Lecture 3: Membrane Potential Part 2

Question 1

What is Membrane Potential

Answer 1

• 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

Question 2

Nernst Equation

Answer 2

allows the calculation of equilibrium potential (Eion) of any ONE ion
-Can predict Vm if only ONE ion is permeable

Question 3

Goldman Equation

Answer 3

used to predict the membrane potential (Vm) only if more than one ion are permeable

Question 4

3 factors contributing to membrane potential

Answer 4

1. Concentration gradient for ions across membrane
2. relative permeability of membrane to these ion
3. charge of ions

Question 5

Membrane potential (Vm) will be established if

Answer 5

1. there is a gradient of concentration for ions across the membrane
2. the ions have different permeability constants

Question 6

the higher the permeability of the ion RELATIVE to the other ions,

Answer 6

the greater its influence on the membrane potential it has

Question 7

The ion with the highest permeability

Answer 7

sets the polarity of the MP

sets most of its value

Question 8

If the membrane is permeable to one ion only,

Answer 8

• this ion sets the MP

- Vm equals Eion and can be calculated using Nernst equation

Question 9

Most real live animal cells

Answer 9

• 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

Question 10

Resting Membrane Potential

Answer 10

no stimulation or information carried in cell

Question 11

of potassium and sodium channels

Answer 11

most cells have many more K+ channels than Na+

Question 12

Channels and Vm

Answer 12

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+

Question 13

Questions to ask yourself: is electro-neutrality respected?

Answer 13

• 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

Question 14

How come the charges on either side of membrane do not dissipate in the intra and extra cellular compartments

Answer 14

because the membrane acts like a capacitor and keeps the charges close to each other

Question 15

Four contributors to resting potential:

Answer 15

1. concentration of some ions inside cells are diff from fluids outside the cells (Na+out>Na+in;Kin>Kout)
2. the fact that some permeable ions are way more permeable than others (most permeable ion contributes most to establishment of resting membrane potential)
3. Na/K electrogenic pumps keep system in balance
4. Plasma membrane acts like a capacitor, keeping the charges on the edge of the membrane and keeps them from dissipating

Question 16

Equilibrium potential/Ion potential/Reversal potential (Eion) of an ion

Answer 16

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

Question 17

Nernst Equation and comparing Equilibrium potential (Eion) with actual measured membrane potential (Vm) allows us to…

Answer 17

• 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)

Question 18

Depolarization

Answer 18

MP becomes less negative, may also reach 0mv or become positive (UP on a graph)

Question 19

Hyperpolarization

Answer 19

MP becomes more negative (DOWN on a graph)

Question 20

Repolarization

Answer 20

MP returns to resting potential

Question 21

Vm=Eions IF:

Answer 21

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

Question 22

Types of questions: “if this ions channels are open, or closed, will the membrane depolarize or hyperpolarize?” to solve…

Answer 22

Compare Eion with Vm

Question 23

If Pion increases…

Answer 23

the ion pulls Vm towards its Eion

Question 24

If Pion decreases…

Answer 24

this ion will not pull Vm toward its Eion as much; Vm goes away from Eion

Question 25

Type of question: what is the direction of the net flux of this ion?

Answer 25

quickly compare Eion with Vm

Question 26

Brain dead vector method for determining direction of net flux

Answer 26

1. draw an arrow representing movement of K+ driven b the concentration gradient (c)
2. draw an arrow equal but opposite direction, which represents elec gradient (e) IF the MP equaled Ek+
3. Compare Ek+=-76mv with actual Vm=-60mv and use this to draw the arrow representing actual (e)
4. Compare the two dark arrows, the longer one is the net movement of K+

Question 27

“be careful” vector method

Answer 27

1. draw the flux of K+ driven by (c)
2. draw the flux of K+ driven by (e)
3. 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
4. Compare length of the (c) arrow and (e) arrow to conclude the net movement