So mm is Millimolar. So basically, this cell membrane is impermeable to ions. And that's because these ions carry charge. Now, if we have some type of protein channel that opens up inside our cell membrane, then our sodium or any other ion can move across the cell membrane and they will always move down their electrochemical gradient. So, for example, if our sodium channels open up that carry sodium channels across the membrane, then the sodium channels will move down their electrochemical gradient from a higher concentration to a lower concentration from the outside to the inside. The same thing is true for calcium.