Final answer:
A. Membrane depolarization first opens sodium channels and then opens potassium channels. Membrane depolarization in a neuron first opens voltage-gated sodium channels, subsequently inactivating them and then opening potassium channels, leading to a sequence of depolarization and repolarization.
Step-by-step explanation:
The effect of membrane depolarization on voltage-gated sodium and potassium channels in a neuron is sequential opening rather than simultaneous. Upon reaching the threshold of excitation, depolarization occurs as voltage-gated sodium channels open first, allowing Na+ ions to rush into the cell, reversing the membrane potential from approximately -70mV to about +30mV. Subsequently, these sodium channels inactivate, which prevents further Na+ entry. In the meantime, voltage-gated potassium channels open, and K+ ions begin to exit the cell, contributing to repolarization where the cell's membrane potential is restored towards a negative value. Eventually, the potassium channels close, and the neuron returns to its resting state with the help of the Na+/K+ pump.