1.5. The development of the resting membrane potential. The development and properties of the electrotonic potential.

Question 1

I. Resting membrane potential (Em)
1. What is the definition of Resting Membrane potential (Em)?

Answer 1

potential difference between the intra- and extracellular space when a cell is at rest
-> it is between action potentials, not performing any special functions

Question 2

I. Resting membrane potential (Em)
2. What are the values of Em for Skeletal muscle, Neuron and RBC? (different cell types)

Answer 2

1/ Skeletal muscle: - 90 mV
2/ Neuron: -70 mV
3/ RBC: - 10 mV

Question 3

I. Resting membrane potential (Em)
3. What can cause membrane potential deviate from the resting value?

Answer 3

Deviation of the Em can be caused by
- Electrotonic potentials (alterations)
- Action potentials

Question 4

I. Resting membrane potential (Em)
4. Origin of membrane potential

Answer 4

Origin of membrane potential
- Diffusion potential (appx. 95%)
- Pump potential (3%)
- Donnan potential (2%)

Question 5

II. Diffusion potential
1/ Definition of diffusion potential

Answer 5

the diffusion potential is any potential (voltage) difference generated across a membrane when an ion diffuses down its concentration gradient

Question 6

II. Diffusion potential
2/ Characteristics of diffusion potential

Answer 6

1/ Its magnitude depends on the size of the concentration gradient
2/ Its sign depends on the charge of the diffusing ion

Question 7

II. Diffusion potential
3/ How is diffusion potential made?

Answer 7

1/ Diffusion of charged particles (ions) in water
2/ There is different mobility of diffusing ions results in the generation of diffusion potential
3/ The generated potential gradient retards the diffusion of the positive ions and accelerates the diffusion of negative ions

Question 8

II. Diffusion potential
4/ An example of diffusion potential

Answer 8

1/ Diffusion potential generated by the diffusion of an HCl (H+ and Cl-) drop in water
2/ There will be a semipermeable membrane (K+ can pass but Cl- and H20 cannot)
3/ K+ transport is driven by the concentration gradient
4/ The generated potential prevents further transport which leads to equilibrium
5/ Therefore, Em stabilizes

Question 9

III. Equilibrium potential
1/ Definition of Equilibrium potential

Answer 9

equilibrium potential is the membrane potential where the net flow of ions through any open channel is 0

Question 10

III. Equilibrium potential
2/ Characteristics of Equilibrium potential

Answer 10

1/ Compensates for the chemical gradient of the given ion
2/ Uneven distribution of only a very small quantity/number of ions (charges) is sufficient for the generation of the potential difference (the ion concentrations on the 2 sides of the membrane do not change significantly)
3/ At the equilibrium potential, equal number of the given ions move to 1 direction (driven by the concentration gradient) as to the other direction (driven by electrical gradient) -> lead to dynamic steady state

Question 11

III. Equilibrium potential
3A. When do we need to use Nernst equation?

Answer 11

If a membrane is permeable to a certain ion, and the ion’s concentration on either sides of the membrane are known
=> its equilibrium potential can be calculated during the Nernst equation

Question 12

III. Equilibrium potential
3B. What is the formula of Nernst equation?

Answer 12

E : membrane potential (volts)
R: universal gas constant ( ~ 8.31 J/mol/K) T: absolute temperature (oK)
z : charge number (e.g. for K+ is +1)
F : Faraday-constant (~ 96500 C/mol)

Question 13

III. Equilibrium potential
3C. Validation of Nernst equation?

Answer 13

1/ Permeability only affects the time in which the dynamic equilibrium will be established
2/ Potential difference will not affect the original concentrations

Question 14

III. Equilibrium potential
4A. What is the Equilibrium Potential of K+, Na+ and Cl- in case of Skeletal Muscle?

Answer 14

Question 15

III. Equilibrium potential
4B. What is the Equilibrium Potential of K+, Na+ and Cl- in case of Neuron?

Answer 15

Question 16

III. Equilibrium potential
4C. What is the Relative Permeability of K+, Na+ and Cl-?

Answer 16

Question 17

IV. Development of resting membrane potential
1. Which factors determine the membrane potential?

Answer 17

1/ The different ion concentrations
2/ The permeability of the membrane to different ions

Question 18

IV. Development of resting membrane potential
2. What is the general mechanism of development of resting membrane potential?

