6.021/Notes/2006-10-16


 * $$V_m^o = \sum_n \frac{G_n}{G_m}V_n$$
 * Rest is not equal to equilibrium
 * eventually all ions would equilibrate inside and outside and there would be no membrane potential
 * For this to not happen, must have pumps (primary active transport)
 * Pumps allow us to go from rest to equilibrium
 * treat as current source
 * $$J_n^a+J_n^p=0$$ for all $$n$$ where $$J_n^a$$ is the active current (pump) and $$J_n^p$$ is the passive current (electro-diffusion).
 * Define quasi-equilibrium: no net flux but requires energy
 * $$J_m = 0 = \sum_n G_n(V_m^o-V_n) + \sum_n J_n^a$$
 * $$V_m^o = \sum_n \frac{G_n}{G_m}V_n - \frac{1}{G_m}\sum_n J_n^a$$
 * The first term is the "indirect effect" whereas the second term is the "direct effect" of the pump
 * Both terms depend on the pump as without the pump, both would be 0
 * All pumps have indirect effect but only some pumps have direct effect
 * A non-electrogenic pump has no net charge change (e.g. trade one sodium ion for one potassium ion)
 * An electrogeneic pump such as 3 sodium for 2 potassium would have net active current
 * Active transport gets energy from glucose metabolism (namely ATP)
 * Experiment shows that sodium and potassium transport are linked
 * Sodium is needed for potassium transport and vice-versa
 * Sodium is pumped out, potassium pumped in
 * The direct effect on membrane potential is negative