than gK+, which renders Em close to Eeq,K+. Medical conditions such as hyperkalemia in which blood serum potassium (which governs [K+]o) is changed are very dangerous since they offset Eeq,K+, thus affecting Em. This may cause arrhythmias and cardiac arrest. The use of a bolus injection of potassium chloride in executions by lethal injection stops the heart by shifting the resting potential to a more positive value, which depolarizes and contracts the cardiac cells permanently, not allowing the heart to repolarize and thus enter diastole to be refilled with blood. Although the GHK voltage equation and Millman's equation are related, they are not equivalent. The critical difference is that Millman's equation assumes the current-voltage relationship to be ohmic, whereas the GHK voltage equation takes into consideration the small, instantaneous rectifications predicted by the GHK flux equation caused by the concentration gradient of ions. Thus, a more accurate estimate of membrane potential can be calculated using the GHK equation than with Millman's equation. Measuring resting potentials In some cells, the membrane potential is always changing (such as cardiac pacemaker cells). For such cells there is never any "rest" and the "resting potential" is a theoretical concept. Other cells with little in the way of membrane transport functions that change with time have a resting membrane potential that can be measured by inserting an electrode into the cell. Transmembrane potentials can also be measured optically with dyes that change their optical properties according to the membrane potential. Summary of resting potential values in different types of cells History Resting currents in nerves were measured and described by Julius Bernstein in 1902 where he proposed a "Membrane Theory" that explained the resting potential of nerve and muscle as a diffusion potential. See also Action potential Depolarization Hyperpolarization (biology) Membrane potential References External links Neuroscience - online textbook by Purves, et al. Basic Neurochemistry Molecular, Cellular, and Medical Aspects by Siegel, et al. Bertil Hille Ion channels of excitable membranes, 3rd ed., Sinauer Associates, Sunderland, MA (2001). Resting Membrane Potential - Online lecture notes on the resting membrane potential The Origin of the Resting Membrane Potential - Online interactive tutorial Neurophysiology Membrane biology Potentials