MCQ – 89339

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:

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:

• Resting membrane potential before action potential.

• At rest, membrane is “Polarized”.(This stage is not a part of action potential).

• Since RMP is negative (-70 mV).

2) Depolarization phase:

• Potential rises to +35 mV in less than a millisecond.

• Due to “Voltage-gated Na2+ channels” opening.

• Causing sodium influx,

• Changes negative to positive charge.

• From -70 mV to -55 mV (“Firing level/Threshold point”)

• Partial opening of voltage-gated sodium channels.

• This AP portion (-70 mV to -55 mV) also called “Pre-Potential”/”Foot of AP”.

2a) Depolarization:

• On threshold level (-55 mV),Membrane depolarization occurs. 

• Complete opening of voltage-gated Na2+channels occurs.

• Leading to Na2+ influx 

• Produced by sudden & brief increasing Na2+ permeability.

• Thus, increases potential suddenly.

• Representing loss of original polarity of cell membrane

3) Repolarization phase:

• When, potential drops to near resting level repolarization occurs.

• Ie., Reversal to original polarity

• Events during start repolarization/depolarization end:Causing inside positive charge again turning negative.

• Voltage-gated Na+ channels close.

• Voltage-gated K+ channels open.

• Increases K+ permeability.

• Causing K+efflux out of cell

4) After depolarization:

• Until 70%, repolarization maintains faster pace.

• Point after is slower repolarization phase.

• Referred as “After Depolarization”.

5) After-hyperpolarization :

• During repolarization phase, 

• K+ channels open with K+ efflux.

• Membrane voltage falls back/repolarizes to resting potential (Ie, -70 mV).

• At this point, K+ channels close slowly.

• Hence, more K+ ions leak out.

• Causes “Hyperpolarization”.

• Brief period of voltage fall below -70 mV

• K+/Na2+ pump works to re-establish resting potential.

• By pumping Na2+ back out & K+ back into cells.