NN 02. Resting Membrane, Graded, Action Potentials
Neurology, MBBS Second Year. This chapter is split into 5 topics and has
202 practice questions. The topics covered are listed below, followed by
10 free sample questions from the chapter.
Topics in this chapter
Resting membrane potential & the Nernst equation51 questions
Action potentials, refractory periods & secretion51 questions
Nernst equation and weighted-permeability calculations28 questions
Clinical and integrated neuronal electrophysiology scenarios22 questions
Sample questions: NN 02. Resting Membrane, Graded, Action Potentials
10 sample questions with the correct answer and an explanation. Try to answer each one before reading the explanation.
Q1. Lorazepam is given to abort a seizure. By which mechanism does it act?
A. It increases EPSP amplitude
B. It is a glutamate agonist that depolarizes neurons
C. It blocks sodium channels permanently
D. It is a GABA agonist, causing anion influx that hyperpolarizes neurons, moving the RMP away from threshold so they cannot generate action potentials(correct answer)
Answer: D.Seizures are characterized by asynchronous and excessive electrical activity in the CNS; lorazepam is a GABA agonist, so it causes anion influx, hyperpolarizes the cell, moves the RMP further from threshold potential, prevents neurons from generating action potentials and helps abort seizure activity.
Q2. Why is summation essential for normal neuronal signalling?
A. Because graded potentials are all-or-none and need no summation
B. Because a single synaptic input produces only a small graded potential that cannot reach threshold, so many inputs must be summed temporally or spatially at the axon hillock(correct answer)
C. Because summation occurs in the axon terminal
D. Because threshold is reached by one EPSP alone
Answer: B.Individual EPSPs and IPSPs are small graded changes; only by summing many inputs — repeatedly from one neuron (temporal) or simultaneously from several neurons (spatial) — can the axon hillock reach the -55 mV threshold and trigger an action potential.
Q3. Which sequence correctly describes the behaviour of voltage-gated Na+ channel gates during an action potential?
A. The gates never reopen after the first action potential
B. The inactivation gates open at -55 mV and the activation gates close at +30 mV
C. Both gates open at -55 mV and stay open throughout
D. At -55 mV the activation gates open allowing Na+ in while the inactivation gates begin slowly closing; at peak depolarization (~30 mV) the inactivation gates close prohibiting further Na+ entry; when the voltage returns to RMP the inactivation gates reopen and activation gates reclose(correct answer)
Answer: D.When -55 mV is reached the activation gates open allowing Na+ to enter while the inactivation gates start slowly closing; at peak depolarization (~30 mV) the activation gates are still open but the inactivation gates close, prohibiting further Na+ entry; once the voltage returns to RMP the inactivation gates reopen and the activation gates reclose, so the channel can be re-stimulated.
Q4. What defines the relative refractory period?
A. The time from the brief overshoot (hyperpolarization) until RMP is just reclaimed, during which the cell can be re-stimulated if a significant stimulus pushes the voltage to threshold(correct answer)
B. The time from threshold to peak depolarization, when the cell cannot be stimulated at all
C. The period during which Na+ inactivation gates are closed
D. The period when K+ channels are fully closed
Answer: A.The relative refractory period runs from the brief overshoot/hyperpolarization until the RMP is just reclaimed; during this time the cell is still repolarizing but can be re-stimulated if a significant amount of stimulus is provided to push the voltage to threshold.
Q5. What happens at the axon terminal at the secretory stage of the action potential?
A. Sodium enters and directly releases neurotransmitter
B. Voltage-gated K+ channels open and K+ enters the terminal
C. Calcium leaves the terminal, preventing vesicle fusion
D. Voltage-gated calcium channels open at +30 mV, Ca2+ rushes in, activates synaptic proteins on vesicles and synapse membranes, causing vesicle fusion and exocytosis of neurotransmitter(correct answer)
Answer: D.The depolarizing wave activates voltage-gated calcium channels in the axon terminal, which open when the voltage inside the cell is +30 mV; calcium rushes in, activates synaptic proteins on vesicles and synapse membranes, leading to fusion of vesicles with membranes and exocytosis of neurotransmitters.
Q6. How is neurotransmitter release stopped after the action potential passes the terminal?
A. The vesicles are destroyed
B. Calcium channels remain open permanently
C. Return to RMP inactivates the voltage-gated calcium channels, preventing Ca2+ entry, vesicle fusion and exocytosis, so the terminal rests and recycles neurotransmitter(correct answer)
D. Potassium enters and fuses the vesicles
Answer: C.The return to RMP inactivates the voltage-gated calcium channels in the axon terminal, preventing calcium entry; this prevents vesicle fusion with the membrane and exocytosis, allowing the axon terminal to start resting and recycling neurotransmitters for the next action potential.
Q7. Assertion: Action potentials are all-or-none and propagate without decrement. Reason: Once threshold is reached, voltage-gated Na+ channels open fully and the inward Na+ current depolarizes the adjacent membrane.
A. Both are true, but the Reason does not explain the Assertion
B. Both Assertion and Reason are true, and the Reason correctly explains the Assertion(correct answer)
C. Assertion is true but Reason is false
D. Assertion is false but Reason is true
Answer: B.Once the axon hillock reaches threshold (-55 mV) the voltage-gated Na+ channels open, Na+ enters and the depolarization wave travels down the axon depolarizing adjacent membrane, so the action potential is regenerated at each point rather than decaying.
Q8. Which sequence correctly summarises an action potential?
A. Threshold → Ca2+ influx → hyperpolarization → depolarization
B. Depolarization → RMP → threshold → overshoot
C. RMP → graded potentials reach -55 mV → voltage-gated Na+ channels open (depolarization) → +30 mV with K+ channels opening (repolarization) → overshoot/hyperpolarization → return to RMP(correct answer)
D. RMP → K+ channels open → Na+ channels open → return to RMP
Answer: C.Graded potentials sum to bring the axon hillock to -55 mV; voltage-gated Na+ channels then open and depolarize the cell; at +30 mV voltage-gated K+ channels open and repolarize it; the delayed K+ closure produces a brief overshoot before the RMP is restored.
Q9. Which pairing of refractory period with its property is correct?
A. Absolute — can be re-stimulated easily; relative — cannot be re-stimulated
B. Both allow re-stimulation with the same threshold stimulus
C. Absolute — cannot be re-stimulated at all; relative — can be re-stimulated by a significant stimulus(correct answer)
D. Both occur before depolarization
Answer: C.During the absolute refractory period (peak of depolarization to RMP) the cell cannot be re-stimulated regardless of stimulus strength; during the relative refractory period (overshoot to RMP) a significant stimulus can still push the voltage to threshold.
Q10. Why is the action potential described as all-or-none yet still able to vary in its information content?
A. Because the amplitude increases with stimulus strength
B. Because the refractory period shortens with strong stimuli
C. Because once threshold is reached the amplitude is fixed (all-or-none), but the frequency of action potentials and the number of axons activated encode stimulus strength(correct answer)
D. Because the threshold changes with stimulus strength
Answer: C.The action potential amplitude is fixed once threshold is reached (all-or-none) and refractory periods limit maximum frequency, so stimulus intensity is encoded by the frequency of firing and by the number of neurons recruited rather than by action potential size.