The Role of Ions
The concentration of ions around the nerve cell controls the action potential.
There are two ions that are essential to a normal action potential. These
ions are Sodium (Na+) and Potassium (K+). The Depolarization phase is controlled
by Sodium. An external stimulus causes an influx of Sodium in the nerve
cell. This depolarization, or action potential continues down the neural
pathway, until it reaches its destination. After the cell depolarizes,
it must repolarize to its resting potential before it can depolarize again.
This repolarization phase is controlled by Potassium. An efflux of Potassium
causes the potential to return to its resting state.
The influx of Sodium ions and the efflux of Potassium ions are controlled
by protein gates in the plasma membrane. The influx
and efflux of these ions occurs by diffusion. Diffusion is the process
by which ions move from an area of higher concentration to lower concentration.
For example, after the nerve cell has been stimulated by an external stimulus,
an influx of Sodium occurs because the extracellular concentration is higher
than that of the intracellular.
The Sodium-Potassium Pump is a separate protein channel that replenishes
the extracellular environment with Sodium, and the intracellular environment
with Potassium. This protects the extracellular environment from becoming
saturated with Potassium and the intracellular with sodium.