Biological Systems MCAT Practice Exam

What maintains the resting membrane potential in neurons?

1. Only Na+/K+ pumps

2. Only K+ leak channels

3. Na+/K+ pumps and K+ leak channels

4. Voltage-gated ion channels

The correct answer is: Na+/K+ pumps and K+ leak channels

The resting membrane potential in neurons is predominantly maintained through the combined action of sodium-potassium (Na+/K+) pumps and potassium (K+) leak channels. The Na+/K+ pump plays a crucial role by actively transporting three sodium ions out of the neuron for every two potassium ions it brings in. This creates a concentration gradient where the concentration of sodium is higher outside the cell and the concentration of potassium is higher inside the cell. This gradient is essential for establishing the resting membrane potential. On the other hand, K+ leak channels allow potassium ions to move out of the neuron down their concentration gradient. As potassium ions exit the cell, they carry positive charge with them, which contributes to the negative charge inside the neuron relative to the outside. The outflow of potassium ions through these channels is vital because it further reinforces the negative resting membrane potential established by the Na+/K+ pump. The combination of these mechanisms results in a stable resting membrane potential, typically around -70 mV, which is critical for the neuron’s ability to respond to stimuli and generate action potentials.