Cardiovascular System – Heart Anatomy, Function, And Circulatory Pathways Practice Test

The resting membrane potential of cardiac muscle cells is set mainly by potassium leak channels. At rest, the cell membrane is highly permeable to K+, so the interior tends to approach the potassium equilibrium potential. For cardiac myocytes, that equilibrium potential is around -90 mV, created and maintained by the balance of K+ ions leaving the cell and the activity of the Na+/K+ ATPase that keeps the gradients for Na+ and K+ across the membrane. This makes the inside of cardiac muscle cells very negative at rest, which is crucial for stabilizing the cell until an action potential is triggered. That’s why -90 mV fits best. Values like -60 mV or -70 mV are less negative and more typical of other cell types (such as many neurons), while -80 mV is less characteristic for ventricular cardiac cells. The strong negativity around -90 mV reflects the heart’s reliance on K+ conductance at rest to maintain readiness for rapid, coordinated excitation.