Membrane Potentials

Question 1

What is excitability

Answer 1

a property that allows cells to respond to changes in membrane potential, resulting in signalling and communication inside the cell and w/ other cells

Question 2

What is resting membrane potential (RMP)

Answer 2

the electric potential between the inside and outside of a cell that acts a baseline for a cell to be excited

Question 3

What cells rely on excitability and membrane potential to perform their function

Answer 3

muscle cells and neurons

Question 4

What is RMP primarily dependent on?

Answer 4

The permeability of plasma membrane to K+ (i.e. impacts to NAK ATPase will have minor impact on RMP but change to K+ will have HUGE impact

Membrane is somewhat permeable to K+ but not to Na+ or Ca2+

Question 5

What is are the ion concentrations inside/outside the cell for K+ and Na+ in skeletal muscle?

Answer 5

K+ = 5 / 150

Na+ = 150 / 5

Question 6

What ions have high concentrations inside the cell? What ions are high omn the outside?

Answer 6

Inside = K+, Ca2+ , proteins

Outside = Na+, Cl-, HCO3-

Question 7

What types of channels are involved in Membrane Potential and how do they function

Answer 7

Open Channels (leak) - nongated, for K+

Voltage Gated - large enough change in potential opens

Ligand-gated – neurotransmitters like acetylcholine bind to receptor and open

Signal-gated – intracellular molecules trigger opening

Question 8

When has a RMP been reached

Answer 8

When electrostatic forces balance inside and outside the cell and K+ stops moving (in large amounts)

Question 9

What is the RMP for: 1) skeletal/cardiac muscle 2) Smooth muscle and 3) Neurons

Answer 9

1) -80 to -90
2) -60
3) -60-70

Question 10

Why are ion channels important in membrane potential?

Answer 10

They form pores in the membrane to permit movement and they are highly specific to what moves through those pores

Question 11

What does NA/K ATPase do

Answer 11

Maintains proper concentraions of K and Na inside and outside the cell

3Na sent out in exchange for 2K inside

Question 12

Briefly explain Leak Channels

Answer 12

Remain open all the time for ion movement

K+ Leak channels present at a 100:1 ration to Na+ Leak Channels

Question 13

What forces act on ions to create membrane potential?

Answer 13

1) Diffusion forces (chemical gradients)
2) Electrostatic forces (charge based – as ions movement between membrane, pos and neg change develops)
3) Electrochemical Forces = Diffusion forces + Electrostatic forces
4) Equilibrium potential (Eion)

Question 14

What is Equilibrium Potential and how do you calculate

Answer 14

Point when electrical and chemical forces are equal and NO FURTHER MOVEMENT OCCURS

DIFFERENT FROM RMP

Question 15

What is the equilibrium potential for 1) K+ and 2) Na+

What can the Eq for K tell us about the cell?

Answer 15

K+ = -91mV

Na+ = +66mV

since K is close to RMP, it is a good measuring stick for estimating a cell’s RMP

Question 16

What is the Nernst Equation and what does it allow us to do?

Answer 16

Find Equilibrium Potential!

KNOW ONLY THE BOXED IN PORTION!!

Question 17

What is the Driving Force and it’s equation

Answer 17

The driving force takes into account the electrical and chemical forces to predict ion movement

(Vm) - (Eion)

Vm = RMP

Eion = equilibrium potential

Question 18

What is an efflux vs an influx with respect to Driving Force?

Answer 18

Efflux – ions moving out (POSITIVE charge in equation)

Influx — ions moving in (NEGATIVE charge in equation)

Question 19

What are the Eion estimates for K+ / Na+ / Ca2+ / Cl-

Answer 19

K = -91 mV

Na = +60 to +66

Ca = +123

Cl = -66

NOTE: in a driving force equation, those here with positive numbers will reeeeallllyy go inside the cell

Question 20

Since Na and Ca have such a high influx according to the Driving Force equationm, why don’t they rush into the cell?

Answer 20

1) membrane in impermeable to them
2) their ion channels are closed (until acted on to open by a stimulus)

Question 21

What does the Goldman Equation allow us to do?

Answer 21

Determine the RMP by taking into account different ion concentrations and permeability via Pearmeability Coefficients

Question 22

What is the permeability of Na, Ca and Cl relative to K+ when K+ has a permeability coefficient of 1.0? What does this mean?

Answer 22

Na = 0.02

Ca = 0.01

Cl = 0.5

Cl permeability, if altered, will have more of an impact on RMP than the others since it is closer to K’s permeability coefficient

Question 23

What are the primary contributers of RMP

Answer 23

1) K+ diffusion potential (-91mV)
2) Na diffusion (+5mV)
3) Na/K ATPase Pump (-4mV)

Question 24

What impact does being more positive or negative to the RMP have?

