Question
Which of the following statements are correct regarding mechanisms of Depolarization, Repolarization & Hyperpolarization in a skeletal muscle action potential?
A-Depolarization due to influx of Na+ through voltage gated channels.
B-Depolarization due to influx of Na+ through ligand gated channels
C-Depolarization due to influx of Na+ through ligand gated channels
D-Repolarization due to efflux of Na+ through ligand gated channels.
E-Hyperpolarization due to efflux of K+.
Choose the correct options:
| A. | B, C and E |
| B. | A, B and C |
| C. |
C and D |
| D. |
A, C and E |
|
Correct Answer � D Explanation |
 |
Ans: D. A, C & E
Phase of Action Potential:
Action potential, recorded using an intracellular electrode, has following phases:
1) Resting stage:
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Resting membrane potential before action potential.
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At rest, membrane is “Polarized”.(This stage is not a part of action potential).
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Since RMP is negative (-70 mV).
2) Depolarization phase:
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Potential rises to +35 mV in less than a millisecond.
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Due to “Voltage-gated Na2+ channels” opening.
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Causing sodium influx,
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Changes negative to positive charge.
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From -70 mV to -55 mV (“Firing level/Threshold point”)
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Partial opening of voltage-gated sodium channels.
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This AP portion (-70 mV to -55 mV) also called “Pre-Potential”/”Foot of AP”.
2a) Depolarization:
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On threshold level (-55 mV),Membrane depolarization occurs.
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Complete opening of voltage-gated Na2+channels occurs.
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Leading to Na2+ influx
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Produced by sudden & brief increasing Na2+ permeability.
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Thus, increases potential suddenly.
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Representing loss of original polarity of cell membrane
3) Repolarization phase:
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When, potential drops to near resting level repolarization occurs.
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Ie., Reversal to original polarity
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Events during start repolarization/depolarization end:Causing inside positive charge again turning negative.
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Voltage-gated Na+ channels close.
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Voltage-gated K+ channels open.
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Increases K+ permeability.
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Causing K+efflux out of cell
4) After depolarization:
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Until 70%, repolarization maintains faster pace.
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Point after is slower repolarization phase.
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Referred as “After Depolarization”.
5) After-hyperpolarization :
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During repolarization phase,
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K+ channels open with K+ efflux.
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Membrane voltage falls back/repolarizes to resting potential (Ie, -70 mV).
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At this point, K+ channels close slowly.
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Hence, more K+ ions leak out.
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Causes “Hyperpolarization”.
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Brief period of voltage fall below -70 mV
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K+/Na2+ pump works to re-establish resting potential.
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By pumping Na2+ back out & K+ back into cells.