Resting and Action Potentials Practice Questions
20 free Resting and Action Potentials practice questions for the Physiology. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
Which of the following ions has the highest permeability in a typical resting nerve cell membrane?
- A Potassium (K+)
- B Sodium (Na+)
- C Calcium (Ca2+)
- D Magnesium (Mg2+)
Correct answer: Potassium (K+)
At rest, the membrane contains many open non-gated 'leak' potassium channels. This high relative permeability to K+ allows the resting membrane potential to sit close to the equilibrium potential of potassium.
The Nernst equation is used to calculate which of the following?
- A The total membrane potential
- B The equilibrium potential for a single ion
- C The threshold for an action potential
- D The rate of sodium-potassium pump activity
Correct answer: The equilibrium potential for a single ion
The Nernst equation determines the electrical potential that exactly opposes the net diffusion of a specific ion down its concentration gradient. It assumes the membrane is permeable to only that one ion.
What is the primary function of the Na+/K+ ATPase pump in nerve physiology?
- A To initiate the depolarization phase of an action potential
- B To cause the rapid repolarization phase of the membrane potential
- C To maintain the concentration gradients of sodium and potassium
- D To selectively block the voltage-gated calcium channels
Correct answer: To maintain the concentration gradients of sodium and potassium
The pump uses ATP to move 3 Na+ out and 2 K+ into the cell against their gradients. While it contributes slightly to the negative potential, its main role is maintaining the ionic gradients necessary for excitability.
During the upstroke of a neuronal action potential, which event occurs first?
- A Opening of voltage-gated K+ channels
- B Closure of the Na+ inactivation gate (h-gate)
- C Opening of the Na+ activation gate (m-gate)
- D Activation of the sodium-potassium pump
Correct answer: Opening of the Na+ activation gate (m-gate)
Depolarization to threshold causes the rapid opening of the m-gates (activation gates) of voltage-gated Na+ channels. This leads to a massive influx of Na+ ions, creating the rapid upstroke.
The 'overshoot' of an action potential refers to the period when the membrane potential is:
- A Below the normal resting membrane potential
- B Exactly equal to the neuron's threshold potential
- C Positive relative to the extracellular fluid
- D Returning gradually from peak back to rest
Correct answer: Positive relative to the extracellular fluid
In most neurons, the action potential peaks above 0 mV, reaching positive values (e.g., +30 mV). This positive range is termed the overshoot because the potential has 'shot over' the zero mark.
Which mechanism is responsible for the absolute refractory period?
- A Delayed opening of potassium channels
- B Inactivation of voltage-gated Na+ channels
- C Increased activity of the Na+/K+ pump
- D Sustained hyperpolarization of the membrane
Correct answer: Inactivation of voltage-gated Na+ channels
During and immediately after the peak of the action potential, Na+ channels enter an inactivated state where the h-gates are closed. They cannot be reopened by a second stimulus until the membrane repolarizes.
Hyperkalemia (elevated extracellular K+) has what immediate effect on the resting membrane potential?
- A Hyperpolarizes the resting membrane
- B Depolarizes the resting membrane
- C Has no effect on membrane potential
- D Raises the threshold for firing
Correct answer: Depolarizes the resting membrane
Increasing extracellular K+ reduces the concentration gradient for K+ to leave the cell. According to the Nernst equation, this makes the equilibrium potential for K+ (and thus the resting potential) less negative.
The repolarization phase of the action potential is primarily produced by:
- A Inward movement of Calcium
- B Outward movement of Sodium
- C Outward movement of Potassium
- D Inward movement of Chloride
Correct answer: Outward movement of Potassium
Repolarization is caused by the closure of Na+ inactivation gates and the delayed opening of voltage-gated K+ channels. K+ ions flow out of the cell, removing positive charge and restoring the negative resting potential.
Which of the following describes the 'all-or-none' law of action potentials?
- A Every stimulus applied produces an action potential
- B The amplitude of the action potential rises with the strength of the stimulus applied
- C Once threshold is reached, the action potential occurs at a constant size
- D Action potentials can summate to reach much higher peaks
Correct answer: Once threshold is reached, the action potential occurs at a constant size
If a stimulus is strong enough to reach threshold, an action potential of standard magnitude and duration is triggered. Increasing the stimulus strength further does not increase the size of the individual action potential.
What is the typical resting membrane potential of a large mammalian nerve fiber?
- A -30 mV
- B -70 mV
- C -90 mV
- D +60 mV
Correct answer: -90 mV
Large nerve fibers typically have a resting potential of approximately -90 mV. This is very close to the equilibrium potential of K+, reflecting the high resting permeability to K+.
