The physics of membrane potentials

the membrane is selectively permeable flor one or a flew ion species, 

You will note that most terms in that equation are constants (the temperature is virtually constant at 37 °C = 310 

so that the potential is essentially a function of the ion concentration gradient alone. However, this simple relationship will hold only when there is only one diffusible ion species. In a cell, we have several ion species, and finite permeabilities for several of them. The two dominant cations, Na and K, have roughly opposite distributions across the plasma membrane. Application of the Nemst equation to Na and would yield membrane potentials of -1-60 mV and -90 mV, respectively. Yet, the actual resting 

the contribution of each ion to the overall membrane potential depends not only on its concentration but it is weighted for the permeability of the membrane for that ion. Since the concentrations and gradients for and Na are of similar magnitude, we may infer that the membrane permeability must be larger for potassium than for sodium (Pk P g), since the actual membrane potential is much closer to the than to the Na equilibrium potential. 

Now what s the molecular basis of different permeabilities for different ions This is where the channels come in. Without a specific channel, no ion can effectively cross the membrane, so its permeability will be very small only ions for which specific channels exist will therefore have a say in determining the membrane potential. Furthermore, as ion channels open and close, the changing permeabilities can shift the weight from one ion to the other. The most important example is the transient opening of sodium channels, which according to the Goldman equation will cause the 

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