How does a cell membrane become polarized?
there is a flux of ions that are pump from a side of the membrane to the other making it more negative or positive depending on the ions used
there are membrane pump that, using energie, pass ions trough the cell against the graident of concentration. that is why, once bridge in the cell are open, the ions travel back in order to get the electrical concentration to be neutrale again. That is how the cells in the neural systeme work, creating ions concentration withs ions pump to get polarized witch get open when a message pass from cell to cell.