Lecture 3: Membrane Potential Part 2 Flashcards
What is Membrane Potential
- difference of voltage/electrical potential across the membrane (Vm)
- is the electro component of the electro chemical gradient that can be harnessed to do work for transport and signaling
Nernst Equation
allows the calculation of equilibrium potential (Eion) of any ONE ion
-Can predict Vm if only ONE ion is permeable
Goldman Equation
used to predict the membrane potential (Vm) only if more than one ion are permeable
3 factors contributing to membrane potential
- Concentration gradient for ions across membrane
- relative permeability of membrane to these ion
- charge of ions
Membrane potential (Vm) will be established if
- there is a gradient of concentration for ions across the membrane
- the ions have different permeability constants
the higher the permeability of the ion RELATIVE to the other ions,
the greater its influence on the membrane potential it has
The ion with the highest permeability
sets the polarity of the MP
sets most of its value
If the membrane is permeable to one ion only,
- this ion sets the MP
- Vm equals Eion and can be calculated using Nernst equation
Most real live animal cells
- membrane is permeable to more than one ion
- all these ions contribute to the MP (higher the permeability, the greater their contribution
- Vm has to be calculated using the goldman equation that takes into account all permeable ions
Resting Membrane Potential
no stimulation or information carried in cell
of potassium and sodium channels
most cells have many more K+ channels than Na+
Channels and Vm
membrane is permeable to an ion only if channels for the ion type are present and they are open
- leak channels have no gates, and therefore are always open
- gated channels gates are OPEN at rest
- Many potassium channels are LEAK channels, and fewer sodium leak channels; therefore membrane permeability to K+»Na+
Questions to ask yourself: is electro-neutrality respected?
- ions are localized right next to the membrane
- few ions have to cross the membrane to make large change in the voltage across membrane
- overall concentration of ion inside and outside do not change detectably
How come the charges on either side of membrane do not dissipate in the intra and extra cellular compartments
because the membrane acts like a capacitor and keeps the charges close to each other
Four contributors to resting potential:
- concentration of some ions inside cells are diff from fluids outside the cells (Na+out>Na+in;Kin>Kout)
- the fact that some permeable ions are way more permeable than others (most permeable ion contributes most to establishment of resting membrane potential)
- Na/K electrogenic pumps keep system in balance
- Plasma membrane acts like a capacitor, keeping the charges on the edge of the membrane and keeps them from dissipating
Equilibrium potential/Ion potential/Reversal potential (Eion) of an ion
the membrane potential value at which there there is no net flux of the ion across a membrane permeable to that ion
-ie. diffusion due to electrical gradient is counter-acted by diffusion due to the concentration gradient
Nernst Equation and comparing Equilibrium potential (Eion) with actual measured membrane potential (Vm) allows us to…
- know if the membrane will depolarize or hyperpolarise when an ion channel opens (Pion increases) or closes (Pion decreases)
- to quickly establish if there is a net passive flux of this ion or not, and which direction
- lets us know if the net flux of the ion has changed direction when Vm is changing (hence Eion’s other name, Reversal potential)
Depolarization
MP becomes less negative, may also reach 0mv or become positive (UP on a graph)
Hyperpolarization
MP becomes more negative (DOWN on a graph)
Repolarization
MP returns to resting potential
Vm=Eions IF:
One ion only can cross the membrane and the concentration of the ioin is the same on both sides OR there is a concentration gradient for this ion across the membrane
Types of questions: “if this ions channels are open, or closed, will the membrane depolarize or hyperpolarize?” to solve…
Compare Eion with Vm
If Pion increases…
the ion pulls Vm towards its Eion
If Pion decreases…
this ion will not pull Vm toward its Eion as much; Vm goes away from Eion
Type of question: what is the direction of the net flux of this ion?
quickly compare Eion with Vm
Brain dead vector method for determining direction of net flux
- draw an arrow representing movement of K+ driven b the concentration gradient (c)
- draw an arrow equal but opposite direction, which represents elec gradient (e) IF the MP equaled Ek+
- Compare Ek+=-76mv with actual Vm=-60mv and use this to draw the arrow representing actual (e)
- Compare the two dark arrows, the longer one is the net movement of K+
“be careful” vector method
- draw the flux of K+ driven by (c)
- draw the flux of K+ driven by (e)
- look at the polarity of the membrane and look at the charge of K+: positive K+ is attracted by the minus charges in the cell, therefore the direction of the electrical flu is inward
- Compare length of the (c) arrow and (e) arrow to conclude the net movement
