Membrane Potential II Flashcards
The Principle of Electrical Neutrality
bulk solutions (inside and out) have to be electrically neutral: the total cation concentration in the external solution must equal the total anion concentration in the external solution. The same holds for the internal solution: [cations]i = [anions]i.
Donnan Rule:
[K+]o[Cl-]o = [K+]i.[Cl-]i
Osmotic balance:
Osmotic balance: [K]i + [Na]i + [Cl]i + [A] = [K]o + [Cl]o + [Na]o
Because the concentration of Na+ outside is higher than inside, the concentration gradient is directed
into the cell.
Because the inside of the cell is electrically negative, the electrical gradient is
directed into the cell.
given the opportunity, Na+ is going to
leak into the cell.
If Na pump is blocked,
Na+ enters the cell, water follows, and the cell swells.
The Na pump can be blocked by
interfering with ATP production or by specific drugs
examples of specific drug that blocks Na pump works by:
that selectively block the Na+ pump by binding to a site on the outside surface of the membrane.
block Na pump by interfering with ATP production examples
low temperature, cyanide, hypoxia
The Na+ pump also pumps
potassium into cells
the Na pump can’t extrude Na+ from the cell very well if the _______.
potassium concentration in the ECF is low
For the Na pump to work,
both Na+ and K+ ions must be present simultaneously, or the pump won’t work.
Cells with a poisoned pump lose _____ as they gain_____
K+
Na+
(they still swell, because they also gain Cl- as Na+ leaks in).
The sodium pump is really an ________
obligatorily coupled sodium-potassium exchange pump.
The Na/K pump is “_______”, that is, it has an ____________ cycles per second.
“saturable”
an easily demonstrable maximum rate of activity of only about 100
“saturable” is characteristic of ________,
carrier mediated transport
the Na/K ratio is _______
3 Na+ for 2 K+
Thus, the pump is __________. This has the effect of making Vm __________.
electrogenic, and not electro-neutral
a little negative
The effect in most cases is very small, so that for our purposes we can ignore its electrogenicity
the ion binding affinity of the channel switches between
Na and K, depending on which gate is open.
______ provides the energy for the transitions (gates swinging and affinity changing).
ATP
The pump contains a large _____ subunit, and a smaller _____ subunit
alpha
beta
The Na/K pump cycle.
A. Both gates are closed, 2 K+ ions are inside.
B. ATP binds, the inner gate opens, and affinity changes from K+ to Na+. So K+ leaves and Na+ enters.
C. ATP is split, leaving the pump
phosphorylated. The inner gate closes.
D. Spontaneously, the outer gate
opens and affinity changes from Na+ to K+.
D to A. The pump loses its phosphate group, and the outer gate closes, completing the cycle.
Na+ is constantly _____ into the cell, trying to get to equilibrium
leaking
The real cell is in a steady state, which means that,
the ion concentrations aren’t changing over time,
constant input of energy is needed
What is equilibrium for Na+?
Vm to equal ENa.
For potassium, equilibrium is achieved when________. So a struggle ensues between the two ions
Vm is equal to EK
Na+ leaking in, trying to pull Vm up to ENa, and K+ leaking out, trying to pull Vm down to EK.
What are the differences in resting membrane potentials due to?
relative permeability.
A cell with many more K+ channels than Na+ channels will have a membrane potential close to _____.
EK
Conversely, a cell with relatively more Na+ channels will have a membrane potential closer to ____.
ENa
Glial cells are nearly perfect :
‘potassium electrodes’ (permeable only to K)
while red blood cells are
about equally permeable to Na and K.
A second determinant of Vm is
ion concentration
A small change in ECF potassium concentration has a big effect on
EK and (in nerve, muscle, and other cells highly permeable to potassium) a big effect on Vm.
Ohm’s Law,
V=I.R
G=1/R
V is the ‘driving force’ on the ion, I is the current carried by the ion, and R is the membrane resistance to the ion G, which is the reciprocal of resistance
Ohm’s Law for sodium is as follows:
for K+:
INa= GNa.(Vm-ENa).
IK= GK.(Vm-EK).
more channels, more ______, more _______
current, and more driving force, more current.
When these two currents are equal and opposite (INa = -IK) there will be:
no net movement of charge across the membrane (we are ignoring other ions), so Vm will be at its resting value.
Gr is the
the relative conductance of the membrane to sodium and potassium.
membrane potential is determined by
relative conductance.
One (leaky membrane) has
100 K channels and 10 Na channels
The other (tight membrane) has
only a tenth as many channels (10 K and 1 Na).
the tight and leaky have membrane potential
the same!
The leaky cell will have ten times more sodium leaking in, and ten times more potassium leaking out, compared to the tight cell, and so will have to expend 10 times more energy to keep the Na/K pump working. Thus, the tight cell is ten times more efficient, energetically speaking.
if the Na/K pump is completely blocked by drugs,
nothing much happens initially (in many cells).
Gradually, of course., the cell will fill up with Na, and lose its K.
As these changes occur, both ENa and EK move towards zero, and cells will depolarize.
Ions crossing a membrane are driven by
both an electrical force (strong) and a concentration gradient (weak).
Like charges repel and opposite charges attract, so a negatively charged Cl- ion diffusing across the cell membrane will be repelled by _________. Eventually these forces will reach an equilibrium whereby for every Cl entering the cell, _______. Together these two forces make up _______
the internal negativity of the cell but will also be attracted by the lower concentration of Cl within the cell
another leaves
an electrochemical gradient.
Equilibrium potential is the:
the electrical potential difference across the membrane that must exist if the ion is to be a equilibrium
Equilibrium potentials are not real voltages. They are:
what the voltage would be for a particular ion to be at equilibrium across a membrane.
Equilibrium potential relates a concentration gradient to
an electrical force.
Membrane potential (Vm) is a:
real voltage that exists across a membrane
Vm is a reflection of the:
equilibrium potentials of all the ions and their given equilibrium state across a membrane.
If Vm is not the same as a given ion’s equilibrium potential, then we know the
membrane is either impermeable to that ion, or that ion must be pumped across the membrane.
Equilibrium potential is ultimately dependent on _______ while Vm is dependent on the _______
concentration (see the Nernst equation)
electric force
Every ion in solution has its own equilibrium potential based solely on
its concentration inside and outside the membrane.
In acute hyperkalemia, the main danger concerns the .
the reliable conduction of electircal signals (action potentials) in the heart.
These synchronized electrical signals can become disrupted during acute hyperkalemia, causing cardiac arrhythmias as conduction blocks occur and maverick pacemakers arise in various locations of the conduction system.
The causes of hyperkalemia mostly concern
loss of potassium from cells.
Crush injuries, burns, and other trauma that disrupt cell membranes can do it.
So can immunological attack of red blood cells (causing hemolysis).
One of the most important determinants of the clinical course of the hyperkalemia is the
status of the kidney,
whose normal job it is to excrete excess potassium.
If kidney function is compromised, hyperkalemia can be much more serious than if the kidney is functioning normally.
The diagnosis of hyperkalemia usually is via an
electrocardiogram (EKG) to detect cardiac arrhythmias, followed by measuring plasma potassium ion concentration.
