The voltage of a neuron is measured as the voltage difference between the inside and outside, with the outside defined as 0 Volts. When the neuron is at rest, the voltage is called the resting potential, and is typically around -40 mV to -80 mV. In this section, we will explore how the resting potential is generated.

To model a neuron, we'll use a tank divided in half by a biological membrane (yellow). The left side of the tank represents the inside of the neuron, and the right side represents the outside. The pink color represents potassium chloride (KCl), which is found in and around neurons. KCl dissociates into equal proportions of K+ (potassium) and Cl– (chloride) ions.

A schematic of a tank water tank filled with purple liquid, and partitioned into left and right halves with a yellow divider. The left side is a deeper purple than the right side.

First, assume that the membrane is not permeable to ions, and that the concentration of KCl is higher inside the neuron (left) than outside (right). On each side, the number of K+ ions equals the number of Cl– ions. Therefore, each side has zero net charge, and the voltage between the sides is 0 mV.

A schematic of a tank water tank filled with purple liquid, and partitioned into left and right halves with a yellow divider. The left and right side are both light purple.

Next, assume the membrane is permeable to both K+ and Cl– ions. The ions will naturally move down the concentration gradient. They will flow to the right, through the membrane, until their concentrations are the same on both sides. Again, on each side, the number of K+ ions equals the number of Cl– ions, and the voltage between the two sides is still 0 mV.

A schematic of a tank water tank filled with purple liquid, and partitioned into left and right halves with a yellow divider. The left side is a deeper purple than the right side.

Now assume that the membrane is permeable to only K+ ions. If the concentration of KCl is higher on the left, both K+ and Cl– ions would normally flow right. K+ ions can flow to the right because the membrane is permeable to them. However, Cl– ions are stuck on the left because the membrane is impermeable to them.

A schematic of a tank water tank filled with purple liquid, and partitioned into left and right halves with a yellow divider. The left side is a deeper purple than the right side. In addition, near the yellow divider, there is a cloud of blue particles on the left side with a negative symbol, and there is a cloud of red particles on the right side with a positive symbol.

The left side now has more Cl– ions than K+ ions, so it has a net negative charge. The right side has more K+ ions than Cl– ions, and has a net positive charge. As this charge imbalance increases, fewer K+ ions are able to enter the right side, because they are repelled by the other K+ ions there.

Eventually, the tendency for K+ ions to move down the concentration gradient will be balanced by the tendency for positive charges to repel each other. The system will reach an equilibrium with no net charge movement. At this point, there will be a steady voltage between the two sides.

This steady voltage is the resting potential. Since the left side has a net negative charge, and the right side has a net positive charge, the voltage is negative on the left compared to the right. Remember that the left side represents the inside of a neuron, and the right side represents the outside. The neuron's resting potential would thus be negative on the inside compared to the outside.