Answer 24

More positive = EASIER to DEPOLARIZE

More negative = HARDER to DEPOLARIZE

Question 25

How do we solve this question?

What is the “driving force” of chloride ions across the plasmas membrane of a neuron where [Cl-] interior is 10mM and [Cl-] external is 120mM?

Answer 25

1) concetration higher on outside = Log will be positive
2) Valence of Cl is -1 so it is NEGATIVE
3) We know ECl (eq. potential of Cl) is -66
4) We know neurons have RMP of 65 for use is driving force equation

SOLVE

+1.4

Question 26

Explain the following terms

1) Polarization
2) Depolarization
3) Hyperpolarization
4) Repolarization

Answer 26

Polarization - any deviation from 0mV

Depolarization - When membrane potential becomes more positive/less negative but it is NOT and action potential

Hyperpolarization - when membrane potential become MORE negative

Repolarization - When membrane potential returns to RMP

Question 27

What is the threshold for an action potential?

Answer 27

55mV

Question 28

What is an action potential and what causes it?

Answer 28

A large depolarization event that leads to further depolarization of membrane potential across a plasma membrane

Deviation from RMP and the length of an action potential varies between cell types

Question 29

What are some key properties of an action potential?

Answer 29

All or nothing event

Propogates/self-reinforcing downstream

Non-decremental (the strength and speed of an action potential does not change, only the frequency does)

Question 30

What is a graded action potential?

Answer 30

Changes in membrane potential that are small and localized and thus dissipate after some distance due to K+ channels being open

Strength of initial graded potential is directly correlated with the strength of the triggering event

Question 31

What are the phases of an action potential

Resting (phase 4)

Depolarization (Phase 0)

Repolarization (Phase 3)

Hyperpolarization (refractory period)

Question 32

What occurs during and action potential?

Answer 32

Increase in Na+ permeability

Voltage-gated Na channel open (BOTH the activation and inactivation gates are open)

Channel’s close after minimal delay

Question 33

Explain the two gastes of an Na voltage-gated channel and when they are open and closed

Answer 33

voltage-gated Na channel have an activation gate and inactivation gate

Resting = activation closed, inactivation open

Depolarization = activation gate open, inactivation gate open

Repolarization = activation and inactivation are both closed

IACTIVATION GATE WILL NOT REOPEN UNTIL NEAR RMP

Question 34

Positive Feedback Loop of Voltage-gated Na channels

Answer 34

A few localized events will open a few channels but neesd a larger event to trigger a membrane-wide event

Question 35

What occurs during repolarization?

Answer 35

1) voltage gated na channel closed
2) K+ still leaks out via K+ leak channels
3) voltage-gated K+ channels SLOWLY open so K+ can go out of cell to bring back to RMP

Question 36

What occurs during hyperpolarization?

Answer 36

1) Voltage-gated K channels stay open too long
2) more difficult to have another action potential – NEED LARGE STIMULUS

Question 37

Difference between Absolute Refractory and Relative Refractory

Answer 37

Absolute – Na channels are open or the inactivation gate is closed and NO AMOUNT OF STIMULUS will lead to another action potential (OCCURS DURING SPIKE)

Relative —- occurs between repolarization and during hyperpolarization —- inactivation gate is open and activation gate is closed but because RMP is still more negative due to K+, a LARGER STIMULUS is needed to cause action potential

Question 38

What is occuring to Na and K in this graph?

Answer 38

A - Na permeability increases rapidly

B - K permeability increase slowly

C - Na permeability necrease rapidly

D - K permeability decreases slowly

Question 39

What happens as we increase or decrease the concentration of K+ as shown in the Frog Graph?

Answer 39

Increase in K+ = RMP closer to threshold, making it easier of action potential

Decrease in K+ = RMP further from threshold, making it more difficult to get an action potential

Question 40

How does Hypokalemia Periodic Paralysis impact action potentials and what factors aggrevate it?

Answer 40

• Paralysis of shouklder and back muscles
• Dips in blood K+ as K+ goes into the cell lead to membrane being hyperpolarized (RMP further from threshold), making it more difficult to get an action potential

Moreover, when an action potential does occur, it has a high driving force for Na and K —–> FASTER CONDUCTION, SHORTER LENGTH OF POTENTIAL

high sugar diets, high salt diets, exercise

Question 41

How does Hyperkalemic Periodic Paralysis impact action potentials and what are factors that aid in stopping it?

Answer 41

• Excessive K+ in blood
• Prolonged action potentials and longer absolute refractory periods
• made better by high sugar diets, potassium-wasting diuretics and exercise

Threshold MUCH easier to hit!!