when a muscle cell or nerve cell is stimulated na channels quickly open and the membrane becomes very permeable to na for a brief time figure 7.4 step 2 the na concentration is much greater outside the cell than inside and therefore a few positively charged na quickly diffuse down their concentration gradient into the cell causing the inside of the cell membrane to become more positive than the outside of the cell this change is called depolarization near the end of depolarization na channels close and additional k channels open figure 7.4 step 3 consequently the thedency for na to enter the cell is decreased and the thedency for k to leave the cell is increased these changes cause the inside of the cell membrane to become more negative than the outside once again additional k channels then close as the charge across the cell membrane returns to its resting condition return to figure 7.4 step1the change back to the resting membrane potential is called repolarization the rapid depolarization and repolarization of the cell membrane is called an actionpotential in a muscle fiber an action potential results in muscle contraction the resting membrane potential and action potential are described in more detail in chapter 8