In living cells, there is generally a higher concentration of positively charged ions
on the outside of the cell and a higher concentration of negatively charged ions on the
inside of the cell.
Thus, there is a concentration gradient (an electrical potential or polarity) across the membrane
that we call the membrane potential that creates an electrical gradient.
Resting Potential. When the cell is in a resting state, the membrane
potential is maintained by the sodium/potassium pump. The sodium/potassium pump
actively transports 3 positive sodium ions (Na+) to the outside of the cell membrane and
2 potassium ions to the inside of the cell membrane. This results in a negative charge
inside the cell and a positive charge outside the cell, producing a potential or polarity
across the membrane.
Action Potential. The electrical activity that occurs in a stimulated neuron or
muscle fiber is called the action potential. This involves depolarization and subsequent
repolarization. First, sodium ions move into the cell by diffusion. This reverses the
polarity (depolarization). Second, potassium moves out of the cell by diffusion that
causes repolarization. The sodium/potassium pump then restores the ionic balance by
actively (energy required) pumping sodium back out and potassium back into the cell.
These various electrical potentials can be measured with appropriate instruments.
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