Correct option is A
Detailed Explanation:
Statement A: The membrane potential is brought to the threshold potential (firing level) due to the opening of some voltage-gated sodium channels in response to a threshold depolarizing stimulus.
- This is correct. The action potential begins when the membrane potential reaches a certain threshold. This depolarization occurs when a threshold stimulus opens some voltage-gated sodium channels, allowing Na+ ions to enter, bringing the membrane potential to the threshold value. This sets up the action potential.
Statement B: The rapid depolarization after the firing level is caused by opening of more voltage-gated sodium channels and entry of Na+ into the nerve fibre.
- This is correct. Once the threshold potential is reached, the membrane undergoes rapid depolarization. This is due to the opening of more voltage-gated sodium channels, allowing a large influx of Na+ ions, causing the membrane potential to become more positive. This is the rising phase of the action potential.
Statement C: The reversal of membrane potential (overshoot) at the peak of action potential occurs as membrane potential moves towards the equilibrium potential of K+.
- This is incorrect. While it is true that the overshoot of the action potential moves the membrane potential toward positive values, it does not directly move toward the equilibrium potential of K+. Instead, it moves toward the equilibrium potential of Na+ as a result of the influx of sodium ions. The membrane potential then begins to repolarize, which involves moving toward the equilibrium potential of K+ due to the efflux of K+.
Statement D: The peak voltage of action potential does not reach the equilibrium potential of K+ primarily because the increase of K+ conductance is short-lived.
- This is incorrect. The equilibrium potential of K+ is not reached because the K+ conductance only increases after the Na+ channels close, but it does not completely overpower the remaining charge. The action potential overshoots because of Na+ influx, but the repolarization phase causes the membrane potential to approach the equilibrium potential of K+, not the peak of the action potential.
