Answer
A difference in electrical charge between two points is called an electrical potential (difference) or a voltage difference. The electrical activity of cells is based on differences in concentration of ions between the intracellular fluid in the cell ( cytosol) and the extracellular fluid (ECF/ISP) outside the cell-- near the cell plasma membrane ( sarcolemma). Usually, the the ICF contains a greater number of negative ions -- especially proteins, nucleic acids, and phosphates. These anions are trapped in the cytosol because the selectively permeable plasma membrane will hardly allow them to diffuse out. Regarding cations, the concentration of sodium ions [Na+] in the interstitial fluid is normally higher that the concentration within the cell. Conversely, the concentration of potassium ions [K+] in the cytosol is greater than its concentration in the ECF/ISF. Because the anions cannot diffuse ou, this gives the interior of the cell a negative charge with respect to the outside( of the cell membrane) This is the normal condition in a resting unstimulated cell. Some sodium ions (Na+) normally leak into the cell down their electrochemical gradient, and some K+ ions leak out of the cell down their chemical gradient. Nevertheless, the potential difference is maintained because of the actions of the N+/K+ ATPase pump; this pump ejects three Na+ ions from the cytosol for every two K+ ions that it brings into the cell form the ISF. This continuous activity of the Na+/K+ ATPase pump maintains the resting potential of the cell. Therefore, a normal unstimulated cell has a resting membrane potential, in other words, it is polarized
The value of the resting potential varies among types of cells ( -20 mV to 100 mV). In neurons the RMP is usually about -70 mV, and in skeletal muscle this is a little greater, about -90 mV. This means that a normal resting cell is polarized.
Work Step by Step
When a myocyte is stimulated electrically , sodium ion channels open and sodium ions (N+) diffuse into the cell down their electrochemical gradient. This ingress of sodium decreases the negative charge in the cell from about -90 MV to about -50 mV. This is threshold for most skeletal muscle cells. As the Na+ ions continue to pour in, the membrane potential goes to zero (0.0 mV) and the cell is depolarized. The Na gates remain open and the Na+ ions continue to diffuse in until the potential difference peaks at a positive +30 to +35 mV.
The potassium ion gates are slower to open than the Na+ ion gates ; they open in time, and the K+ ions diffuse out down their chemical gradient. This decreases the positivity of the internal cell environment and the potential difference falls from the peak of about 30-40 mV, past zero, past threshold; recovers normal RMP, and falls still lower ( hyperpolarizes) to about 100 mV. The normal resting potential of 85-90 mV is slowly restored as the Na+/K+ ATPase pump ejects more Na+ anons than the number of K+ cations that it brings in.
