Membrane potential is the difference in electrical charge between the inside and outside of a cell. It arises because charged particles, or ions, are distributed unevenly across the cell membrane and because the membrane allows some ions to cross more easily than others.
Almost all living cells maintain some membrane potential. In nerve and muscle cells, changes in membrane potential are especially important because they allow these cells to communicate and produce rapid, coordinated responses. But membrane potential is not limited to the nervous system. It is a fundamental feature of cell physiology.
What creates a membrane potential?
A cell membrane separates two watery environments: the cytoplasm inside the cell and the extracellular fluid outside it. Both contain dissolved ions, including potassium (K⁺), sodium (Na⁺), chloride (Cl⁻), and other charged molecules.
The key point is that these ions are not distributed equally on the two sides of the membrane.
In many animal cells, potassium concentration is higher inside the cell, while sodium concentration is higher outside. Chloride distribution also differs between the two sides. Large negatively charged molecules, including many proteins, are largely trapped inside the cell because they cannot freely cross the lipid membrane.
The membrane itself is a barrier to ions. An ion cannot simply pass through the membrane’s lipid interior whenever it wants. Instead, it generally crosses through specialized membrane proteins called ion channels or through transport proteins.
This combination—unequal ion concentrations plus selective membrane permeability—creates the conditions for an electrical difference across the membrane.
Why does an ion concentration difference produce voltage?
Ions are electrically charged, so moving them from one side of a membrane to the other changes the electrical conditions on both sides.
Consider potassium as an example. Because potassium is more concentrated inside many cells, there is a chemical tendency for K⁺ to move outward when potassium-selective channels are open. As positive potassium ions leave, the inside of the cell becomes relatively more negative.
That developing negative charge creates an electrical force pulling K⁺ back toward the inside. Eventually, the chemical force pushing potassium outward and the electrical force pulling it inward can balance one another.
The voltage at which those forces balance for a particular ion is called that ion’s equilibrium potential.
The membrane potential of a real cell is usually influenced by several ions at once, however. It therefore depends not only on concentration differences but also on which ions the membrane is permeable to and how permeable it is to each one.
What is the resting membrane potential?
A cell that is not undergoing a rapid electrical event can still have a membrane potential. This is commonly called its resting membrane potential.
In many animal cells, the inside of the cell is negative relative to the outside. The exact value varies substantially among cell types and physiological conditions. A commonly cited example for a neuron is roughly −70 millivolts (mV), but this is an approximate value rather than a universal property of neurons.
The resting membrane potential is strongly influenced by potassium because many resting cell membranes are relatively permeable to K⁺. Potassium tends to diffuse outward through potassium leak channels, leaving the cell interior relatively negative.
Sodium also matters. Although sodium generally has a strong electrochemical tendency to enter the cell, the resting membrane is often much less permeable to Na⁺ than to K⁺. As a result, sodium contributes to the resting potential without determining it to the same extent as potassium in many cells.
The sodium-potassium pump maintains the ion gradients
The sodium-potassium pump, also called the Na⁺/K⁺ ATPase, is an important part of the system that maintains the ion concentration differences underlying membrane potential.
Using energy from ATP, this membrane protein transports sodium and potassium in opposite directions. For each cycle, it moves three Na⁺ ions out of the cell and two K⁺ ions into it.
This transport is described as active transport because it moves ions against their concentration gradients and requires metabolic energy.
The pump has two related roles. It maintains the unequal distributions of Na⁺ and K⁺ across the membrane, and its movement of three positive charges out for every two positive charges in makes a smaller direct contribution to the membrane’s electrical potential.
It is important not to picture the pump as simply “creating” the entire resting membrane potential. The ion gradients it maintains provide the driving forces, while the selective movement of ions through channels is a major immediate determinant of the voltage.
Membrane potential reflects electrochemical forces
The movement of an ion across a membrane is governed by two influences.
The chemical gradient results from a difference in concentration. An ion tends to diffuse from an area of higher concentration toward an area of lower concentration.
The electrical gradient results from differences in charge. A positively charged ion is attracted toward a relatively negative region and repelled by a relatively positive one.
Together, these forces form an electrochemical gradient.
