In a neuron at 37 degrees Celsius, intracellular Na+ concentration is 14 mmol/L and extracellular Na+ concentration is 140 mmol/L. Using the Nernst equation with the factor 61 mV per tenfold concentration ratio for a monovalent cation, the equilibrium potential for sodium is approximately:

• A -61 mV
• B +61 mV ✓
• C +30 mV
• D +122 mV

Explanation

ENa = 61 x log([Na+]o/[Na+]i) = 61 x log(140/14) = 61 x log(10) = 61 x 1 = +61 mV. The sign is positive because the concentration gradient drives Na+ into the cell, leaving the inside relatively positive at equilibrium. Option C would result if the ratio were inverted incorrectly, and option D results from forgetting the logarithm and using the raw ratio. This value explains why Na+ influx during the upstroke drives the potential toward roughly +30 to +40 mV but never reaches ENa.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

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