Essentials of Human Physiology for Pharmacy

20 Essentials of Human Physiology for Pharmacyof the plasma membrane to potassium is high compared to that of sodium,the membrane potential approaches –90 mV.Next, consider a condition in which the membrane is permeable only tosodium. Because sodium is in a greater concentration outside the cell, the Na +ions initially diffuse into the cell down their concentration gradient. As a result,an excess of these positively charged ions accumulates in the ICF along theinternal surface of the plasma membrane; an excess of negative charges in theform of the impermeable extracellular anion, chloride (Cl – ), remains outsidethe cell along the external surface of the plasma membrane. This inwardmovement of positive charges creates a positive membrane potential becausethe inside of the cell is now positive relative to the outside. However, as thepositively charged Na + ions continue to diffuse inward, once again an electricalgradient develops.The (+) charges that have accumulated in the ICF begin to repel anyadditional Na + ions and oppose the further movement of (+) charges inward.Instead, the positively charged Na + ions are now attracted to the negativelycharged Cl – ions remaining outside the cell. Eventually, the initial forcemoving Na + ions inward down their concentration gradient is exactly balancedby the subsequent force moving Na + ions outward down their electricalgradient, so there is no further net diffusion of sodium. The membranepotential at this point has reached the equilibrium potential for Na + (E Na+ ) andis equal to +60 mV. Therefore, when the permeability of the plasma membraneto sodium is high compared to that of potassium, the membranepotential approaches +60 mV.At any given time, the membrane potential is closer to the equilibriumpotential of the more permeable ion. Under normal resting conditions, Na +ions and K + ions are permeable; however, potassium is significantly (50 to75 times) more permeable than sodium. Therefore, a large number of K + ionsdiffuse outward and a very small number of Na + ions diffuse inward downtheir concentration gradients. As a result, the comparatively copious outwardmovement of K + ions exerts a powerful influence on the value of theresting membrane potential, driving it toward its equilibrium potential of–90 mV. However, the slight inward movement of Na + ions that would tendto drive the membrane potential toward its equilibrium potential of +60 mVrenders the membrane potential slightly less negative. The balance of thesetwo opposing effects results in a typical neuron resting membrane potentialof –70 mV (see Figure 3.1).The Na + –K + pump also plays a vital role in this process. For each moleculeof ATP expended, three Na + ions are pumped out of the cell into theECF and two K + ions are pumped into the cell into the ICF. The result is theunequal transport of positively charged ions across the membrane such thatthe outside of the cell becomes more positive compared to its inside; in otherwords, the inside of the cell is more negative compared to the outside.Therefore, the activity of the pump makes a small direct contribution togeneration of the resting membrane potential.