COMSAE PHASE 1 NEUROPHYSIOLOGY
PRACTICE EXAM WITH ACTUAL
QUESTIONS AND VERIFIED ANSWERS,
PLUS EXPLAINED RATIONALES/EXPERT
VERIFIED FOR GUARANTEED 100% PASS
2026/LATEST UPDATE/INSTANT
DOWNLOAD PDF
1.
A 24-year-old medical student is studying the resting membrane
potential of a large myelinated neuron. The neuron has a high
intracellular concentration of K⁺ and a high extracellular concentration
of Na⁺. At rest, the membrane is substantially more permeable to K⁺ than
to Na⁺ because of open K⁺ leak channels. Which mechanism most
directly establishes and maintains the ionic gradients underlying the
resting membrane potential?
A. Voltage-gated Na⁺ channels
B. Na⁺/K⁺-ATPase
C. Voltage-gated Ca²⁺ channels
D. Na⁺/Ca²⁺ exchanger
E. Ligand-gated Cl⁻ channels
Answer: B. Na⁺/K⁺-ATPase
Rationale: The Na⁺/K⁺-ATPase uses ATP to transport 3 Na⁺ out of the
neuron and 2 K⁺ into the neuron. This maintains the transmembrane
Na⁺ and K⁺ concentration gradients that permit K⁺ efflux through leak
channels and thereby contribute to the resting membrane potential. K⁺
leak channels are the immediate major determinants of resting
permeability, but the pump maintains the gradients over time. Voltage-
1
,gated channels primarily participate in action potentials rather than
establishing the resting ionic gradients.
2.
A neuron has a resting membrane potential of −70 mV. A small
excitatory postsynaptic potential depolarizes the membrane to −60 mV.
Which statement best describes the change that has occurred?
A. The membrane has undergone an action potential
B. The membrane has become more negative
C. The membrane has undergone graded depolarization
D. The neuron has reached the equilibrium potential for K⁺
E. Voltage-gated K⁺ channels have produced afterhyperpolarization
Answer: C. The membrane has undergone graded depolarization
Rationale: A depolarization is a decrease in the magnitude of the
negative membrane potential, such as a change from −70 to −60 mV.
Postsynaptic potentials are graded: their amplitude varies with
stimulus strength and they decay with distance. An action potential is
an all-or-none event that requires reaching threshold and opening
sufficient voltage-gated Na⁺ channels.
3.
A patient receives a local anesthetic that blocks voltage-gated Na⁺
channels in a peripheral sensory nerve. Which phase of the neuronal
action potential is most directly impaired?
A. Rapid depolarization
B. Resting membrane potential
C. Slow repolarization caused by Cl⁻ influx
2
,D. Hyperpolarization caused by Ca²⁺ influx
E. Maintenance of the Na⁺ concentration gradient by the Na⁺/K⁺ pump
Answer: A. Rapid depolarization
Rationale: Voltage-gated Na⁺ channels rapidly open when threshold is
reached, causing Na⁺ influx and the rapid upstroke of the action
potential. Local anesthetics prevent this Na⁺ current, thereby
preventing propagation of action potentials. The Na⁺/K⁺-ATPase
maintains ionic gradients but does not produce the rapid depolarizing
phase of an individual action potential.
4.
A researcher experimentally depolarizes a neuronal membrane to
threshold. Voltage-gated Na⁺ channels open, and Na⁺ rapidly enters the
cell. Shortly afterward, the Na⁺ channels become nonconductive despite
continued membrane depolarization. What is the primary mechanism
responsible?
A. Closure of voltage-gated Ca²⁺ channels
B. Opening of ligand-gated Cl⁻ channels
C. Inactivation of voltage-gated Na⁺ channels
D. Activation of Na⁺/K⁺-ATPase
E. Closure of K⁺ leak channels
Answer: C. Inactivation of voltage-gated Na⁺ channels
Rationale: Voltage-gated Na⁺ channels have an activation gate and an
inactivation gate. Depolarization rapidly opens the activation gate, but
shortly afterward the inactivation gate closes, terminating Na⁺ influx.
This produces the absolute refractory period because another action
potential cannot be initiated until sufficient Na⁺ channels recover from
inactivation.
3
, 5.
A neuron is stimulated during the relative refractory period. Which
alteration best explains why a stronger-than-normal stimulus is required
to generate another action potential?
A. All Na⁺ channels are permanently closed
B. The membrane is hyperpolarized and some Na⁺ channels remain
inactivated
C. The Na⁺/K⁺ pump has completely stopped
D. K⁺ concentration has become equal inside and outside the neuron
E. Voltage-gated Ca²⁺ channels are permanently open
Answer: B. The membrane is hyperpolarized and some Na⁺ channels
remain inactivated
Rationale: During the relative refractory period, many Na⁺ channels
have recovered from inactivation, but not all have returned to the
resting state. At the same time, K⁺ conductance remains elevated,
producing hyperpolarization. Consequently, a stronger-than-normal
depolarizing stimulus is necessary to reach threshold.
6.
A neurophysiology experiment compares action potentials in two axons.
Axon A is unmyelinated and 1 μm in diameter. Axon B is myelinated
and 10 μm in diameter. Which feature most strongly explains why action
potentials propagate much faster in axon B?