Tetrodotoxin (TTX), a toxin from pufferfish, blocks action potentials by affecting which structure?
- A Voltage-gated K+ channels
- B Voltage-gated Na+ channels
- C Na+/K+ ATPase
- D Acetylcholine receptors
Correct answer: Voltage-gated Na+ channels
TTX specifically binds to and blocks the extracellular pore of voltage-gated Na+ channels. This prevents the influx of Na+ and makes the generation of an action potential impossible.
The Goldman-Hodgkin-Katz equation differs from the Nernst equation because it accounts for:
- A The temperature of the surrounding solution
- B The relative valence of the ions
- C The total volume of the cell
- D The permeabilities of multiple ions
Correct answer: The permeabilities of multiple ions
While Nernst looks at one ion, the GHK equation calculates the membrane potential by considering the concentrations and relative membrane permeabilities of Na+, K+, and Cl- simultaneously.
After-hyperpolarization (undershoot) occurs because:
- A Voltage-gated sodium channels remain open for far too long
- B Potassium conductance remains elevated after repolarization
- C The sodium-potassium pump suddenly stops working entirely
- D Chloride ions rapidly rush out of the neuron's cytoplasm
Correct answer: Potassium conductance remains elevated after repolarization
Voltage-gated K+ channels are slow to close. This temporary increase in K+ conductance drives the membrane potential even closer to the K+ equilibrium potential than it is at rest.
An increase in extracellular Calcium (hypercalcemia) typically leads to:
- A Increased excitability of the nerve membrane
- B Decreased excitability of the nerve membrane
- C Spontaneous firing of action potentials
- D Immediate depolarization of the membrane
Correct answer: Decreased excitability of the nerve membrane
Calcium ions interact with the exterior of sodium channels, increasing the voltage level required to open the activation gates. Thus, hypercalcemia makes it harder to reach threshold, decreasing excitability.
Which of the following describes the state of the voltage-gated Na+ channel at the resting membrane potential?
- A Activation gate open, inactivation gate closed
- B Both gates open
- C Both gates closed
- D Activation gate closed, inactivation gate open
Correct answer: Activation gate closed, inactivation gate open
At rest, the activation gate (m-gate) is closed, preventing Na+ entry. However, the inactivation gate (h-gate) is open, meaning the channel is 'ready' to respond to a depolarizing stimulus.
Accommodation in a nerve fiber refers to:
- A A rise in threshold caused by a slowly rising stimulus
- B The increase in conduction velocity caused by myelination
- C The ability of a nerve to store electrical charge
- D The jumping of the impulse between Nodes of Ranvier
Correct answer: A rise in threshold caused by a slowly rising stimulus
If a membrane is depolarized slowly, many Na+ channels inactivate and K+ channels open before the threshold is reached. This effectively raises the threshold, requiring a stronger stimulus to fire an action potential.
Which ion is primarily responsible for the plateaus seen in cardiac action potentials but not in typical nerve action potentials?
- A Calcium
- B Magnesium
- C Chloride
- D Phosphate
Correct answer: Calcium
The plateau in cardiac muscle is caused by the slow opening of L-type voltage-gated Calcium channels. Nerve action potentials lack this phase because they rely primarily on rapid Na+ and K+ channel kinetics.
The movement of which ion is responsible for the 'calcium spike' in some dendrites?
- A Influx of Ca2+
- B Efflux of Ca2+
- C Inactivation of Ca2+ channels
- D Exchange of Ca2+ for Na+
Correct answer: Influx of Ca2+
In certain excitable tissues, voltage-gated Ca2+ channels can generate action potentials or 'spikes' through the influx of Ca2+ ions down their steep electrochemical gradient into the cell.
What is the valence (z) of the chloride ion used in the Nernst equation?
- A +1
- B +2
- C -2
- D -1
Correct answer: -1
Chloride (Cl-) is a monovalent anion. Therefore, its charge or valence used in electrochemical equations is -1.
If the permeability to Sodium (PNa) were to suddenly increase to 1000 times that of Potassium (PK), the membrane potential would:
- A Approach +61 mV
- B Approach -90 mV
- C Remain at 0 mV
- D Become infinitely negative
Correct answer: Approach +61 mV
The membrane potential always moves toward the equilibrium potential of the ion to which it is most permeable. Since the equilibrium potential for Na+ is roughly +61 mV, a massive increase in PNa would drive the potential to that value.