Which is NOT listed as a factor generating the cardiac membrane potential?

• A. Membrane surface area
• B. Ion concentration gradients
• C. Ion conductances
• D. Electrogenic ion transport

Answer: (A)

Membrane potential is set by ion gradients and how permeable the membrane is to those ions, with electrogenic transport helping maintain that state. The ion concentration differences across the membrane create the driving forces for Na+, K+, Ca2+, and Cl−, and the actual potential depends on how permeable the membrane is to these ions via channels—this is captured by the idea that conductances determine the resting voltage. Electrogenic transport, like the Na+/K+-ATPase pump, adds a net current that helps sustain the gradient and stabilize the resting potential over time. The membrane surface area does not set the voltage itself; it mainly affects capacitance, so a larger area slows how quickly the membrane potential can change in response to currents, but it doesn’t establish the resting value. Therefore, the factor that does not generate the cardiac membrane potential is membrane surface area.