The nerve cell, just like a muscle cell, normally has more positively charged sodium ions outside than inside. This difference is maintained by pumps that push positively charged sodium ions out of the cell through the nerve cell membrane. The excess of external positive charges provides a voltage difference across the cell membrane of about one-hundredth of a volt. Although this doesn’t sound like much, you have to remember that cell membranes are just a few nanometers thick, so it is a voltage across a very short distance. This means that we have an electrical gradient (what voltage actually
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