Resting membrane potential
[Ref: KB2:p232; WG21:p7-8; PK1:p3-4; BL8:p10]
RMP
... is the steady state potential which exists across the cell membrane
Typical values
- myelinated peripheral nerve axon
= -70mV
- myocyte
= -90mV
Production of RMP
- Na-K ATPases pump Na+ out of cells and K+ into cells
- A concentration gradient across the membrane is produced
- Na+ and K+ leak down the gradient
- BUT, the membrane is much more permeable to K+, so much more K+ leaves the cell than Na+ enters the cell
- This results in net movement of positive charge out of the cell, causing the inside of the cell to be more negative than outside
--> this potential difference is the resting membrane potential
Other contributors
- The Na-K ATPase pump itself is electrogenic because it moves 3 Na+ ions out of cells for every 2 K+ ions.
i.e. adds a little to RMP
- Gibbs-Donnan effect: large intracellular proteins and organic phosphates are not diffusible, and it results in some redistribution of diffusible ions (e.g. K+, Cl-)
i.e. also adds a little to RMP
Nernst equation
... calculates the potential difference that any ion (with a concentration gradient across the membrane) would produce if the membrane was permable to it.
E = RT/zF x In{[ion]o/[ion]i}
= 61.5 x log{[ion]o/[ion]i} @37C
- E = equilibrium potential
- R = gas constant
* (8.314JK-1mol-1)
- F = Faraday's constant
* (96500 coulombsmol-1)
- z = ionic valency
- [ion]o = concentration outside
- [ion]i = concentration inside
Goldman equation
(Full name: Goldman-Hodgkin-Katz form of Nernst equation)
E = 61.5
x log{(PK[K]o+PNa[Na]o+PCl[Cl]o)/(PK[K]i+PNa[Na]i+PCl[Cl]i)}
- Modified from Nernst to take into account the relative concentration and permeability of the important ions
Contribution by ions
An electrolyte have greater influence on RMP when:
- high concentration
- large concentration gradient
- high membrane permeability
Also, as an ion's permeability increases, the membrane potential moves towards its equilibrium potential
Other notes
Nernst equation application
For Cl (i:9-10, o:125)
ECl = -61.5 x log(125/10)
= -61.5 x 1.0969
= -66.91
~ -70mV
For K (i: 135-150, o: 4-5.5)
EK = 61.5 x log(4.5/150)
= 61.5 x -1.523
= -93.66
~ -94mV
For Na (i:10-15, o:145-150)
ENa = 61.5 x log(150/15)
= 61.5 x 1
~ +60mV
Note
- If Na-K pump stops, the leakage of ions will gradually diminish the RMP
- Na-K pump is inhibited by digitalis
Factors affecting RMP
- Relative conductance to Na and K determines the RMP
- EK as per Nernst equation
= -94mV (depends on [K] used)
- RMP is smaller (in absolute term) than EK because of a small inward leakage of Na.
When extracellular [K+] decreases below 5mM, this small leakage of Na has greater influence on the final RMP. RMP will still become more negative as extracelluar [K+] decreases, but will deviate more from the predicted value (EK)
[BL8:p12]