Sodium, Not Potassium

I noticed what appears to be an error in “Why I Love Physics” (essay, March 2): “Where a biologist might study how potassium atoms enter the outer wall of a nerve cell and start an electrical pulse shuddering through the cell” should be “Where a biologist might study how sodium ions pass through the membrane of a nerve cell ... .” Potassium ions then leave the cell to return it to its resting potential; they enter the cell primarily via the sodium-potassium pump.