difference of charge across the membrane the larger the membrane potential (voltage). Recall that the average neuron cell has a RMP of -70mV but there is a range of -40 to -90 mV. A cell that exhibits membrane potential is said to be Polarized .
RMP arises from 3 major factors:
Unequal distribution of ions in the ECF and cytosol:
ECF is rich in Na+ and Cl-. The cytosol mainly has the cation K+. The plasma membrane typically has more K+ leakage channels and therefore the amount of K+ diffusing out of the cell is higher than the amount of Na+ coming into the cell. As more K+ exits, the inside of the cell becomes increasingly more negative and the outside of the cell becomes more positive.
Inability of most anions to leave the cell:
Most anions in the cell cannot leave the cell easily with K+ because they are attached to non- diffusible molecules such as ATP and large proteins.
Electro-genic nature of the Na+/K+ ATPases
Membrane permeability to Na+ is very low since there are fewer Na+ leakage channels. However, Na+ does slowly leak into the cell. Left unchecked, this would destroy the RMP. To help offset this leakage, the Na+/K+ pump maintains the RMP by pumping Na+ out as fast as it leaks in. Concurrently, the pump is bringing K+ back into the cell. Even so, the K+ will still continue to leak out . The Na+/K+ pump expels 3 Na+ for each 2 K+. Therefore, since the pump removes more (+) from the cell than they bring into the cell, they are Electro-genic meaning they contribute to the negativity of the RMP. Their total contribution is small however, being only about -3mV of the total -70mV in a typical neuron. REVIEW: When ion channels are open, they allow specific ions to flow across the plasma membrane down their Electrochemical Gradient (a chemical concentration difference plus an electrical difference). Also recall that ions move from areas of higher concentration to areas of lower concentration (chemical part of the gradient). Also recall that positive cations move toward a more negatively charged anions and the anions move toward the more positive cations area (the electrical part of the gradient). As these ions flow, they create electrical current that can change the membrane potential.
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