Question
Action potential of cardiac muscle involves four (phases 0 – 4).
- During phase 0 & 1, the voltage-gated calcium channel slowly opens.
- During Phase 2, calcium channels completely open & potassium channels close.
Assertion: Phase 2 of the cardiac muscle action potential is plateau shaped.
Reason: Plateau-shape is due to decreased calcium ion permeability & increased potassium ion permeability.
| A. |
Both A & R are correct and R is correct explanation for A.
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| B. |
Both A & R are correct and R is not correct explanation for A.
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| C. |
A is correct but R is incorrect.
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| D. |
A is incorrect but R is correct.
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Show Answer
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Correct Answer » C
Explanation
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The ventricular cardiac muscle action potential consists of five phases (Phase 0–4). It is classified as a fast-response action potential and is fundamentally different from skeletal muscle due to the presence of a prolonged plateau phase.
Phases of Ventricular Cardiac Action Potential
Phase 0 – Rapid Depolarization
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Caused by rapid opening of fast voltage-gated sodium channels.
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Massive influx of Na⁺ into the cell.
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Membrane potential rises sharply from approximately −90 mV to +20 mV.
Phase 1 – Initial Repolarization
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Transient outward potassium current (Ito).
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Sodium channels inactivate.
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Slight fall in membrane potential.
Phase 2 – Plateau Phase
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Characteristic feature of cardiac muscle.
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L-type (slow) voltage-gated calcium channels open.
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Calcium enters the cell.
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Simultaneously, delayed rectifier potassium channels allow K⁺ to leave.
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Inward Ca²⁺ current is balanced by outward K⁺ current.
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This balance produces a prolonged plateau at near 0 mV.
Phase 3 – Rapid Repolarization
Phase 4 – Resting Membrane Potential
Analysis of Assertion
Assertion: Phase 2 of the cardiac muscle action potential is plateau shaped.
This statement is correct. Phase 2 is the plateau phase and is the defining feature of ventricular cardiac action potential. It prolongs depolarization and increases the refractory period.
Analysis of Reason
Reason: Plateau shape is due to decreased calcium ion permeability and increased potassium ion permeability.
This statement is incorrect.
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