Answer 18

1/ Resting membrane potential is created as a result of movement of ions across the membrane, down their concentration gradients (established by primary & secondary active transport mechanisms), eager to reach their equilibrium potentials

2/ Each ion ‘’wants’’ to drive the membrane potential toward its own equilibrium potential. Therefore:
-> The movement of ions to which the membrane is most permeable to, will have the greatest effect on the resting membrane potential (in this case K+-ions)

Question 19

IV. Development of resting membrane potential
3. How do ions participate in the development of resting membrane potential?

Answer 19

1/ The membrane is more permeable to K+ than it is to Na+
=> K+-channels are open and Na+-channels are closed

2/ Equilibrium potential for K+ is -94mV, and since some of them leak out, the membrane potential will also be -94mV

3/ But since some Na+-channels are still open, Na+-ions (+65mV) come into the neuron, which increases the resting membrane potential to -87mV

Question 20

V. Goldman-Hodkin-Katz equation
1. What is the purpose of Goldman-Hodkin-Katz equation?

Answer 20

For the calculation of the membrane potential

Question 21

V. Goldman-Hodkin-Katz equation
2. What is the formula of Goldman-Hodkin-Katz equation?

Answer 21

For the calculation of the membrane potential
-> Em is between EK and ENa

Question 22

V. Goldman-Hodkin-Katz equation
2. Which ions are the main contributors to Goldman-Hodkin-Katz equation?

Answer 22

In many cells, K+, Na+ and Cl- are the main contributors to the membrane potential

Question 23

V. Goldman-Hodkin-Katz equation
3. What is the formula of Goldman-Hodkin-Katz equation?

Answer 23

For the calculation of the membrane potential
-> Em is between EK and ENa

Question 24

V. Goldman-Hodkin-Katz equation
4. What is the interpretation of Goldman-Hodkin-Katz equation?

Answer 24

We use the concentrations of ions and their membrane permeability to calculate the membrane potential
- [Cl-]ic is on the bottom, because its charge is negative, and so its driving force direction is opposite to cations-> Cl- will try to leave negative areas and join more positive ones

Question 25

V. Goldman-Hodkin-Katz equation
5A. What happen if we change the permeability of ions?

Answer 25

it will have effect on Em -> it will go in the direction of the equilibrium potential of the same ion

Question 26

V. Goldman-Hodkin-Katz equation
5B. If we change the permeability of ions, what are the consequences to the present K+, Na+ and Cl-?

Answer 26

1/ [K+]EC ↑ = depolarization

2/ [K+]EC ↓ = hyperpolarization

3/ pK+ ↑ = hyperpolarization

4/ pNa+ ↑ = depolarization (only change when [Na+]IC)

5/ In case of increase the permeability of Cl- ions, nothing will happen. => Because Ecl (-88mV) is almost the same as Em (-87 mV)

Question 27

VI. Na+/ K+ - ATPase
1. What is the role of Na+/ K+ - ATPase?

Answer 27

Maintenance of the Em and prevent the concentration equalization of an alive cell

Question 28

VI. Na+/ K+ - ATPase
2. What is the mechanism of Na+/ K+ - ATPase?

Answer 28

1/ Actively (ATP hydrolysis) transports Na+ and K+ against their concentration gradient -> 3 Na+ are pumped out of the cell, and 2 K+ are brought into the cell
2/ Since there are 3 Na+ exported for every 2 K+ are brought into the cell, the pump makes the membrane potential slightly negative than it would be
3/ The pump maintains the steady Na+ and K+ gradient, which contributes to the movement of the respective ions through leak channels