What is the typical resting membrane potential of neurons?

• A. -60 mV
• B. -70 mV
• C. -50 mV
• D. -80 mV

Answer: (B)

A neuron’s resting membrane potential comes from an electrochemical balance: the inside of the cell is negative relative to the outside because ions are distributed across the membrane and the membrane is more permeable to some ions than others. Specifically, potassium channels are open at rest, so K+ tends to leak out, pulling the inside negative. The Na+/K+ pump helps maintain the concentration gradients by pumping three Na+ out and two K+ in, which keeps the gradients from dissipating. When you combine these factors, the resting potential lands around -65 to -70 mV in many neurons, so -70 mV is the typical value.

If the membrane were mostly permeable to K+ with little Na+ leak, the resting potential would be closer to the K+ equilibrium potential (about -90 mV). If there were less leak or different conditions, the resting potential could be less negative (around -60 mV) or more negative, but the standard, common resting value used in biology is about -70 mV. A more depolarized value like -50 mV is closer to threshold for firing, while -80 mV is more hyperpolarized than usual. The key idea is the balance of ion gradients and membrane permeability that centers around -70 mV.