Final answer:
The opening of sodium channels during an action potential is caused by the presence of positive charge due to membrane depolarization. Voltage-gated sodium channels are key in initiating the action potential, with subsequent opening of potassium channels to repolarize the membrane.
Step-by-step explanation:
When sodium channels open during an action potential, the opening is caused by the presence of positive charge. This occurs when the membrane depolarizes, causing voltage-gated sodium channels to activate. As sodium (Na+) ions rush into the cell, the internal charge becomes more positive, leading to the further opening of these channels. This positive feedback loop continues until the membrane potential reaches approximately +40 mV, after which potassium (K+) channels open to allow potassium ions to exit the cell, repolarizing the membrane and ending the action potential.
At the resting potential, all channels are closed, but when an action potential is triggered, Na+ channels open first, followed by the opening of K+ channels, each controlled by changes in voltage across the neuronal membrane. During the action potential, the Na+ channels inactivate shortly after opening, preventing further Na+ entry until after the cell repolarizes and resets to resting potential.