Intracellular fluid and extracellular fluid are electrically neutral solutions, in that each has an equal number of positively and negatively charged ions. A simple but important concept is that these opposite charges are attracted to each other and ions of the same charge repel each other. In an unstimulated or resting cell, a slight accumulation of negative charges (–) on the internal surface of the plasma membrane is attracted to an equal number of positive charges (+) that have accumulated on the external surface of the membrane. Therefore, all cells at rest are electrically
; that is, the inside of the cell is slightly negative relative to the outside. This separation of charge across the plasma membrane is referred to as the
. The magnitude of the membrane potential depends primarily on the
number of opposite charges separated by the membrane. The greater the separation of charge then, the greater the membrane potential is. Because the actual number of charges involved is quite small, the potential is measured in millivolts (mV). Furthermore, the sign (+ or –) of the potential is defined by the predominant charge on the internal surface of the cell membrane. Therefore, the membrane potential under resting conditions is negative. As will be discussed, nerve cells and muscle cells rely on changes in
this membrane potential for their functions. In other words, changes in the membrane potential convey information to these types of cells.