A. Continuous opening of ligand-gated Na⁺ channels
B. Saltatory conduction between nodes of Ranvier
C. Increased intracellular Na⁺ concentration
D. Reduced membrane resistance caused by myelin
E. Continuous regeneration of action potentials throughout the myelin
sheath
4
PRACTICE EXAM WITH ACTUAL
QUESTIONS AND VERIFIED ANSWERS,
PLUS EXPLAINED RATIONALES/EXPERT
VERIFIED FOR GUARANTEED 100% PASS
2026/LATEST UPDATE/INSTANT
DOWNLOAD PDF
1.
A 24-year-old medical student is studying the resting membrane
potential of a large myelinated neuron. The neuron has a high
intracellular concentration of K⁺ and a high extracellular concentration
of Na⁺. At rest, the membrane is substantially more permeable to K⁺ than
to Na⁺ because of open K⁺ leak channels. Which mechanism most
directly establishes and maintains the ionic gradients underlying the
resting membrane potential?
A. Voltage-gated Na⁺ channels
B. Na⁺/K⁺-ATPase
C. Voltage-gated Ca²⁺ channels
D. Na⁺/Ca²⁺ exchanger
E. Ligand-gated Cl⁻ channels
Answer: B. Na⁺/K⁺-ATPase
Rationale: The Na⁺/K⁺-ATPase uses ATP to transport 3 Na⁺ out of the
neuron and 2 K⁺ into the neuron. This maintains the transmembrane
Na⁺ and K⁺ concentration gradients that permit K⁺ efflux through leak
channels and thereby contribute to the resting membrane potential. K⁺
leak channels are the immediate major determinants of resting
permeability, but the pump maintains the gradients over time. Voltage-
1
,gated channels primarily participate in action potentials rather than
establishing the resting ionic gradients.
2.
A neuron has a resting membrane potential of −70 mV. A small
excitatory postsynaptic potential depolarizes the membrane to −60 mV.
Which statement best describes the change that has occurred?
A. The membrane has undergone an action potential
B. The membrane has become more negative
C. The membrane has undergone graded depolarization
D. The neuron has reached the equilibrium potential for K⁺
E. Voltage-gated K⁺ channels have produced afterhyperpolarization
Answer: C. The membrane has undergone graded depolarization
Rationale: A depolarization is a decrease in the magnitude of the
negative membrane potential, such as a change from −70 to −60 mV.
Postsynaptic potentials are graded: their amplitude varies with
stimulus strength and they decay with distance. An action potential is
an all-or-none event that requires reaching threshold and opening
sufficient voltage-gated Na⁺ channels.
3.
A patient receives a local anesthetic that blocks voltage-gated Na⁺
channels in a peripheral sensory nerve. Which phase of the neuronal
action potential is most directly impaired?
A. Rapid depolarization
B. Resting membrane potential
C. Slow repolarization caused by Cl⁻ influx
2
,D. Hyperpolarization caused by Ca²⁺ influx
E. Maintenance of the Na⁺ concentration gradient by the Na⁺/K⁺ pump
Answer: A. Rapid depolarization
Rationale: Voltage-gated Na⁺ channels rapidly open when threshold is
reached, causing Na⁺ influx and the rapid upstroke of the action
potential. Local anesthetics prevent this Na⁺ current, thereby
preventing propagation of action potentials. The Na⁺/K⁺-ATPase
maintains ionic gradients but does not produce the rapid depolarizing
phase of an individual action potential.
4.
A researcher experimentally depolarizes a neuronal membrane to
threshold. Voltage-gated Na⁺ channels open, and Na⁺ rapidly enters the
cell. Shortly afterward, the Na⁺ channels become nonconductive despite
continued membrane depolarization. What is the primary mechanism
responsible?
A. Closure of voltage-gated Ca²⁺ channels
B. Opening of ligand-gated Cl⁻ channels
C. Inactivation of voltage-gated Na⁺ channels
D. Activation of Na⁺/K⁺-ATPase
E. Closure of K⁺ leak channels
Answer: C. Inactivation of voltage-gated Na⁺ channels
Rationale: Voltage-gated Na⁺ channels have an activation gate and an
inactivation gate. Depolarization rapidly opens the activation gate, but
shortly afterward the inactivation gate closes, terminating Na⁺ influx.
This produces the absolute refractory period because another action
potential cannot be initiated until sufficient Na⁺ channels recover from
inactivation.
3
, 5.
A neuron is stimulated during the relative refractory period. Which
alteration best explains why a stronger-than-normal stimulus is required
to generate another action potential?
A. All Na⁺ channels are permanently closed
B. The membrane is hyperpolarized and some Na⁺ channels remain
inactivated
C. The Na⁺/K⁺ pump has completely stopped
D. K⁺ concentration has become equal inside and outside the neuron
E. Voltage-gated Ca²⁺ channels are permanently open
Answer: B. The membrane is hyperpolarized and some Na⁺ channels
remain inactivated
Rationale: During the relative refractory period, many Na⁺ channels
have recovered from inactivation, but not all have returned to the
resting state. At the same time, K⁺ conductance remains elevated,
producing hyperpolarization. Consequently, a stronger-than-normal
depolarizing stimulus is necessary to reach threshold.
6.
A neurophysiology experiment compares action potentials in two axons.
Axon A is unmyelinated and 1 μm in diameter. Axon B is myelinated
and 10 μm in diameter. Which feature most strongly explains why action
potentials propagate much faster in axon B?
A. Continuous opening of ligand-gated Na⁺ channels
B. Saltatory conduction between nodes of Ranvier
C. Increased intracellular Na⁺ concentration
D. Reduced membrane resistance caused by myelin
E. Continuous regeneration of action potentials throughout the myelin
sheath
4
