What is the equilibrium potential for sodium (ENa)?

What is the equilibrium potential for sodium (ENa)?

• A. -96 mV
• B. +52 mV
• C. +134 mV
• D. 0 mV

Answer: (B)

The key idea is that the equilibrium potential for an ion is the membrane voltage at which the chemical gradient pulling the ion in is exactly balanced by the electrical gradient pushing it out (or in), so there’s no net current for that ion. For sodium, which is far more concentrated outside the cell than inside, this balance point sits on the positive side of the membrane. You get ENa from the Nernst equation. At physiological temperature, it’s often written as E_Na ≈ 61.5 mV × log10([Na+]o/[Na+]i). With typical values used in many physiology contexts—extracellular Na+ around 140 mM and intracellular Na+ around 10–15 mM—the ratio is about 9–14, yielding a positive potential in the vicinity of +50 to +70 mV. The commonly taught classroom number of about +52 mV reflects those standard assumptions and approximations. So a positive value around +50 mV is expected for ENa, consistent with sodium’s large extracellular gradient. The other options don’t fit: a negative value would imply the opposite gradient, zero would imply no gradient, which isn’t the case for Na+.

The key idea is that the equilibrium potential for an ion is the membrane voltage at which the chemical gradient pulling the ion in is exactly balanced by the electrical gradient pushing it out (or in), so there’s no net current for that ion. For sodium, which is far more concentrated outside the cell than inside, this balance point sits on the positive side of the membrane.

You get ENa from the Nernst equation. At physiological temperature, it’s often written as E_Na ≈ 61.5 mV × log10([Na+]o/[Na+]i). With typical values used in many physiology contexts—extracellular Na+ around 140 mM and intracellular Na+ around 10–15 mM—the ratio is about 9–14, yielding a positive potential in the vicinity of +50 to +70 mV. The commonly taught classroom number of about +52 mV reflects those standard assumptions and approximations.

So a positive value around +50 mV is expected for ENa, consistent with sodium’s large extracellular gradient. The other options don’t fit: a negative value would imply the opposite gradient, zero would imply no gradient, which isn’t the case for Na+.