Questions: Resting Membrane Potential

Question 1

At rest, the neuron membrane is most permeable to which ion, and what is the consequence for membrane potential?

• A. Na+, driving the potential toward +60 mV
• B. K+, driving the potential toward −90 mV because K+ leaks out down its concentration gradient
• C. Cl−, driving the potential toward 0 mV
• D. Na+ and K+ equally, so the potential sits at 0 mV

Answer: (B)

At rest, leak K+ channels dominate conductance. K+ is concentrated inside and flows out down its gradient, leaving net negative charge inside. The resting potential (~−70 mV) sits between the K+ equilibrium potential (−90 mV) and the Na+ equilibrium potential (+60 mV), pulled toward K+ because K+ permeability is far greater.

Question 2

The Na+/K+ ATPase directly generates most of the resting membrane potential by actively electrogenic pumping.

• T. True
• F. False

Answer: False

The Na+/K+ ATPase contributes a small direct electrogenic effect (pumps 3 Na+ out and 2 K+ in per cycle, net −1 charge per cycle), but its primary role is maintaining the concentration gradients that K+ and Na+ leak channels then exploit. The gradients — not the pump's direct current — account for the bulk of the −70 mV resting potential.

Question 3

Explain why the resting membrane potential is closer to the K+ equilibrium potential than to the Na+ equilibrium potential.

Model answer: Because resting membrane permeability to K+ far exceeds permeability to Na+. The Goldman equation weights each ion's contribution by its conductance; since K+ conductance dominates, the membrane potential is pulled strongly toward E_K (~−90 mV) and only weakly toward E_Na (+60 mV), yielding a net resting potential near −70 mV.

The Goldman equation (which generalizes the Nernst equation to multiple ions) shows that each ion's equilibrium potential is weighted by that ion's permeability. A high K+ permeability means K+ movement dominates the electrical behavior of the resting membrane.