9/12 Membrane Potentials

Question 1

what’s the purpose of a cell having the Excitability

Answer 1

allows cells to establish a means of communicating to their own interior or to other cells

Question 2

Resting membrane potential establish what baseline?

Answer 2

a starting point for a cell to be potentially excited

Question 3

Resting membrane potential will deviate from rest based on changes in charge across the membrane, list two possible reasons?

Answer 3

Different ions

Direction of electrochemical gradient

Question 4

membrane potential

Question 5

resting membrane potential is due to the permeability of the plasma membrane of which ion?

Answer 5

K+

Question 6

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

Movement across membrane governed by various channels/pumps. Which two methods that allow K+ to pass through the membrane?

Answer 6

K+ leakage channel

Na+/k+ ATPase

Question 7

Na+/K+ ATPase exchange how many Na+ ion and K+ ion through the channel? what is the purpose of this?

Answer 7

Exchanges 3 Na+ ions to outside of cell in exchange for 2 K + ions to the inside of the cell * Requires ATP (=Energy)

Maintains concentrations in proper place

Question 8

K+ leak channel

Leak channels are open _____ ; permit mostly unregulated passage of ions.

K+ leak channels are present at ____ ratio to Na+ leak channels

Answer 8

all the time

100:1

Overall, passively K+ is more likely to leave cell than Na+ to enter. Main component in permeability of K+ to the plasma membrane.

Question 9

what is used to measure the membrane negativity?

Answer 9

silver-silver chloride electrode

Question 10

Diffusion Forces (Chemical Gradients) is based on what?

Electrostatic Forces (Electrical Gradients)is based on what?

Answer 10

Concentration gradient

Charge based, opposites attract (During movement of ions across a plasma membrane, charge will develop on either side; this charge opposes further diffusion)

Question 11

Electrochemical Forces = ____+_____

Equilibrium potential (Eion) is what?

Answer 11

Diffusion Forces + Electrical Forces

membrane potential when electrical and chemical forces are equal, no further movement occurs (≠ resting membrane potential)

Question 12

1. Movement of Sodium Ions alone:

 Na+ are freely permeable with unlimited movement, movement occurs until Electrochemical Gradient is Equilibrated = Equilibrium Potential = ENa+ = +66 mv Movement of

2. Potassium Ions alone:

 K+ are freely permeable with unlimited movement, movement occurs until Electrochemical Gradient is Equilibrated = Equilibrium Potential = EK+ = -91 mv

Question 13

Nernst equation for calculating the Equilibrium potential

Question 14

[Resting membrane potential, Vm] – [EION] = “Driving force”

when the driving force is positive, what does it mean?

Answer 14

efflux

Question 15

Question 16

Nernst equation

Question 17

K+ ion Muscles

Start with resting membrane potential in muscle -120 mv

 [Vm] – [EION] = “Driving force”  -120 mv – (-91 mv) = -29 mv  Represents net ____?

Answer 17

influx

Question 18

for K+

Now we are neurons  Resting membrane potential is -65 mv

 EK+ = {(61.5 mv) / (+1)} x {log ([5 mM]/[150 mM])} = -91 mv

 [Vm] – [EION] = “Driving force”

 -65 mv – (-91 mv) = +26 mv

 Represents net_____?

Answer 18

efflux

Question 19

For Na+

Start with resting membrane potential in muscle -85 mv  ENa+ = {(61.5 mv) / (+1)} x {log ([150 mM]/[15 mM])} = +61.5 mv  [Vm] – [EION] = “Driving force”  -85 mv – (61.5 mv) = -146.5 mv

 Represents influx

 Why no influx?

Answer 19

The membrane in mostly impermeable to Na+ at rest

Question 20

Goldman Equation - Takes into account different ion concentrations and permeability

Question 21

which ion has the highest and lowest membrane permeability?

Answer 21

highest = K+

lowest = Na+ and Ca2+

Question 22

what is the main contributor for the membrane resting potential?

Answer 22

K+ diffusion potential

Question 23

why is the Contribution of Na+ Diffusion to the resting potential minimal?

Answer 23

low Na+ membrane permeability

Question 24

Contribution of Na+ - K+ ATP Pump to the membrane resting potential is ?

