The electrical potential difference across a cell’s membrane when it is not actively transmitting a signal is known as the resting potential. This state is a fundamental property of excitable cells, such as neurons and muscle fibers, providing the baseline electrical readiness necessary for communication. It represents a stored form of energy, a slight electrical imbalance, which allows the cell to respond rapidly when stimulated.
The Electrical Foundation
A living cell is encased in a lipid bilayer membrane, a fatty barrier that prevents most charged particles from passing freely between the inside and outside of the cell. The maintenance of the resting potential relies on an unequal distribution of positively and negatively charged particles, called ions, across this membrane. Specifically, the fluid outside the cell has a high concentration of sodium ions (\(\text{Na}^+\)), while the fluid inside the cell contains a high concentration of potassium ions (\(\text{K}^+\)) and large, negatively charged proteins and organic molecules. The difference in ion concentration between the intracellular and extracellular spaces creates a concentration gradient, which is a chemical driving force where ions tend to move from an area of high concentration to an area of low concentration. Chloride ions (\(\text{Cl}^-\)) and large, negatively charged intracellular proteins also play a role in the overall charge balance.
The Role of the Sodium-Potassium Pump
The initial, steep concentration gradients for sodium and potassium ions must be actively created and maintained by a specialized protein called the sodium-potassium pump. This pump is a \(\text{Na}^+/ \text{K}^+\)-ATPase that uses the energy stored in adenosine triphosphate (ATP) to perform its work. The active transport performed by this pump is essential for the cell’s survival, as the ion gradients would eventually dissipate without it. In a single cycle of operation, the pump binds to three sodium ions from inside the cell and transports them out into the extracellular space, while simultaneously binding two potassium ions from outside the cell and moving them into the cell’s interior. The pump’s action of moving three positive charges out for every two positive charges moved in is electrogenic, meaning it contributes a small, direct electrical difference across the membrane, and establishes the enduring concentration gradients necessary for the cell’s resting potential.
Maintaining the Negative Charge
While the sodium-potassium pump sets up the necessary ion gradients, the actual negative voltage of the resting potential, typically around \(-70\) millivolts (\(\text{mV}\)), is primarily achieved and maintained by selective membrane permeability. The cell membrane, even at rest, contains a number of channels that are always open, known as “leakage channels.” The cell membrane is far more permeable to potassium ions than it is to sodium ions, largely because there are significantly more open potassium leakage channels. Because of the high internal concentration of potassium established by the pump, \(\text{K}^+\) ions tend to leak out of the cell down their concentration gradient through these channels. As the positively charged potassium ions leave the cell, they leave behind the large, negatively charged proteins and organic molecules that cannot cross the membrane. This net outflow of positive charge establishes the negative electrical potential inside the cell relative to the outside. The negative charge inside the cell then begins to pull the positive potassium ions back in, creating an electrical gradient that opposes the chemical concentration gradient. The resting potential is the stable point where the force of the potassium concentration gradient pushing \(\text{K}^+\) out is nearly balanced by the electrical force pulling \(\text{K}^+\) back in.
Why Resting Potential Matters
The existence of a stable resting potential, with the inside of the cell maintained at a negative charge, represents a state of readiness for electrically excitable cells. This steady voltage difference, or polarization, stores energy that can be rapidly released. The resting potential is the necessary baseline from which all electrical signaling begins. When a neuron or muscle cell receives an adequate stimulus, the membrane potential rapidly changes, creating an electrical signal known as an action potential. This signal is generated by momentarily opening specific ion channels, allowing a rapid influx of positive ions that quickly shifts the voltage away from the resting state. The established ion gradients and the negative internal charge are the fuel that powers these rapid electrical shifts, allowing nerve cells to transmit information and muscle cells to contract almost instantaneously.