The amplitude of the action potential (the potential change during phase o)varies linearly with the logarithm of INato When Nato is increased E-20 from about 20% ofits normal value to about 150% of its normal value, the transmembrane potential at Resting membrane the peak of the action potential increases from 8101520 100150 about -20 mv to about +40 External Na* concentration(% of normall FIGURE 16-7. The [Na'l in the external medium is mWhen [Na+ ]o is increased from about 20% of its normal value to about 150% of its normal value, the transmembrane potential at the peak of the action potential increases from about -20 mV to about +40 mV. The amplitude of the action potential (the potential change during phase 0) varies linearly with the logarithm of [Na+ ]o