The cell membrane functions as a selective barrier, separating the watery environment inside the cell from the fluid outside. This separation establishes a difference in electrical charge between the intracellular and extracellular spaces, known as the membrane potential. This electrical potential is the foundation of cellular excitability, allowing cells like neurons and muscle fibers to generate and transmit electrical signals. The resting membrane potential represents the baseline electrical charge of the cell when it is in an inactive, or “resting,” state.
Defining the Resting State
The polarity of the resting membrane potential is negative. This means the inside of the cell carries a negative electrical charge relative to the fluid outside the cell. The potential, measured in millivolts (mV), typically falls within a range of approximately -60 mV to -90 mV for excitable cells such as neurons and skeletal muscle fibers.
A negative resting potential signifies that the cell’s interior is polarized, much like a charged battery. This established charge difference is fundamental to the ability of a cell to quickly generate an electrical impulse, or action potential. The magnitude of this negative charge provides the necessary electrical gradient for fast communication within the nervous and muscular systems.
The Role of Unequal Ion Distribution
The negative polarity of the resting membrane potential originates from the unequal distribution of specific ions across the cell membrane. The two cations that play the largest role are Potassium (\(\text{K}^+\)) and Sodium (\(\text{Na}^+\)). Their concentrations are kept out of balance between the intracellular fluid (ICF) and the extracellular fluid (ECF).
Potassium ions are maintained at a much higher concentration inside the cell, while Sodium ions are kept at a significantly higher concentration outside the cell. This imbalance creates a concentration gradient for each ion. For example, the concentration of \(\text{K}^+\) inside a cell can be 30 times greater than its concentration outside, and the concentration of \(\text{Na}^+\) can be five times greater outside than inside.
This concentration difference means that each ion tends to move across the membrane to equalize the concentration. This movement is driven by diffusion, where particles move from an area of high concentration to an area of low concentration. The uneven distribution of these ions powers the generation of the resting membrane potential.
Selective Permeability and Driving Forces
The concentration gradients established by the unequal ion distribution are converted into a negative voltage through selective permeability. At rest, the cell membrane possesses numerous non-gated channels, often called leak channels, that allow ions to pass through. The membrane is far more permeable to Potassium (\(\text{K}^+\)) than to Sodium (\(\text{Na}^+\)) because there are many more \(\text{K}^+\) leak channels open.
The concentration gradient for \(\text{K}^+\) pushes it to move out of the cell, down its gradient. As positively charged \(\text{K}^+\) ions leave, they move away from the negatively charged molecules, like large proteins and organic anions, trapped inside the cell. This efflux of positive charge causes a net accumulation of negative charge on the inner surface of the membrane.
As the interior becomes increasingly negative, an electrical force pulls the positively charged \(\text{K}^+\) ions back into the cell, opposing the chemical force pushing them out. The resting potential is established when the outward chemical force on \(\text{K}^+\) is balanced by the inward electrical force. Because the membrane is slightly permeable to \(\text{Na}^+\), a small number of \(\text{Na}^+\) ions leak into the cell, preventing the potential from reaching the maximum negativity dictated by \(\text{K}^+\) alone.
Active Maintenance of the Potential
While ion leakage establishes the potential, an active mechanism is required to restore the ion gradients that are slowly lost. The Sodium-Potassium ATPase pump, or \(\text{Na}^+/\text{K}^+\) pump, performs this task. This protein is a primary active transport mechanism, meaning it uses energy (ATP) to move ions against their concentration gradients.
The pump works by binding three \(\text{Na}^+\) ions inside the cell and two \(\text{K}^+\) ions outside the cell. For every molecule of ATP consumed, the pump transports three \(\text{Na}^+\) ions out and two \(\text{K}^+\) ions back into the cell. This action restores the high extracellular \(\text{Na}^+\) concentration and the high intracellular \(\text{K}^+\) concentration, which are the foundational gradients for the resting state.
The pump is electrogenic because it moves a net positive charge of one ion out of the cell during each cycle. This unequal exchange contributes a small amount to the negative resting potential. However, the pump’s primary function is the continuous maintenance of the concentration gradients that prevent the resting membrane potential from collapsing due to ion leakage.