Membrane potential [細胞膜(跨膜)電位]: an electrical potential difference across a membrane. The ions inside or outside a cell are different. Most of these ions carry charges. Cell membrane is impermeable to these charged ions; thus results a separation of charge across the membrane. The movement of ions will be facilitated by ion channels. Membrane potential is a dynamic equivalence. • Every cell has a voltage across its plasma membrane. • Changes in membrane potential act as signals. • Voltage can be measured by electrodes placed inside and another outside the axon. A difference of electrical charge between these two points creates a difference in electrical potential.