Which factors generate membrane potentials in cardiac cells?

• A. Ion concentration gradients, ion conductances, electrogenic ion transport
• B. Mechanical stretch and pressure
• C. Hormonal signaling only
• D. Random fluctuations in ion channels

Answer: (A)

Membrane potentials in cardiac cells come from electrochemical gradients and how permeable the cell membrane is to those ions. The inside of a cardiac cell sits negative largely because of a high potassium gradient and the presence of potassium leak channels that favor K+ leaving the cell. If you look at each ion, the Nernst potential tells you the voltage that would exist if the membrane were only permeable to that ion, and in the heart the actual potential reflects a weighted combination of these permeabilities for potassium, sodium, calcium, and chloride. The Goldman-Hodgkin-Katz framework captures this nicely, showing how multiple ions together determine the resting potential and the shape of the action potential.

During excitation, channels open in a sequence that changes conductances: rapid Na+ entry depolarizes the cell, Ca2+ entry helps sustain the plateau in ventricular myocytes, and K+ efflux brings the membrane back toward resting after repolarization. Electrogenic transporters also contribute by moving net charge across the membrane, which helps maintain gradients and subtly shifts the membrane potential over time. So the fundamental generators are the ion concentration gradients, the membrane’s ion conductances, and electrogenic transport processes.

Mechanical stretch or hormonal signals can modulate excitability and channel behavior, but they do not create the membrane potential themselves. Random fluctuations in channel opening add noise but do not establish the baseline potential.