Physiology Topics
The plasma membrane creates a difference in the distribution of positively and negatively charged ions between one side of the membrane and the other called the electrical gradient or membrane potential. The concentrations of Na+ and Cl- are higher outside the cell, while the concentration of K+ and organic anions are higher inside the cell. These differences are due to the relatively higher permeability of the membrane to K+ but not to Na+ or Cl- and to the Na+/K+ ATPases that pump Na+ out of the cell and K+ in. Most cells have more K+ channels open than for any other ion, therefore the K+ concentration outside and inside the cell are at equilibrium.
The membrane (average?) potential that just balances K+ concentration at equilibrium is -90 mV (?). The electrical effect of K+ outflow could be canceled if Na+ could flow inwards as easily but this is not possible due to the lower number of open channels and the activity of the Na+/K+ ATPases. Adding to the negative membrane potential are the organic ions, mostly proteins and phosphate which cannot freely leave the cytosol The inward leakage of Cl- down its concentration gradient could only add to the already negative membrane potential.
The resulting buildup of anions in the cytosol along the membrane and the similar buildup of cations in the extracellular fluid along the membrane result in an overall membrane potential of about -70 mV. This can be calculated using the Nernst equation:.
Eion = RT log [ion]out
zF [ion]in
Where Eion is the equilibrium potential for an ion across the membrane, R is the gas constant, T is the absolute temperature, z the ion valence and F the Faraday constant (at 20ºC, RT/zF = 58). Therefore the equilibrium potential for each ion can be calculated from their average concentrations in and out of the cell (A- represents all other anions):
EK+ = 58 mV • log
4 mM = -90 mV
1
140 mM
ENa+ = 58 mV • log
150 mM = 64 mV
1
12 mM
ECl- = 58 mV • log
120 mM = -86 mV
-1
4 mM
EA- = 58 mV • log
34 mM = 37 mV
-1
148 mM
If we account for the differences in permeability using a permeability constant P for each ion and assuming that PK+ ~ 100 PNa+ ~ 100 PCl-
Ecell = RT • log 100
[K+]out + [Na+]out
+ [Cl-]out
zF
100 [K+]in
+ [Na+]in
+ [Cl-]in
Ecell = 58 mV • log 100
(4) + 150 + 120 = -77 mV
1
100 (140)
+ 12
+ 4
(I know, it is supposed to be -70 mV but this is what I get with the sample
values from my class, close enough for me...)
Thus the average resting membrane potential is -70 mV, very close to the K+ potential (-90 mV). Membrane potentials are measured in millivolts (mV) according to the Ohm's law:
V = i R
where V is the difference in potential between the inside and the outside of the cell (millivolts), i is the movement of ions or current in amperes and R is the resistance.
Continue to "Action Potential" or take a quiz: [Q1] [Q2].
Also check out this cool sites about electrophysiology I found on the web:
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