Membrane Potential Flashcards
relatively which is stronger, the electric or osmotic force?
electric! (~10^18 more powerful)
What are the 2 forces acting on an ion moving across a membrane?
- concentration difference
- electrical potential difference (membrane potential)
=>electrochemical gradient
What produces a membrane potential?
an imbalance of cations and anions within the cell
What is the typical charge of a bulk solution?
Bulk solutions can safely be considered neutral
What is the Nernst equation (original and at body conditions)
BODY:
E(mV) = (60/z)log(Cout/Cin)
ORIGINAL:
E = (RT/zF)ln(Cout/Cin)
What is equilibrium potential?
electrical potential difference across a membrane that must exist if the ion is to be at equilibrium at a given concentration
What is the difference between equilibrium potential and recorded membrane potential?
Equilibrium potential is a theoretical value (electromotive force) for a single ion
Membrane potential is measure with a micro electrode and is the voltage difference measured between the tip of a micro-electrode place inside a cell and a micro-electrode outside the cell
How many equilibrium potentials are there?
One for each ion species # equilibrium potentials = # ion species
What does it mean if membrane potential does not equal equilibrium potential for an ion?
one of two things:
- the membrane is impermeable to that ion
- the ion must have a pump
What is the principle of electrical neutrality?
Bulk solutions inside and out must be electrically neutral
Total cations in cell = total anions incell
same for outside of cell
How does can membrane potential exist without contradicting the principle of electrical neutrality?
The excess anions or cations in a cell is very small compared to the total number of ions:
a cell with a resting membrane potential of -80mV:
has 100,000 cations
100,001 anions
What is the Donnan rule?
[K+out][Cl-out]=[K+in][Cl-in]
ions are distributed at equilibrium
What does the general charge balance equation state?
[K]i + [Na]i = [Cl]i + n[A]i
n[A] concentration of proteins, SO4, and HPO4
And [K]o + [Na]o = [Cl]o
What does the general osmotic balance equation look like?
[K]i + [Na]i + [Cl]i + [A]i = [K]o + [Cl]o + [Na]o
How many ions, and in which direction, are moved with each cycle of the potassium/sodium pump?
3Na+ out
2K+ in
uses ATP
Describe 1 cycle of the Na/K pump
A) both gates closed, 2 K+ ions inside
B) ATP binds -> inner gate opens-> affinity changes to Na+ -> K+ leaves and Na+ enters
C) ATP hydrolyzed -> Inner gate closes
D) Outer gate opens spontaneously -> affinity changes from Na+ to K+ -> Na+ leaves and K+ enters
D->A: pump loses phosphate group, outer gate closes => cycle is complete
What does steady state mean?
ion concentrations are not changing over time, but unlike model cel, constan input of energy is needed (in the form of ATP) to drive the Na/K pump
v. equillibrium of ideal cell which required no energy
What does relative permeability mean?
Different cells, depending on the concentration of K+ and Na+ channels have different relative permeability to those ions
What is the primary short term determinant of membrane potential?
relative permeability
A small change in the concentration of which ion in the extracellular fluid can have important clinical implications?
K+
How does the Na+/K+ pump relate to membrane potential?
less impact on short term membrane potential
BUT
If non-functional:
- cell gradually depolarizes
- speed of depolarization depends on surface area (faster over larger surface area)
What is the goldman equation?
Vm =
60log {Ko +Pr[Na]o…}/{Ki +Pr[Na]i…}
What is the effect of a change in the ECF level of Na+ on the membrane potential of a cell?
Relatively little:
- membrane is relatively impermable to Na
- & Na is a major component of ECF- major reduction would be accompanied by large volume of fluid loss
What is the effect of a change in the ECF level of K+ on the membrane potential of a cell?
HUGE
- reduces eflux of K+ from cell-> cell gradually depolarizes
- Ek moves closer to 0 (-60mV from -90mV)
WHEREVER EK GOES, VM FOLLOWS B/C MEMBRANE IS RELATIVELY MORE PERMEABLE TO K
What is C BIG K used for and what do the letters stand for?
Treat hyperkalemia
C: Ca2+ (usually IV to treat cardia arrythmias_
B: Bicarb (to encourage K+ uptake)
I: Insulin (Energy -ATP- for Na+/K+ pump)
G: Glucose (Energy -ATP- for Na+/K+ pump)
K: Kayexalate (ion exchanger oral/enema removes K)
What is a long term treatment for chronic hyperkalemia?
Dialysis
