Some of the molecules in and around the axon carry an electrical charge, either positive or negative.
The fluid inside the axon contains protein molecules with a negative charge.
These particular protein molecules are found only in the intracellular fluid.
Other electrically charged ions are present in the fluid both inside and outside the axon.
Sodium ions and potassium ions have a positive charge, while chloride ions have a negative charge.
When the axon is in its resting state, there are more negative particles inside the axon than outside the axon,
largely because of the presence of protein molecules inside but not outside the axon.
This produces an electrical charge (called the resting potential) across the membrane.
The interior of the axon is electrically negative—or “polarized”—compared to the exterior.
It's important to remember that there are several types of positively charged
and negatively charged molecules on both sides of the axon membrane.
But from this point on we will concentrate only on the sodium and potassium ions,
because their movement through the membrane is the central issue.
Although sodium ions are present on both sides of the membrane,
they are more concentrated in the extracellular fluid.
Potassium ions are more concentrated in the intracellular fluid.