Action Potential in Pacemaker Cells (SA Node)

Multiple Choice Questions (MCQs)

1. What is the resting membrane potential (RMP) of the SA node?
   a) –90 mV
   b) –85 mV
   c) –55 mV
   d) –40 mV
   Answer: c) –55 mV
2. Which ion is responsible for the spontaneous depolarization of pacemaker cells?
   a) Potassium (K⁺)
   b) Sodium (Na⁺)
   c) Calcium (Ca²⁺)
   d) Chloride (Cl⁻)
   Answer: b) Sodium (Na⁺)
3. What occurs during the plateau phase (Phase 2) of the cardiac muscle action potential?
   a) Rapid Na⁺ influx
   b) K⁺ efflux and Ca²⁺ influx balance each other
   c) Closure of Ca²⁺ channels
   d) Rapid repolarization
   Answer: b) K⁺ efflux and Ca²⁺ influx balance each other
4. What is the primary cause of the rapid depolarization in Phase 0 of cardiac muscle action potential?
   a) Opening of slow Ca²⁺ channels
   b) Rapid Na⁺ influx through voltage-gated sodium channels
   c) K⁺ efflux
   d) Slow delayed rectifier channels
   Answer: b) Rapid Na⁺ influx through voltage-gated sodium channels
5. Which phase of the cardiac action potential is associated with the closure of Na⁺ channels?
   a) Phase 0
   b) Phase 1
   c) Phase 2
   d) Phase 3
   Answer: b) Phase 1
6. What prevents tetanization in cardiac muscle?
   a) Rapid depolarization
   b) Plateau phase overlapping the twitch duration
   c) High resting membrane potential
   d) Continuous K⁺ efflux
   Answer: b) Plateau phase overlapping the twitch duration
7. What is the threshold potential of SA node cells?
   a) –55 mV
   b) –40 mV
   c) –85 mV
   d) –90 mV
   Answer: b) –40 mV
8. What channels are responsible for the final rapid repolarization (Phase 3) in cardiac muscle cells?
   a) Slow Ca²⁺ channels
   b) Na⁺-Ca²⁺ channels
   c) Rapid delayed rectifier K⁺ channels
   d) Gap junctions
   Answer: c) Rapid delayed rectifier K⁺ channels
9. Which ion channels maintain the resting membrane potential in cardiac muscle?
   a) Na⁺ channels
   b) K⁺ channels
   c) Ca²⁺ channels
   d) Na⁺-Ca²⁺ channels
   Answer: b) K⁺ channels
10. What is the typical duration of a cardiac muscle action potential?
    a) 50 ms
    b) 100–150 ms
    c) 250 ms
    d) 500 ms
    Answer: b) 100–150 ms

Fill in the Blanks

1. The threshold potential of SA node cells is ________.
   Answer: –40 mV
2. The plateau phase of cardiac muscle action potential is primarily maintained by the influx of ________ ions and the efflux of ________ ions.
   Answer: Calcium (Ca²⁺), Potassium (K⁺)
3. Cardiac muscle action potential lasts ________ milliseconds, enabling prolonged contraction.
   Answer: 100–150
4. Resting membrane potential of ventricular muscle cells is approximately ________ mV.
   Answer: –100
5. The ________ phase of cardiac muscle action potential is characterized by rapid Na⁺ influx.
   Answer: Phase 0

True or False

1. The resting membrane potential of SA node cells is more negative than ventricular muscle cells.
   Answer: False
2. The spontaneous depolarization in pacemaker cells is due to leaky Na⁺ channels.
   Answer: True
3. The plateau phase of the cardiac muscle action potential prevents tetanization.
   Answer: True
4. K⁺ efflux is the primary ion movement during Phase 3 of the cardiac muscle action potential.
   Answer: True
5. The action potential in cardiac muscle cells is shorter than that in skeletal muscle cells.
   Answer: False

Subjective Questions

1. Explain the sequence of events in the action potential of pacemaker cells in the SA node.
   Answer:
   • Resting Potential: The resting membrane potential starts at –55 mV. Due to leaky Na⁺ channels, the membrane potential gradually rises to the threshold at –40 mV (spontaneous depolarization).
   • Threshold Potential: At –40 mV, Na⁺-Ca²⁺ channels open, initiating rapid depolarization. This leads to the self-excitation required for initiating a heartbeat.
   • Repolarization: K⁺ channels open, causing K⁺ efflux, which hyperpolarizes the cell to –55 to –60 mV.
   • Reinitiation: K⁺ channels close, and Na⁺ ions leak back into the cell, restarting the cycle.
2. Describe the phases of action potential in cardiac muscle cells and their significance.
   Answer:
   • Phase 0 (Rapid Depolarization): Na⁺ channels open, allowing a rapid influx of Na⁺ ions, making the membrane potential positive.
   • Phase 1 (Early Repolarization): Na⁺ channels close, and K⁺ efflux begins through rapid delayed rectifier channels.
   • Phase 2 (Plateau): Slow Ca²⁺ channels open, allowing Ca²⁺ and Na⁺ influx, balancing K⁺ efflux. This phase prolongs depolarization, ensuring sustained contraction for effective blood ejection.
   • Phase 3 (Final Repolarization): Slow Ca²⁺ channels close, and K⁺ efflux through delayed rectifiers restores resting potential.

Phase 4 (Resting Potential): Membrane potential stabilizes at –90 mV. This phase prepares the cell for the next action potential.
These phases ensure the heart contracts rhythmically and effectively, preventing tetanization and maintaining proper blood circulation