What is the Nernst potential? How is the magnitude determined?

Nernst potential: Diffusion potential level across membrane that exactly opposes net diffusion through membrane. Ratio of concentrations of ion on 2 sides of the membrane. Greater ratio = greater tendency for ion to diffuse in one direction = greater nernst potential required. Equation: v_s = (rt/zf) ln(s_out/s_in), v_s = potential difference, r = gas constant, t = abs. temp., f = faradays const, z = ion charge, s_in = intracellular ion concentration, s_out = extracellular ion concentration. Extension of nernst equation with na+ k+ cl+ - v_m = (-rt/f) ln(p_k c_kin + p_na c_nain + p_cl c_clout / opposite direction) p = permeability, c = concentration.