Answer 24

Minimal direct contribution

4 mv negative contribution

Indirectly contribution to maintain ion concentration gradients

Question 25

More _____ RMP makes it easier to depolarize cell (closer to threshold) More _____ RMP makes it more difficult to depolarize cell (cell is hyperpolarized, and further away from the threshold

Answer 25

positive negative

Question 26

Resting membrane potential –90 mv in skeletal muscle Polarization – deviation from 0 mv Depolarization – when membrane potential becomes less negative (A) Hyperpolarization – when membrane potential becomes more negative (B) Repolarization – when membrane potential is returning towards resting membrane potential (C)

Question 27

action potential  Large depolarization that elicits a further depolarization and complete reversal of membrane potential across plasma membrane  Deviation from resting membrane potential varies between cell types  Length of action potential varies between cell types

Answer 27

Question 28

Three key properties of action potential

Answer 28

All-or-none Propagating or self-reinforcing Non-decremental

Question 29

what is graded potential? what is the main characteristic?

Answer 29

Changes in membrane potential that are **small and local**  Excitatory or inhibitory Graded potential dissipate with distance because K+ leak channels are always open

Question 30

for the graded potential, Strength of initial graded potential correlates with _____ of triggering event

Answer 30

strength Stronger triggering event, more channels will open to change polarity of membrane

Question 31

phases of action potential

Question 32

what are the two ions and three channels play critical roles in the action potential?

Answer 32

K+ Na+ Voltage-gated Na + channels Voltage-gated K + channels K+ leak channels (lesser degree) Note: Ca2+ ions are important for many cell types too (e.g., cardiac pacemaker cells)

Question 33

key channels in the action potential Mostly passive (do not require Energy) Open Channels (non-gated) = ions move down concentration gradient  “Leak” (non-gated) channels Gated Channels – Restrict ion movement  Voltage-gated  Ligand-gated  Signal-gated  Mechanically-gated

Answer 33

Question 34

Depolarization which ion channel dictates this action?

Answer 34

increase in permeability of membrane to Na+ Voltage-gated Na+-channels open rapidly After minimal delay, these channels close automatically

Question 35

for the Na voltage gated channel, what are the two phases?

Answer 35

activation and inactivation Phases of opening:  Resting – activation gate is closed and inactivation gate is open  Activation – activation gate opens during initial depolarization  Inactivation – inactivate gate closes rapidly after activation phase  Cannot be moved until membrane potential returns to near resting

Question 36

\_\_\_\_\_ Feedback Loop of voltage-gated Na+-channels Loop is broken at ____ mv because Na+- channels close

Answer 36

Positive +30  Local triggering event opens some channels  If event is large enough, there will be a greater and wide area of increased membrane potential  More Na+-channels open  More spread

Question 37

repolarization phase which channel is closed? and is open?

Answer 37

Voltage gated Na+-channels are closed Voltage-gated K+-channels **slowly open** further increasing the membrane permeability to K+ Potassium leaks out still via K + “leak” channels

Question 38

Voltage-gated K+-channels Not to be confused with K+ leak channels Ability to close Selectivity – via **selectivity filter**

Question 39

commons in the ion channels

Question 40

hyperpolarization is caused by what? this leads to what condition?

Answer 40

Voltage-gated K+ channels stay open a little too long refractory period

Question 41

summary of ion channels during the action potential

Question 42

what are the two types of refractory periods? what about the gate status for each period?

Answer 42

**1. Absolute refractory period** – Na+-channels are either open or the **inactivation gate is closed** and cannot reopen.  Another action potential cannot be generated. **2.** **Relative refractory period** – **inactivation gate is now open and activation gate is closed**.  K+ permeability is still fairly high and overshoots potassium leaving the cell so the membrane becomes slightly more negative than resting membrane potential.  In addition, not all voltage-gated Na+-channels are in the same state at the same time yet, varying the potential response.  Action potential may be initiated but requires a **stronger stimulus.**

Question 43

Answer 43

Question 44

what happens when the threshold is reached?

Answer 44

Increase in membrane potential locally  Rapid opening of activation gate of local Na+-channels  Increase in membrane Na+ permeability locally  Delayed closure of Na+-channel inactivation gate locally  Change in membrane potential initiates opening of more Na+-channels

Question 45

Depolarization of the membrane, what happens?

Answer 45

Membrane potential rapidly increases  Rapid opening of activation gate of Na+- channels (positive feedback loop)  Na+ permeability dominates membrane Slight delay in closure of Na+-channel inactivation gate Slow opening of local voltage-gated K+- channels

Question 46

repolarization of the membrane, what happens?

Answer 46

Peak action potential is reached and membrane potential begins to return to resting Na+-channel inactivation gates are closed Voltage-gated K+-channel are open Membrane permeability to Na+ decreases and permeability to K+ continues to rise

Question 47

repolarization of the membrane, what happens?

Answer 47

Membrane potential drops below resting  Voltage-gated K+-channels are slowly closing  Permeability of K+ still dominates, but is decreasing towards K+ equilibrium

Question 48

Answer 48