Membrane Resting Potentials
B. L. Krilowicz Biology 436 Spring 2009
I. The Neuron Membrane is Permeable to K+, Na+ and Cl- at rest
A. Evidence from radioactive tracer studies tells us that all major ionic species can pass the resting neuronal membrane, i.e. there are "leak channels" for these ions
Outside K+
Na+
Cl-
Inside
K+
Na+
Cl-
However, not all ionic species traverse the membrane at the same rate
1) Not all channels allow the same number of ions to pass per unit time (i.e. carry the same current), maybe due to structural differences in the channels? ex1. The more narrow the channel the harder it is for an ion to traverse the channel (bumps into the wall and decreases the kinetic energy) ex2. There are different numbers of the different channels in the membrane (i.e. there are more K+ leak channels
Permeability
A measure of how easy it is for an ion to cross the membrane; takes into account ease to traverse a channel/ # of channels/ etc. At rest, for giant squid axon Pk+ : PNa+ : PCl- = 1 : 0.04 : 0.45 Ratios differ for different species and different cell types within a species
What happens to the Vm when leak + channels for Na and Cl are introduced into the membrane?
Exercise #1 = Draw equilibrium in a resting neuron. Start with a liver cell in equilibrium (see the following slide) and add Na+ leak channels to the membrane. (Hint #1: See Time0+1 when Na+ is just starting to enter the cell - the second image to follow this slide). (Hint #2 : at equilibrium Vm = -60 mV and [K+]I>[K+]O and [Na+]I<[Na+]O
Timeeq for non-excitable cell; Vm = -75 mV = Nernst equilibrium potential for K+ +
= Membrane
is impermeable to ion "chemical force" magnitude and direction of net diffusion Hypothetical membrane freely permeable to
+
K+
K
"electrical force" = - "chemical force"; no net movement
+
------Na+
+++ +++ +++
Na+ Cl-
Cl-
of K across the membrane
Organic
inside
-
Organic-
only K outside
Time0+1 when Na+ leak channels are introduced into the membrane
= Membrane
is impermeable to ion "chemical force" magnitude and direction of net diffusion Hypothetical membrane permeable to
+ +
K
"electrical force" = - "chemical force"; no net movement
+
+
------+++ +++ +++
K+
Na+
Na+ Cl-
Cl-
of K across the membrane
Organic
inside
-
Organic-
K and Na
outside
Timeeq
Rate of exit or entry
= Membrane
K+ is impermeable to ion "chemical force" magnitude and direction of net diffusion Hypothetical permeable membrane to
+ +
K
"electrical force" =
resting
+
------+++ +++ +++
V
Na+
++ +
Na+ Cl-
Cl-
Organic
inside
-
Organic-
K and Na
outside
Explanation
1) Na+ enters the cell and depolarizes the membrane. The rate of entry is low and is set by the # and structure of the Na+ leak channels and the Na+ concentration gradient. 2) The Vm no longer balances the K+ concentration gradient. 3) A small amount of K+ is now able to leave the cell. 4) At equilibrium, the rate of entry of Na+ is equal to the rate of exit of K+ and the Vm is constant at a new value of -60 mV; i.e. more + than for "liver cell" equilibrium. At equilibrium there is no net movement of charge.
Explanation
5) The Vresting is much closer to EK+ than ENa+ because the permeability of the membrane to K+ is much greater than the permeability of the membrane to Na+.
Use of Sodium-Potassium ATP Coupled Pumps to Counter the Resting Ionic Fluxes
2 x K+ +++
++ = ATP
ADP + Pi
3 x Na+
At rest, the low level influx of Na+ is balanced by the low level efflux of K+. This leak must be countered over the long term by the ATPcoupled pumps that maintain the ionic concentration gradients across the membrane.
The ATP-coupled Pump is Electrogenic
1) At rest, the passive ion fluxes (due to concentration gradients and Vm) are countered by the active ionic fluxes (due to the ATP-coupled pumps). 2) Consequently, Vm is constant at -60 mV and the K+ and Na+ concentration gradients are also constant through ...