Membrane potential is the difference in electric potential between the two sides of a biological membrane, especially the plasma membrane surrounding a cell. It arises from unequal distributions of ions, selective membrane permeability, and the activity of membrane transporters. Usually expressed in millivolts (mV), it is conventionally defined as the potential inside the cell minus that outside. A negative value therefore means that the interior is electrically negative relative to the exterior. Membrane potential underlies electrical signaling and also contributes to transport and biological energy conversion. (ncbi.nlm.nih.gov)
Physical basis
A membrane separates conducting aqueous solutions with a thin, relatively insulating lipid layer. Transport proteins, including ion channels, provide selective pathways through this barrier. Ion movement reflects both diffusion down concentration gradients and electrical forces acting on charged particles; together these constitute an electrochemical gradient. An ion can consequently move against its concentration gradient if the electrical force favoring that movement is sufficiently strong. (ncbi.nlm.nih.gov)
Only a small fraction of the ions on either side must redistribute to establish a substantial voltage. The bulk solutions remain approximately electrically neutral: the relevant excess charge is concentrated near the membrane surfaces. Thus, a negative intracellular potential does not mean that the entire cytoplasm contains a large excess of negative ions. Electrically, the membrane behaves approximately as a capacitor, storing separated charge, while its ion-conducting pathways determine how that charge changes over time. (ncbi.nlm.nih.gov)
Resting potential and active maintenance
The resting membrane potential is the voltage of a cell when it is not undergoing an action potential or another major electrical response. Many neurons have resting values near −70 mV, but there is no universal value: voltage depends on cell type, ionic conditions, and which channels are open. In many animal cells, potassium is more concentrated inside and sodium outside. Resting membranes are commonly more permeable to potassium, so their voltage lies relatively close to potassium’s equilibrium potential. Sodium and chloride permeability also influence it. (ncbi.nlm.nih.gov)
The sodium–potassium pump maintains these gradients by using ATP to export three sodium ions and import two potassium ions per transport cycle. Because it transfers a net positive charge outward, it is electrogenic and can contribute directly to the voltage. Its essential longer-term role is to preserve the gradients that support passive ionic currents. A resting cell is therefore generally in a maintained steady state, not complete thermodynamic equilibrium: ion leakage and active transport can continue even while voltage remains stable. (ncbi.nlm.nih.gov)
Equilibrium potentials and quantitative models
For a particular ion, the equilibrium potential is the voltage at which electrical and chemical driving forces balance, producing no net passive movement of that ion. The Nernst equation expresses this relationship:
[ E_i=\frac{RT}{z_iF}\ln\frac{a_{i,\mathrm{out}}}{a_{i,\mathrm{in}}}. ]
Here (R) is the gas constant, (T) is absolute temperature, (z_i) is the ion’s signed charge number, (F) is the Faraday constant, and (a_i) denotes ionic activity. Concentrations often approximate activities in physiological calculations. The charge sign matters: anions and cations respond oppositely to the same concentration ratio. (esalq.usp.br)
When several ions are permeant, a single equilibrium potential generally does not describe the membrane voltage. The Goldman–Hodgkin–Katz voltage equation incorporates their relative permeabilities. For potassium, sodium, and chloride, its familiar concentration-based form is
[ V_m=\frac{RT}{F} \ln\frac{ P_K[K^+]{\mathrm{out}}+ P{Na}[Na^+]{\mathrm{out}}+ P{Cl}[Cl^-]{\mathrm{in}} }{ P_K[K^+]{\mathrm{in}}+ P_{Na}[Na^+]{\mathrm{in}}+ P{Cl}[Cl^-]_{\mathrm{out}} }. ]
The chloride concentrations appear in reversed positions because chloride is negatively charged. This model assumes a constant electric field across the membrane and zero total passive ionic current. It is an approximation, not a universal description of every membrane; active pump currents and additional permeant ions may require separate treatment. (esalq.usp.br)
Changes in voltage and electrical signaling
Depolarization makes membrane voltage less negative or more positive; hyperpolarization makes it more negative. Repolarization describes a return toward the preceding resting voltage after depolarization. These changes commonly result from altered channel opening, rather than large, immediate changes in the total intracellular ion concentrations. Opening a selectively permeable channel tends to drive voltage toward that channel’s reversal potential—the voltage at which its net current reverses direction. (ncbi.nlm.nih.gov)
In excitable cells, voltage-dependent channel activity can produce an action potential. In a typical neuronal spike, sodium-channel opening generates inward current and rapid depolarization; sodium-channel inactivation and increased potassium conductance then favor repolarization. At a synapse, a neurotransmitter can instead change channel activity to produce a local postsynaptic response. Membrane voltage is therefore both an electrical signal and a regulator of the proteins that generate such signals. (ncbi.nlm.nih.gov)
Organelles and measurement
Membrane potentials also occur across intracellular membranes. In a mitochondrion, the electron transport chain pumps protons across the inner membrane, leaving the matrix electrically negative relative to the intermembrane space. This voltage and the proton concentration difference together form the proton-motive force, which drives ATP synthase during oxidative phosphorylation. The voltage and chemical gradient are distinct, interacting components of stored electrochemical energy. (ncbi.nlm.nih.gov)
Plasma-membrane voltage can be measured using an intracellular electrode and an extracellular reference. In patch-clamp experiments, a glass micropipette forms a tight seal with the membrane. Current-clamp recording measures voltage while controlling injected current; voltage-clamp recording holds voltage at a chosen value while measuring the current required to maintain it. Single-channel recordings resolve currents through individual channels, whereas whole-cell recordings characterize the combined electrical behavior of the accessible membrane. (ncbi.nlm.nih.gov)