potential above threshold, such as a typical na+ current, is considered excitatory. a current with a reversal potential below threshold, such as a typical k+ current, is considered inhibitory. a current with a reversal potential above the resting potential, but below threshold, will not by itself elicit action potentials, but will produce subthreshold membrane potential oscillations. thus, neurotransmitters that act to open na+ channels produce excitatory postsynaptic potentials, or epsps, whereas neurotransmitters that act to open k+ or cl− channels typically produce inhibitory postsynaptic potentials, or ipsps. when multiple types of channels are open within the same time period, their postsynaptic potentials summate (are added together). other values from the viewpoint of biophysics, the resting membrane potential is merely the membrane potential that results from the membrane permeabilities that predominate when the cell is resting. the above equation of weighted averages always applies, but the following approach may be more easily visualized. at any given moment, there are two factors for an ion that determine how much influence that ion will have over the membrane potential of a cell: that ion's driving force that ion's permeability if the driving force is high, then the ion is being "pushed" across the membrane. if the permeability is high, it will be easier for the ion to diffuse across the membrane. driving force is the net electrical force available to move that ion across the membrane. it is calculated as the difference between the voltage that the ion "wants" to be at (its equilibrium potential) and the actual membrane potential (em). so, in formal terms, the driving force for an ion = em - eion for example, at our earlier calculated resting potential of −73 mv, the driving force on potassium is 7 mv : (−73 mv) − (−80 mv) = 7 mv. the driving force on sodium would be (−73 mv) − (60 mv) = −133 mv. permeability is a measure of how easily an ion can cross the membrane. it is normally measured as the (electrical) conductance and the unit, siemens, corresponds to 1 c·s−1·v−1, that is one coulomb per second per volt of potential. so, in a resting membrane, while the driving force for potassium is low, its permeability is very high. sodium has a huge driving force but almost no resting permeability. in this case, potassium carries about 20 times more current than sodium, and thus has 20 times more influence over em than does sodium. however, consider another case—the peak of the action potential. here, permeability to na is high and k permeability is relatively low. thus, the membrane moves to near ena and far from ek. the more ions are permeant the more complicated it becomes to predict the membrane potential. however, this can be done using the goldman-hodgkin-katz equation or the weighted means equation. by plugging in the concentration gradients and the permeabilities of the ions at any instant in time, one can determine the membrane potential at that moment. what the ghk equations means is that, at any time, the value of the membrane potential will be a weighted average of the equilibrium potentials of all permeant ions. the "weighting" is the ions relative permeability across the membrane. effects and implications while cells expend energy to transport ions and establish a transmembrane potential, they use this potential in turn to transport other ions and metabolites such as sugar. the transmembrane potential of the mitochondria drives the production of atp, which is the common currency of biological energy. cells may draw on the energy they store in the resting potential to drive action potentials or other forms of excitation. these changes in the membrane potential enable communication with other cells (as with action potentials) or initiate changes inside the cell, which happens in an egg when it is fertilized by a sperm. changes in the dielectric properties of plasma membrane may act as hallmark of underlying conditions such as diabetes and dyslipidemia. in neuronal cells, an action potential begins with a rush of sodium ions into the cell through sodium channels, resulting in depolarization, while recovery involves an outward rush of potassium through potassium channels. both of these fluxes occur by passive diffusion. a dose of salt may trigger the still-working neurons of a fresh cut of meat into firing, causing muscle spasms.