BMS 300 Exam 3 Ultimate Study Guide & Test Bank |High-
Yield Practice Questions with Detailed Explanations &
Rationales (Human Physiology)
SECTION 1: Cardiac Electrophysiology & Action Potentials
Q1. Which of the following ion currents is primarily responsible for the rapid
depolarization phase (Phase 0) of a contractile cardiomyocyte action potential?
A. Influx of calcium ions (\(Ca^{2+}\)) through L-type calcium channels.
B. Efflux of potassium ions (\(K^{+}\)) through transient outward potassium channels.
C. Influx of sodium ions (\(Na^{+}\)) through voltage-gated fast sodium channels.
D. Influx of sodium ions (\(Na^{+}\)) through hyperpolarization-activated cyclic nucleotide-
gated (HCN) channels.
Correct Answer: C
Rationale: In contractile cardiomyocytes, Phase 0 (rapid depolarization) is driven by the
rapid opening of voltage-gated fast \(Na^{+}\) channels, causing a massive influx of sodium
into the cell. Calcium influx through L-type channels (A) is responsible for the Phase 2
plateau phase. Potassium efflux (B) initializes Phase 1 repolarization. HCN channels (D)
drive the pacemaker potential in conductile (pacemaker) cells, not contractile cells.
, Q2. In conductile (pacemaker) cardiomyocytes of the sinoatrial (SA) node, the
unstable resting membrane potential (pacemaker potential) is initialized by which of
the following?
A. The inward \(Na^{+}\) "funny current" (\(I_{f}\)) flowing through HCN channels.
B. The rapid efflux of \(K^{+}\) through leaky potassium channels.
C. The sudden inactivation of T-type calcium channels.
D. The activation of the \(Na^{+}/K^{+}\) ATPase pump pumping 3 \(Na^{+}\) out and 2
\(K^{+}\) in.
Correct Answer: A
Rationale: Pacemaker cells do not have a flat resting membrane potential. Instead, they
exhibit a slow spontaneous depolarization driven by the funny current (\(I_{f}\)), which is an
inward flow of \(Na^{+}\) through HCN channels that open when the membrane
hyperpolarizes at the end of the preceding action potential. Potassium efflux (B)
hyperpolarizes the cell rather than depolarizing it toward threshold. T-type calcium channels
open later in the pacemaker potential to push the cell to threshold, rather than initializing it
(C).
Q3. During the plateau phase (Phase 2) of a contractile cardiomyocyte action
potential, the membrane potential remains depolarized for a prolonged period. This
balance of ion movement is maintained by:
, A. Rapid \(Na^{+}\) influx balanced by rapid \(K^{+}\) efflux.
B. Inward \(Ca^{2+}\) current through L-type channels balanced by outward \(K^{+}\) current.
C. Inward \(Cl^{-}\) current balanced by outward \(Na^{+}\) pumping.
D. Complete cessation of all transmembrane ion movement.
Correct Answer: B
Rationale: Phase 2 (the plateau) allows a prolonged contraction time for effective blood
ejection. It represents a precise equilibrium between the inward movement of \(Ca^{2+}\) via
L-type voltage-gated calcium channels and the outward movement of \(K^{+}\) through
delayed rectifier potassium channels. If \(Na^{+}\) channels were still open rapidly (A), the
voltage would spike further. Ion movement never fully ceases (D).
Q4. Why is the absolute refractory period of a cardiac contractile cell significantly
longer than that of a skeletal muscle fiber?
A. To allow rapid tetanic contractions to maximize cardiac output.
B. Because cardiac cells lack a functional sarcoplasmic reticulum.
C. To prevent summation and tetanus, ensuring the heart fully relaxes to fill with blood
between beats.
D. Because voltage-gated fast \(Na^{+}\) channels reset instantaneously in cardiac tissue.
Correct Answer: C
, Rationale: If a cardiac cell could undergo tetanus (sustained contraction without relaxation)
like skeletal muscle, the heart would lock up, could not fill with blood, and cardiac output
would drop to zero. The prolonged absolute refractory period, caused by the Phase 2
calcium plateau, ensures that the muscle twitch is almost completely over before another
action potential can be fired.
Q5. A physiology student maps an ECG tracing and notes the duration of the P-R (or
P-Q) interval. This interval specifically represents the time required for:
A. The entire ventricle to fully depolarize and contract.
B. The electrical impulse to travel from the SA node, through the atria, and experience a
physiological delay at the AV node.
C. The ventricles to transition from depolarization to repolarization.
D. The atria to fully repolarize and relax.
Correct Answer: B
Rationale: The P-R interval captures the onset of atrial depolarization (the P wave) up to the
onset of ventricular depolarization (the QRS complex). It serves as a vital clinical metric for
the speed of conduction through the AV node. Prolongation of this interval indicates an AV
block. Ventricular contraction is mapped by the QRS and S-T segments (A). Ventricular
repolarization is the T wave (C).
Q6. Under autonomic regulation, how does sympathetic stimulation via
norepinephrine binding to \(\beta _{1}\)-adrenergic receptors increase heart rate?
