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
- $J_m = 0 = \sum_n G_n(V_m^o-V_n) + \sum_n J_n^a$
- - 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