Biological Systems MCAT Practice Exam

At what membrane potential do voltage-gated Na+ channels open during action potential?

1. -70 mV

2. -50 mV

3. 0 mV

4. +35 mV

The correct answer is: -50 mV

Voltage-gated sodium (Na+) channels open when the membrane potential reaches a certain threshold that is typically around -50 mV. This threshold is crucial for the initiation of an action potential in neurons. When the membrane is at resting potential, approximately -70 mV, the voltage-gated Na+ channels are closed. As the membrane depolarizes and the potential becomes less negative (for example, due to stimuli that cause local depolarization), the voltage-gated Na+ channels become increasingly responsive. Once the membrane potential reaches the threshold of around -50 mV, these channels open. The opening of voltage-gated Na+ channels allows Na+ ions, which are in higher concentration outside the cell, to rush into the cell due to both the concentration gradient and the electrochemical gradient. This influx of sodium ions causes further depolarization of the membrane, leading to the rapid rise in membrane potential characteristic of the action potential. The other values listed, such as -70 mV, 0 mV, and +35 mV, do not represent the typical threshold for opening these channels. -70 mV is the resting potential where channels are closed, 0 mV is the point at which the membrane potential crosses zero during action