Introduction

Nerve cells (neurons) are far from idle when they are at “rest” and not firing. Instead, they constantly use ATP to power membrane pumps to unequally distribute Na⁺ and K⁺ ions across the membrane, creating ion concentration gradients. K⁺ leak channels in the membrane allow K⁺ ions to move down their concentration gradient, eventually reaching a dynamic equilibrium called the resting membrane potential. In this state, the neuron is ready to fire.

When a neuron fires, it produces an action potential, which is a rapid and temporary disruption of the resting membrane potential that travels along the axon. The process begins when a threshold stimulus is reached and voltage-gated Na⁺ channels open. Na⁺ ions rush inward down their concentration gradient through the open channels, briefly reversing the membrane potential.

Na⁺/K⁺ Pumps

Protein pumps (ATPases) in the axon membrane use energy from ATP to move ions against their concentration gradients. In each pump cycle, the Na⁺/K⁺ pump transports three Na⁺ ions out of the cell and two K⁺ ions into the cell.

As a result, Na⁺ accumulates outside the cell, and K⁺ accumulates inside, establishing concentration gradients that store potential energy. These gradients do not directly produce the resting membrane potential. Instead, they create the conditions that allow it to develop through the movement of K⁺ ions across the membrane.

During an action potential, the energy stored in these gradients drives the rapid and sequential movement of Na⁺ and K⁺ ions across the membrane.

K⁺ Leak Channels.

The resting membrane is much more permeable to K⁺ ions than to Na⁺ ions. As a result, many K⁺ ions pumped into the cell diffuse back out down their concentration gradient.

As K⁺ ions leave the cell, the interior becomes more negative relative to the exterior. This separation of charge establishes an electrical gradient and polarizes the membrane.

Resting Equilibrium

The outward movement of K⁺ ions does not continue indefinitely. Instead, K⁺ is repelled by the growing positive charge outside the membrane and attracted to the increasingly negative charge inside.

Although the membrane is far less permeable to Na⁺, a small number of Na⁺ ions also leak into the cell down their concentration gradient. This inward movement of positive charge slightly offsets the loss of K⁺.

Eventually, the chemical and electrical forces acting on K⁺ balance. At this point, K⁺ ions continue to move in both directions, but there is no net movement. This state of dynamic equilibrium contributes to a stable resting membrane potential.

How Membrane Potentials Are Measured

The membrane potential is determined by placing a reference electrode outside the cell membrane and a recording electrode inside it.

The charge difference between the two electrodes, the membrane potential, is measured in millivolts (mV) and displayed on an oscilloscope.