Yield Practice Questions with Detailed Explanations &
Rationales (Human Physiology)
SECTION 1: Cardiac Electrophysiology & Action Potentials
Q1. Which of the following ion currents is primarily responsible for the rapid
depolarization phase (Phase 0) of a contractile cardiomyocyte action potential?
A. Influx of calcium ions (\(Ca^{2+}\)) through L-type calcium channels.
B. Efflux of potassium ions (\(K^{+}\)) through transient outward potassium channels.
C. Influx of sodium ions (\(Na^{+}\)) through voltage-gated fast sodium channels.
D. Influx of sodium ions (\(Na^{+}\)) through hyperpolarization-activated cyclic nucleotide-
gated (HCN) channels.
Correct Answer: C
Rationale: In contractile cardiomyocytes, Phase 0 (rapid depolarization) is driven by the
rapid opening of voltage-gated fast \(Na^{+}\) channels, causing a massive influx of sodium
into the cell. Calcium influx through L-type channels (A) is responsible for the Phase 2
plateau phase. Potassium efflux (B) initializes Phase 1 repolarization. HCN channels (D)
drive the pacemaker potential in conductile (pacemaker) cells, not contractile cells.
, Q2. In conductile (pacemaker) cardiomyocytes of the sinoatrial (SA) node, the
unstable resting membrane potential (pacemaker potential) is initialized by which of
the following?
A. The inward \(Na^{+}\) "funny current" (\(I_{f}\)) flowing through HCN channels.
B. The rapid efflux of \(K^{+}\) through leaky potassium channels.
C. The sudden inactivation of T-type calcium channels.
D. The activation of the \(Na^{+}/K^{+}\) ATPase pump pumping 3 \(Na^{+}\) out and 2
\(K^{+}\) in.
Correct Answer: A
Rationale: Pacemaker cells do not have a flat resting membrane potential. Instead, they
exhibit a slow spontaneous depolarization driven by the funny current (\(I_{f}\)), which is an
inward flow of \(Na^{+}\) through HCN channels that open when the membrane
hyperpolarizes at the end of the preceding action potential. Potassium efflux (B)
hyperpolarizes the cell rather than depolarizing it toward threshold. T-type calcium channels
open later in the pacemaker potential to push the cell to threshold, rather than initializing it
(C).
Q3. During the plateau phase (Phase 2) of a contractile cardiomyocyte action
potential, the membrane potential remains depolarized for a prolonged period. This
balance of ion movement is maintained by:
, A. Rapid \(Na^{+}\) influx balanced by rapid \(K^{+}\) efflux.
B. Inward \(Ca^{2+}\) current through L-type channels balanced by outward \(K^{+}\) current.
C. Inward \(Cl^{-}\) current balanced by outward \(Na^{+}\) pumping.
D. Complete cessation of all transmembrane ion movement.
Correct Answer: B
Rationale: Phase 2 (the plateau) allows a prolonged contraction time for effective blood
ejection. It represents a precise equilibrium between the inward movement of \(Ca^{2+}\) via
L-type voltage-gated calcium channels and the outward movement of \(K^{+}\) through
delayed rectifier potassium channels. If \(Na^{+}\) channels were still open rapidly (A), the
voltage would spike further. Ion movement never fully ceases (D).
Q4. Why is the absolute refractory period of a cardiac contractile cell significantly
longer than that of a skeletal muscle fiber?
A. To allow rapid tetanic contractions to maximize cardiac output.
B. Because cardiac cells lack a functional sarcoplasmic reticulum.
C. To prevent summation and tetanus, ensuring the heart fully relaxes to fill with blood
between beats.
D. Because voltage-gated fast \(Na^{+}\) channels reset instantaneously in cardiac tissue.
Correct Answer: C
, Rationale: If a cardiac cell could undergo tetanus (sustained contraction without relaxation)
like skeletal muscle, the heart would lock up, could not fill with blood, and cardiac output
would drop to zero. The prolonged absolute refractory period, caused by the Phase 2
calcium plateau, ensures that the muscle twitch is almost completely over before another
action potential can be fired.
Q5. A physiology student maps an ECG tracing and notes the duration of the P-R (or
P-Q) interval. This interval specifically represents the time required for:
A. The entire ventricle to fully depolarize and contract.
B. The electrical impulse to travel from the SA node, through the atria, and experience a
physiological delay at the AV node.
C. The ventricles to transition from depolarization to repolarization.
D. The atria to fully repolarize and relax.
Correct Answer: B
Rationale: The P-R interval captures the onset of atrial depolarization (the P wave) up to the
onset of ventricular depolarization (the QRS complex). It serves as a vital clinical metric for
the speed of conduction through the AV node. Prolongation of this interval indicates an AV
block. Ventricular contraction is mapped by the QRS and S-T segments (A). Ventricular
repolarization is the T wave (C).
Q6. Under autonomic regulation, how does sympathetic stimulation via
norepinephrine binding to \(\beta _{1}\)-adrenergic receptors increase heart rate?
