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Quiz: The Action Potential

Test your understanding of Chapter 2. Choose the best answer, then click Show Answer to check.

Question 1

What is the typical textbook value of a neuron's resting potential?

  1. About +30 mV
  2. About −70 mV
  3. About −55 mV
  4. About 0 mV
Show Answer

The correct answer is B. A resting neuron's inside is about −70 mV compared with the outside (real neurons vary, often between about −60 and −80 mV). About −55 mV is the typical textbook threshold, not the resting value, and +30 mV is near the peak of a spike.

Concept Tested: Resting Potential Bloom's Level: Remember

Question 2

For each ATP it uses, what does the sodium-potassium pump move?

  1. 2 Na⁺ out and 3 K⁺ in
  2. 3 Na⁺ in and 2 K⁺ out
  3. 1 Na⁺ out and 1 K⁺ in
  4. 3 Na⁺ out and 2 K⁺ in
Show Answer

The correct answer is D. The pump pushes three sodium ions out and brings two potassium ions in, keeping sodium crowded outside and potassium crowded inside. Option A swaps the numbers, a common mix-up.

Concept Tested: Sodium-Potassium Pump Bloom's Level: Remember

Question 3

What is saltatory conduction?

  1. An action potential jumping from one node of Ranvier to the next along a myelinated axon
  2. An action potential spreading slowly, patch by patch, along an unmyelinated axon
  3. The release of chemical messengers at the end of an axon
  4. The return of the membrane to its resting potential after a spike
Show Answer

The correct answer is A. In myelinated axons, current spreads quickly under the insulated stretches and a new spike is triggered at each node of Ranvier, so the signal seems to leap. Option B describes continuous conduction in unmyelinated axons, which is slower.

Concept Tested: Saltatory Conduction Bloom's Level: Remember

Question 4

Why can't a neuron fire a second action potential during the absolute refractory period, no matter how strong the stimulus?

  1. The sodium-potassium pump has stopped working
  2. The neuron has run out of sodium ions outside the cell
  3. Most voltage-gated sodium channels are inactivated and cannot open yet
  4. The potassium channels are all closed
Show Answer

The correct answer is C. During the absolute refractory period, the inactivation gates of the voltage-gated sodium channels are shut, so no stimulus can open them. Option A is wrong because the pump keeps running; the refractory period comes from the sodium channels' own inactivation gates.

Concept Tested: Refractory Period Bloom's Level: Understand

Question 5

Which statement best explains why action potentials are all-or-none?

  1. The sodium-potassium pump always moves the same number of ions per spike
  2. Once threshold is reached, sodium entry opens more sodium channels in a positive feedback loop that runs to completion
  3. Neurons can only detect stimuli of one fixed strength
  4. Myelin limits the size of every spike to the same value
Show Answer

The correct answer is B. Above threshold, incoming Na⁺ depolarizes the membrane, which opens more Na⁺ channels, so a full spike always follows. Option A is wrong because the pump does not create the spike; voltage-gated channels do.

Concept Tested: All-or-None Principle Bloom's Level: Understand

Question 6

During the falling phase (repolarization) of an action potential, what is mainly happening?

  1. Potassium ions flow out through open voltage-gated K⁺ channels
  2. Sodium ions rush in through open voltage-gated Na⁺ channels
  3. The sodium-potassium pump quickly pumps sodium back out
  4. Chloride ions rush into the cell
Show Answer

The correct answer is A. Voltage-gated K⁺ channels open, and potassium leaving the cell carries positive charge out, bringing the voltage down. Option C is a common mistake: the pump restores gradients slowly in the background but does not produce the fast falling phase.

Concept Tested: Repolarization Bloom's Level: Understand

Question 7

A dentist injects lidocaine near a nerve in the lower jaw. The patient feels pressure during the filling but no pain. Which explanation fits best?

  1. Lidocaine destroys the pain-sensing nerve fibers permanently
  2. Lidocaine opens potassium channels so neurons fire faster
  3. Lidocaine blocks sodium channels in the brain's pain center
  4. Lidocaine blocks voltage-gated sodium channels in the nerve, and thin pain fibers are blocked more easily than fibers carrying pressure
Show Answer

The correct answer is D. Lidocaine is a local anesthetic that blocks voltage-gated sodium channels where it is injected, so axons there cannot reach threshold. Pain fibers are more sensitive to local anesthetics than pressure fibers. Option A is wrong because the block is temporary and wears off in a few hours.

Concept Tested: Local Anesthetic Bloom's Level: Apply

Question 8

A sensory neuron fires 10 action potentials per second when touched lightly. The touch becomes much firmer. What do you predict?

  1. Each action potential becomes taller, but the rate stays at 10 per second
  2. The neuron stops firing because the stimulus is too strong
  3. The neuron fires more action potentials per second, each about the same size
  4. Each action potential becomes longer, lasting several seconds
Show Answer

The correct answer is C. Because spikes are all-or-none, stimulus strength is coded mainly by firing rate: a stronger stimulus produces more spikes per second, while each spike keeps the same size. Option A reflects the misconception that stronger stimuli make bigger spikes.

Concept Tested: Firing Rate Coding Bloom's Level: Apply

Question 9

A man eats improperly prepared pufferfish. His lips tingle, then his muscles grow weak, and his breathing becomes difficult. A patient who gets lidocaine at the dentist has only a numb lip. What best explains the difference?

  1. Tetrodotoxin also blocks voltage-gated sodium channels, but it spreads through the body and reaches the nerves that control breathing
  2. Tetrodotoxin blocks potassium channels, while lidocaine blocks sodium channels
  3. Tetrodotoxin speeds up action potentials until neurons become exhausted
  4. Tetrodotoxin works only in the brain, while lidocaine works only in the skin
Show Answer

The correct answer is A. Both are sodium channel blockers. Lidocaine is placed in one small area and wears off, while tetrodotoxin is a neurotoxin absorbed into the body that can block nerves and muscles everywhere, including breathing muscles. Option B is wrong because tetrodotoxin targets voltage-gated sodium channels, not potassium channels.

Concept Tested: Neurotoxin Bloom's Level: Analyze

Question 10

In the chapter's simulator, outside K⁺ is raised to 12 mM. The neuron's resting potential is less negative than normal, yet even strong stimuli produce no spike. Which analysis is correct?

  1. The neuron is now hyperpolarized, so it is too far below threshold
  2. The long-lasting depolarization has left most voltage-gated sodium channels inactivated, so the sodium feedback loop cannot start
  3. The sodium-potassium pump has reversed direction and is pumping sodium in
  4. High potassium has made the myelin sheath fall off
Show Answer

The correct answer is B. Raising outside K⁺ makes the resting potential less negative. A little depolarization makes firing easier, but a sustained depolarization pushes many sodium channels into their inactivated state, so too few are ready to fire. Option A is backwards: the membrane is depolarized, not hyperpolarized.

Concept Tested: Membrane Potential Bloom's Level: Analyze