Membrane potential: difference in electrical charge b/w both sides of the membrane. Two electrodes are placed in the body: one on the outside of the cell and one on the inside of the cell. The intracellular electrode = microelectrode, b/c it must be small enough to pierce the membrane w/o damaging it. Resting membrane potential: membrane potential when the cell is at rest (no aps). Membrane is said to be polarized (carries a charge) Inside of the cell is more negative than the outside of the cell. Composition of cell: more k+ inside and more na+ outside. Ion channels are specific to each ion and transport k+ and na+ into/out of cell passively. Factors contributing to the even distribution of ions: random motion: particles tend to move down their concentration gradient. B/c there is more k+ inside than outside, k+ wants to move out: electrostatic pressure: like repels like and opposites attract.