Question
- 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. |
| B. |
Both A & R are correct and R is not correct explanation for A. |
| C. |
A is correct but R is incorrect. |
| D. |
A is incorrect but R is correct. |
|
Correct Answer » C Explanation |
 |
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
-
Caused by rapid opening of fast voltage-gated sodium channels.
-
Massive influx of Na⁺ into the cell.
-
Membrane potential rises sharply from approximately −90 mV to +20 mV.
Phase 1 – Initial Repolarization
-
Transient outward potassium current (Ito).
-
Sodium channels inactivate.
-
Slight fall in membrane potential.
Phase 2 – Plateau Phase
-
Characteristic feature of cardiac muscle.
-
L-type (slow) voltage-gated calcium channels open.
-
Calcium enters the cell.
-
Simultaneously, delayed rectifier potassium channels allow K⁺ to leave.
-
Inward Ca²⁺ current is balanced by outward K⁺ current.
-
This balance produces a prolonged plateau at near 0 mV.
Phase 3 – Rapid Repolarization
-
Calcium channels close.
-
Potassium efflux predominates.
-
Membrane returns to resting potential.
Phase 4 – Resting Membrane Potential
-
Maintained by high potassium permeability.
-
Resting membrane potential approximately −90 mV.
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.