The difference in electric charge between the inside and outside of a neuron's cell membrane is called the

• A. Action potential
• B. Resting potential
• C. Graded potential
• D. EPSP

Answer: (B)

This is about the resting membrane potential—the voltage difference across a neuron's membrane when it’s not actively signaling. At rest, the inside of the neuron is more negative than the outside, typically around −70 millivolts. This negative interior arises from ion gradients (potassium is higher inside, sodium higher outside) and the membrane’s selective permeability, mainly through potassium leak channels. The Na+/K+-ATPase pump helps maintain these gradients by pumping Na+ out and K+ in, using energy.

This resting difference is what underlies a neuron’s ability to fire an action potential. An action potential is a rapid, temporary change in membrane potential that travels along the membrane, whereas resting potential is the steady-state condition. Graded potentials are small, localized changes that can depolarize or hyperpolarize the cell and may summate to reach threshold, but they describe transient signals rather than the baseline difference. An EPSP is a type of excitatory graded potential that depolarizes the membrane, again a specific local event, not the overall resting difference.

So the difference in charge across the membrane when the neuron is not signaling is the resting membrane potential.