Membrane Potentials Flashcards
What is excitability
a property that allows cells to respond to changes in membrane potential, resulting in signalling and communication inside the cell and w/ other cells
What is resting membrane potential (RMP)
the electric potential between the inside and outside of a cell that acts a baseline for a cell to be excited
What cells rely on excitability and membrane potential to perform their function
muscle cells and neurons
What is RMP primarily dependent on?
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+
What is are the ion concentrations inside/outside the cell for K+ and Na+ in skeletal muscle?
K+ = 5 / 150
Na+ = 150 / 5
What ions have high concentrations inside the cell? What ions are high omn the outside?
Inside = K+, Ca2+ , proteins
Outside = Na+, Cl-, HCO3-
What types of channels are involved in Membrane Potential and how do they function
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
When has a RMP been reached
When electrostatic forces balance inside and outside the cell and K+ stops moving (in large amounts)
What is the RMP for: 1) skeletal/cardiac muscle 2) Smooth muscle and 3) Neurons
1) -80 to -90
2) -60
3) -60-70
Why are ion channels important in membrane potential?
They form pores in the membrane to permit movement and they are highly specific to what moves through those pores
What does NA/K ATPase do
Maintains proper concentraions of K and Na inside and outside the cell
3Na sent out in exchange for 2K inside
Briefly explain Leak Channels
Remain open all the time for ion movement
K+ Leak channels present at a 100:1 ration to Na+ Leak Channels
What forces act on ions to create membrane potential?
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)
What is Equilibrium Potential and how do you calculate
Point when electrical and chemical forces are equal and NO FURTHER MOVEMENT OCCURS
DIFFERENT FROM RMP
What is the equilibrium potential for 1) K+ and 2) Na+
What can the Eq for K tell us about the cell?
K+ = -91mV
Na+ = +66mV
since K is close to RMP, it is a good measuring stick for estimating a cell’s RMP
What is the Nernst Equation and what does it allow us to do?
Find Equilibrium Potential!
KNOW ONLY THE BOXED IN PORTION!!
What is the Driving Force and it’s equation
The driving force takes into account the electrical and chemical forces to predict ion movement
(Vm) - (Eion)
Vm = RMP
Eion = equilibrium potential
What is an efflux vs an influx with respect to Driving Force?
Efflux – ions moving out (POSITIVE charge in equation)
Influx — ions moving in (NEGATIVE charge in equation)
What are the Eion estimates for K+ / Na+ / Ca2+ / Cl-
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
Since Na and Ca have such a high influx according to the Driving Force equationm, why don’t they rush into the cell?
1) membrane in impermeable to them
2) their ion channels are closed (until acted on to open by a stimulus)
What does the Goldman Equation allow us to do?
Determine the RMP by taking into account different ion concentrations and permeability via Pearmeability Coefficients
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?
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
What are the primary contributers of RMP
1) K+ diffusion potential (-91mV)
2) Na diffusion (+5mV)
3) Na/K ATPase Pump (-4mV)
What impact does being more positive or negative to the RMP have?
More positive = EASIER to DEPOLARIZE
More negative = HARDER to DEPOLARIZE
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?
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
Explain the following terms
1) Polarization
2) Depolarization
3) Hyperpolarization
4) Repolarization
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
What is the threshold for an action potential?
55mV
What is an action potential and what causes it?
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
What are some key properties of an action potential?
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)
What is a graded action potential?
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
What are the phases of an action potential
Resting (phase 4)
Depolarization (Phase 0)
Repolarization (Phase 3)
Hyperpolarization (refractory period)
What occurs during and action potential?
Increase in Na+ permeability
Voltage-gated Na channel open (BOTH the activation and inactivation gates are open)
Channel’s close after minimal delay
Explain the two gastes of an Na voltage-gated channel and when they are open and closed
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
Positive Feedback Loop of Voltage-gated Na channels
A few localized events will open a few channels but neesd a larger event to trigger a membrane-wide event
What occurs during repolarization?
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
What occurs during hyperpolarization?
1) Voltage-gated K channels stay open too long
2) more difficult to have another action potential – NEED LARGE STIMULUS
Difference between Absolute Refractory and Relative Refractory
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
What is occuring to Na and K in this graph?
A - Na permeability increases rapidly
B - K permeability increase slowly
C - Na permeability necrease rapidly
D - K permeability decreases slowly
What happens as we increase or decrease the concentration of K+ as shown in the Frog Graph?
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
How does Hypokalemia Periodic Paralysis impact action potentials and what factors aggrevate it?
- 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
How does Hyperkalemic Periodic Paralysis impact action potentials and what are factors that aid in stopping it?
- 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!!