Whether an ion actually tends to enter or leave a cell depends on the combined effect of these two forces. This is why knowing only the ion concentrations is not enough to predict its movement. The membrane voltage must also be considered.
How ion channels change membrane potential
Ion channels are selective pores in the cell membrane. When a particular channel opens, the ions that can pass through it move according to their electrochemical gradients.
Opening potassium channels, for example, often allows K⁺ to leave a cell. This tends to make the inside more negative and can therefore drive the membrane potential toward a more negative value.
Opening sodium channels often has the opposite effect in an excitable cell. Na⁺ tends to move inward, bringing positive charge into the cell and making the membrane potential more positive.
Channels can open or close in response to different signals. Voltage-gated channels respond to changes in membrane voltage. Ligand-gated channels respond to chemical signals, such as neurotransmitters. Other channels respond to mechanical forces, temperature, intracellular signals, or other stimuli.
Through these channels, cells can convert chemical or physical signals into changes in electrical potential.
What is an action potential?
An action potential is a rapid, temporary change in membrane potential that occurs in certain excitable cells, particularly neurons and muscle cells.
In a typical neuronal action potential, a sufficiently large depolarization activates voltage-gated sodium channels. Sodium then enters the cell rapidly, causing further depolarization. This positive feedback produces a rapid rise in membrane potential.
Voltage-gated sodium channels then become inactivated, while voltage-gated potassium channels allow K⁺ to leave the cell. The membrane potential consequently moves back toward its resting level and may briefly become more negative than the resting potential before returning to its usual state.
The action potential is therefore not simply a pulse of electricity traveling through a wire. It is a coordinated sequence of changes in ion-channel permeability and ion movement across the membrane.
In neurons, this changing membrane potential can propagate along the axon and ultimately trigger neurotransmitter release at the nerve terminal.
Depolarization, repolarization, and hyperpolarization
Several terms describe changes in membrane potential.
Depolarization generally means that the membrane potential becomes less negative relative to its resting state. For example, a change from −70 mV toward −50 mV is a depolarization.
Repolarization describes the return of membrane potential toward its resting level after depolarization, particularly during an action potential.
Hyperpolarization means that the membrane potential becomes more negative than its usual resting value. In a neuron, this can occur when potassium continues to leave the cell or when chloride movement produces an appropriate electrical effect.
These terms describe changes relative to a cell’s existing membrane potential, so their meaning depends on the starting state.
Membrane potential is not the same as membrane charge
It can be tempting to imagine that the inside of a cell is packed with negative charge while the outside is packed with positive charge. That is not an accurate picture.
A cell can have a membrane potential even though the bulk solutions inside and outside remain close to electrically neutral. The voltage arises from a very small separation of charge near the membrane surfaces.
This distinction matters because membrane potential is fundamentally a voltage difference, not a statement that the entire cytoplasm has become substantially charged relative to the extracellular fluid.
Why membrane potential matters to cells
Membrane potential gives cells a way to use ion gradients as a form of stored electrochemical energy.
In excitable cells, changes in membrane potential enable rapid signaling. Neurons use them to transmit information, while muscle cells use them as part of the process that leads to contraction.
Membrane potential also contributes to the operation of many transport systems. Some molecules and ions are moved across membranes by transporters that are influenced by the electrical gradient. In mitochondria, an electrochemical gradient across the inner mitochondrial membrane is central to ATP production.
In other words, membrane potential is not merely an electrical measurement made by cell biologists. It is an important part of how cells move substances, respond to signals, communicate, and convert stored energy into useful cellular work.
The central idea
Membrane potential exists because a cell maintains unequal ion concentrations across a membrane that is selectively permeable to those ions. Ion channels allow particular ions to move according to their electrochemical gradients, while active transport systems such as the sodium-potassium pump help maintain the underlying concentration differences.
The resulting voltage is dynamic rather than fixed. Change which channels are open, and ion movement changes. Change ion concentrations or membrane permeability, and the membrane potential changes as well.
That simple relationship between ion gradients, selective permeability, and electrical forces is the foundation for understanding resting potentials, action potentials, neuronal signaling, muscle excitation, and many other processes in cell biology.
