Membrane potential is the difference in electrical charge, or voltage, that exists across the cell membrane of nearly every living cell. This voltage is created by an unequal distribution of positively and negatively charged ions between the inside and the outside of the cell. The cellular membrane acts as a selective barrier, separating these charges and allowing the cell to store electrical energy. This electrical energy is fundamental for communication in excitable cells like nerves and muscles.
Establishing the Electrical Gradient
The foundation of membrane potential relies on an imbalance of ions, primarily sodium (\(\text{Na}^{+}\)) and potassium (\(\text{K}^{+}\)). Sodium ions are maintained at a high concentration outside the cell, while potassium ions are concentrated inside the cell. This concentration difference is actively established and maintained by the Sodium-Potassium Pump (\(\text{Na}^{+}\)/\(\text{K}^{+}\) ATPase), a specialized protein embedded in the cell membrane.
The pump uses energy supplied by adenosine triphosphate (ATP) to move ions against their concentration gradients (active transport). For every ATP consumed, the pump transports three sodium ions out of the cell and simultaneously brings two potassium ions into the cell. This unequal exchange creates a net loss of positive charge from the interior of the cell, setting up the necessary electrochemical gradient.
The Resting Potential
The resting membrane potential is the stable, negative voltage maintained by the cell when it is not actively signaling. For many neurons, this resting voltage is around -70 millivolts (mV), meaning the inside of the cell is 70 mV more negative than the outside. This negativity is largely due to the cell membrane being far more permeable to potassium ions than to sodium ions, even at rest.
The membrane contains numerous “leak” channels that allow potassium ions to diffuse out of the cell, moving down their concentration gradient. As positive potassium ions exit, they leave behind large, negatively charged proteins and organic molecules trapped inside the cell. The resting potential represents a dynamic equilibrium where the electrical force pulling positive ions inward is balanced by the concentration gradient pushing them outward.
Generating the Action Potential
The action potential is a rapid electrical event that allows excitable cells to transmit signals over long distances. It begins when a stimulus causes the membrane potential to become less negative, a process called depolarization. If this depolarization reaches a threshold voltage, around -55 mV, it triggers the opening of voltage-gated sodium channels.
Sodium ions rush into the cell, driven by both their concentration and electrical gradients. This influx of positive charge causes the membrane potential to rapidly reverse polarity, spiking to a positive value, often reaching about +30 mV. This reversal is the peak of the action potential, which is brief before the sodium channels inactivate.
The repolarization phase immediately follows, initiated by the slower opening of voltage-gated potassium channels. Potassium ions flow out, repelled by the positive internal voltage. This outflow of positive charge rapidly restores the negative potential across the membrane, sometimes briefly overshooting the resting potential before the cell returns to its stable resting state.
Role in Physiological Systems
The controlled generation of the action potential underlies the function of the body’s most active tissues. In the nervous system, action potentials are the fundamental unit of communication, transmitting information along the axon of a neuron. The rapid sequence of depolarization and repolarization propagates like a wave, allowing signals to travel throughout the body.
This electrical signal also triggers muscle contraction. When an action potential reaches a muscle cell, it initiates internal events leading to the synchronized movement of contractile proteins. The precise control of membrane potential allows for the rapid and coordinated responses necessary for life, enabling functions like thought, sensation, and movement.