotonin made in the morning and at night comes from different organs.
  4. Platelets make one supply and gut bacteria make the other.
Show Answer

The correct answer is A. Two versions of tryptophan hydroxylase work in two places, TPH1 in EC cells and TPH2 in neurons, and the blood-brain barrier separates the gut and blood pool from the brain pool. Option D is wrong because platelets store gut serotonin rather than make it.

Concept Tested: Tryptophan Hydroxylase Bloom's Level: Understand

Question 6

What does the serotonin transporter (SERT) do in the gut lining?

  1. It carries serotonin across the blood-brain barrier into the brain.
  2. It makes serotonin from tryptophan inside EC cells.
  3. It pumps serotonin back into cells, ending its signal.
  4. It releases serotonin into the gut to trigger peristalsis.
Show Answer

The correct answer is C. SERT is a pump that clears serotonin by moving it back into cells; in the gut it is found on essentially all lining cells, and platelets use it too. Option B describes the job of tryptophan hydroxylase, not SERT.

Concept Tested: Serotonin Transporter Bloom's Level: Understand

Question 7

A 16-year-old starts an SSRI for anxiety and has nausea and loose stools in the first two weeks. Which explanation best fits this chapter?

  1. The medicine sends brain serotonin down into the gut.
  2. The medicine kills helpful gut bacteria that make serotonin.
  3. Nausea means the medicine isn't reaching the brain at all.
  4. The medicine also blocks SERT in the gut, so gut serotonin lingers near nerves that control movement and nausea.
Show Answer

The correct answer is D. SSRIs reach the whole body, and the gut lining and platelets use SERT too. Blocking it leaves gut serotonin active longer near sensory nerves. Option A is backwards: brain serotonin doesn't travel to the gut, just as gut serotonin doesn't travel to the brain. Any change to the medicine should be decided with the prescriber.

Concept Tested: SSRI Bloom's Level: Apply

Question 8

A friend says, "I'm going to eat a lot of turkey so all that tryptophan turns into serotonin in my brain." Using this chapter, what is the best response?

  1. Right, nearly all tryptophan becomes brain serotonin within an hour.
  2. Most tryptophan goes to the kynurenine pathway and protein building, and what reaches the brain competes with other amino acids to cross.
  3. Tryptophan can't cross the blood-brain barrier, so none of it reaches the brain.
  4. Tryptophan only becomes serotonin in the gut, never in the brain.
Show Answer

The correct answer is B. More than 95% of dietary tryptophan is broken down through the kynurenine pathway, and tryptophan competes with other large neutral amino acids for the transporter into the brain. Option C goes too far the other way: tryptophan can cross, which is how the brain makes its own serotonin.

Concept Tested: Kynurenine Pathway Bloom's Level: Apply

Question 9

In the Yano et al. (2015) study, spore-forming bacteria given to germ-free mice raised serotonin in the colon and blood. Which conclusion does this study best support?

  1. Spore-forming bacteria cure depression in people.
  2. Gut bacteria control the amount of serotonin in the human brain.
  3. In mice, certain gut bacteria increase the gut's own serotonin production.
  4. The gut serotonin mood myth has been proven true.
Show Answer

The correct answer is C. The study was done in mice and measured gut and blood serotonin, which can't enter the brain, so it says nothing directly about brain serotonin or mood. Option B is the overstatement often made in ads: it jumps from mice to people and from gut serotonin to brain serotonin.

Concept Tested: Spore-Forming Bacteria Bloom's Level: Analyze

Question 10

A post says: "Gut bacteria make GABA, the brain's calming chemical, so the right bacteria will calm your anxiety." Which analysis is most accurate?

  1. Some gut bacteria make GABA in lab cultures, but it hasn't been shown that microbial GABA reaches the human brain or calms anxiety.
  2. The post is fully proven, because GABA from any source enters the brain easily.
  3. The post is impossible, because no bacteria can make GABA.
  4. The post is proven by the human study that found Coprococcus lower in depression.
Show Answer

The correct answer is A. Strandwitz and colleagues showed that common gut bacteria such as Bacteroides make large amounts of GABA in culture, but most such molecules act locally in the gut, and the human evidence linking microbes to mood is correlational. Option D treats a correlation in a human study as proof of cause, which it isn't.

Concept Tested: Microbe-Made Neurotransmitter Bloom's Level: Analyze