Membrane Potential

Membrane potential (also transmembrane potential or membrane voltage) is the difference in electrical potential between the interior and the exterior of a biological cell. Typical values of membrane potential range from –40 mV to –80 mV.

All animal cells are surrounded by a plasma membrane composed of a lipid bilayer with a variety of types of proteins embedded in it. The membrane potential arises primarily from the interaction between the membrane and the actions of two types of transmembrane proteins embedded in the plasma membrane. The membrane serves as both an insulator and a diffusion barrier to the movement of ions. Ion transporter/pump proteins actively push ions across the membrane to establish concentration gradients across the membrane, and ion channels allow ions to move across the membrane down those concentration gradients, a process known as facilitated diffusion. In the most fundamental example of this, the ion transporter Na+/K+-ATPase pumps sodium cations from the inside to the outside, and potassium cations from the outside to the inside of the cell. This establishes two concentration gradients: a gradient for sodium where its concentration is much higher outside than inside the cell, and a gradient for potassium where its concentration is much higher inside the cell than outside. Transmembrane potassium-selective leak channels allow potassium ions to diffuse across the membrane, down the concentration gradient that was established by the ATPase, creating a charge separation, and thus a voltage, across the membrane. In almost all cases, the ion that determines the so-called "resting" membrane potential of a cell is K+, although other ions do contribute in more minor ways. By convention, the sign of the membrane potential is the voltage inside relative to ground outside the cell. In the case of K+, its diffusion down its concentration gradient (toward the outside of the cell, in this case) creates transmembrane voltage that is negative relative to the outside of the cell, and typically –60 to –80 millivolts (mV) in amplitude.

Virtually all eukaryotic cells (including cells from animals, plants, and fungi) maintain a nonzero transmembrane potential, usually with a negative voltage in the cell interior as compared to the cell exterior. The membrane potential has two basic functions. First, it allows a cell to function as a battery, providing power to operate a variety of "molecular devices" embedded in the membrane. Second, in electrically excitable cells such as neurons and muscle cells, it is used for transmitting signals between different parts of a cell. Signals are generated by opening or closing of ion channels at one point in the membrane, producing a local change in the membrane potential. This change in the electric field can quickly be detected by either adjacent or more distant ion channels in the membrane. Those ion channels can then depolarize, reproducing the signal.

In non-excitable cells, and in excitable cells in their baseline states, the membrane potential is held at a relatively stable value, called the resting potential. For neurons, typical values of the resting potential range from –70 to –80 millivolts; that is, the interior of a cell has a negative baseline voltage of a bit less than one tenth of a volt. The opening and closing of ion channels can induce a departure from the resting potential. This is called a depolarization if the interior voltage becomes more positive (say from –70 mV to –60 mV), or a hyperpolarization if the interior voltage becomes more negative (say from –70 mV to –80 mV). In excitable cells, a sufficiently large depolarization can evoke an action potential, in which the membrane potential changes rapidly and significantly for a short time (on the order of 1 to 100 milliseconds), often reversing its polarity. Action potentials are generated by the activation of certain voltage-gated ion channels.

In neurons, the factors that influence the membrane potential are diverse. They include numerous types of ion channels, some that are chemically gated and some that are voltage-gated. Because voltage-gated ion channels are controlled by the membrane potential, while the membrane potential itself is influenced by these same ion channels, feedback loops that allow for complex temporal dynamics arise, including oscillations and regenerative events such as action potentials.