9/12 Membrane Potentials Flashcards
what’s the purpose of a cell having the Excitability
allows cells to establish a means of communicating to their own interior or to other cells
Resting membrane potential establish what baseline?
a starting point for a cell to be potentially excited
Resting membrane potential will deviate from rest based on changes in charge across the membrane, list two possible reasons?
Different ions
Direction of electrochemical gradient
membrane potential
resting membrane potential is due to the permeability of the plasma membrane of which ion?
K+
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?
K+ leakage channel
Na+/k+ ATPase
Na+/K+ ATPase exchange how many Na+ ion and K+ ion through the channel? what is the purpose of this?
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
K+ leak channel
Leak channels are open _____ ; permit mostly unregulated passage of ions.
K+ leak channels are present at ____ ratio to Na+ leak channels
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.
what is used to measure the membrane negativity?
silver-silver chloride electrode
Diffusion Forces (Chemical Gradients) is based on what?
Electrostatic Forces (Electrical Gradients)is based on what?
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)
Electrochemical Forces = ____+_____
Equilibrium potential (Eion) is what?
Diffusion Forces + Electrical Forces
membrane potential when electrical and chemical forces are equal, no further movement occurs (≠ resting membrane potential)
- 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
- Potassium Ions alone:
K+ are freely permeable with unlimited movement, movement occurs until Electrochemical Gradient is Equilibrated = Equilibrium Potential = EK+ = -91 mv
Nernst equation for calculating the Equilibrium potential
[Resting membrane potential, Vm] – [EION] = “Driving force”
when the driving force is positive, what does it mean?
efflux
Nernst equation
K+ ion Muscles
Start with resting membrane potential in muscle -120 mv
[Vm] – [EION] = “Driving force” -120 mv – (-91 mv) = -29 mv Represents net ____?
influx
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_____?
efflux
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?
The membrane in mostly impermeable to Na+ at rest
Goldman Equation - Takes into account different ion concentrations and permeability
which ion has the highest and lowest membrane permeability?
highest = K+
lowest = Na+ and Ca2+
what is the main contributor for the membrane resting potential?
K+ diffusion potential
why is the Contribution of Na+ Diffusion to the resting potential minimal?
low Na+ membrane permeability
Contribution of Na+ - K+ ATP Pump to the membrane resting potential is ?
Minimal direct contribution
4 mv negative contribution
Indirectly contribution to maintain ion concentration gradients
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
positive
negative
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)
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
Three key properties of action potential
All-or-none
Propagating or self-reinforcing
Non-decremental
what is graded potential? what is the main characteristic?
Changes in membrane potential that are small and local
Excitatory or inhibitory
Graded potential dissipate with distance because K+ leak channels are always open
for the graded potential, Strength of initial graded potential correlates with _____ of triggering event
strength
Stronger triggering event, more channels will open to change polarity of membrane
phases of action potential
what are the two ions and three channels play critical roles in the action potential?
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)
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
Depolarization
which ion channel dictates this action?
increase in permeability of membrane to Na+ Voltage-gated Na+-channels open rapidly
After minimal delay, these channels close automatically
for the Na voltage gated channel, what are the two phases?
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
_____ Feedback Loop of voltage-gated Na+-channels
Loop is broken at ____ mv because Na+- channels close
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
repolarization phase
which channel is closed? and is open?
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
Voltage-gated K+-channels
Not to be confused with K+ leak channels
Ability to close
Selectivity – via selectivity filter
commons in the ion channels
hyperpolarization is caused by what? this leads to what condition?
Voltage-gated K+ channels stay open a little too long
refractory period
summary of ion channels during the action potential
what are the two types of refractory periods?
what about the gate status for each period?
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.
what happens when the threshold is reached?
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
Depolarization of the membrane, what happens?
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
repolarization of the membrane, what happens?
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
repolarization of the membrane, what happens?
Membrane potential drops below resting
Voltage-gated K+-channels are slowly closing
Permeability of K+ still dominates, but is decreasing towards K+ equilibrium
