Membrane Potential (Em)
1. What is it?
2. How does the membrane
potential arise?
3. How do you estimate the
Membrane Potential with the
Nernst Equation?
4. Why study membrane
potential?= see next slide
Page 191, 203 (ion channels), 208-209 (CF), 365368 (Understanding mem pot, Nernst equa)
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Why study the membrane potential Em?
(Don’t memorize!!):
Our cells make ATP (possibly the most important reaction in the
body) with use of the membrane potential.
Epilepsy is thought to be due to bad "voltage-gated" potassium
channels (voltage-gated means that the channels are opened or
closed by the membrane voltage Em).
Cystic fibrosis is due to bad chloride movement across membranes.
Heart drugs such as cardiotonic steroids (i.e., cardiac glycosides) are
Na-K pump (active transport) inhibitors- this can effect Em
Neurotransmitters act by changing ion fluxes across membranes-this
changes Em.
Ions move across membranes in the eye- changing Em, allowing us
to see.
Ion movement across membranes and changes in Em is important
for each muscle contraction and for each nerve impulse.
One of the most deadly poisons known (tetrodotoxin) is from the
Puffer fish (a specialty in the Japanese diet); the poison acts by
blocking sodium channels- this prevents action potentials (which are
changes in Em).
Some anesthetics work by altering Em.
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FIG. 5-2 The cell spends a tremendous
amount of energy maintaining the
Membrane Potential
CHEMICAL WORK
CHEMICAL WORK
Note positive ion moving out
Makes the membrane potential
With an excess of + outside, - inside
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The membrane potential:
VOLTAGE OF A MEMBRANE: Vm OR Em is the
symbol
WHAT IS A VOLTAGE? SINCE MEMBRANES ARE
HYDROPHOBIC AND PREVENT IONS FROM
CROSSING, A POTENTIAL DIFFERENCE (IN
UNITS OF VOLTAGE) CAN BUILD UP ACROSS
THE MEMBRANE.
That is, + charges build up on one side of a
Membrane, and – charges build up on the other
CELLS ARE REGULATED BY CHANGES IN THE
MEMBRANE POTENTIAL (ESP. NERVE CELLS)
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Membrane Voltage: IS THE INSIDE OF THE
CELL POSITIVE OR NEGATIVE? FIG. 13-11
-4O MILLIVOLTS
CELL HAS
HI [K]
AND LOW
[Na]
We will
measure
membrane
potential this
way
+
-
CELL
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If a Positive ion like K moves out of
the cell, a membrane potential
develops- where is the plus sign?
Inside the cell or out?
K+
K+
- +
Voltage becomes very negative== -50 mV (or more)
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For most cells (cells are “rest” or
resting cells), as K moves out of
the cell, this makes the membrane
potential
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Go through these web sites to review
(see them on my web site for cell
biology 3611)
1) http://carbon.cudenver.edu/~bstith/membrpotential.gif
(D:\cell biol 3611\ch 7 8 9 membrane
trans\membrpotential.gif)
2) http://carbon.cudenver.edu/~bstith/nernst.mov
(D:\cell biol 3611\ch 7 8 9 membrane trans\nernstshort.mov)
3) http://distance.stcc.edu/AandP/AP/AP1pages/nervssys/
unit10/resting.htm#what%20is%20RP
4) http://www.taumoda.com/web/nernstjava/
5) For the computational biology students: view this paper:
http://carbon.cudenver.edu/~bstith/koch.pdf
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Then, if a Negative Ion like
Chloride (Cl-) moves out of the
cell, the membrane potential
decreases toward zero
Cl-
Cl-
+-
Em changes from -50 mV to a less negative number
(Cl- are leaving)--Such as -20 mV
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How do Ions (like
sodium or Na,
potassium or K,
Chloride or Cl) cross
membranes?
IONS CROSS
MEMBRANES
THROUGH
CHANNELS (A
PROTEIN THAT
CROSSES THE
MEMBRANE)
FIG 13-8
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How does the membrane potential
develop?
Most cells are negative inside, about – 40 to -60
mV
This potential is typically due to potassium
moving out of the cell—there are more K
channels open than channels for other ions
K moves out because K concentration is very
high in the cell (and low outside)-so K moves
from high to low conc. (due to the NaK pump)
What would happen to the membrane potential if
negative Cl moved out of the cell? We will see...
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ION CHANNELS allow ions to
move across the membrane
ION CHANNELS ALLOW ONLY IONS TO CROSS,
CHANGING MEMBRANE POTENTIAL OF CELL
CHANGE Em, TURN ON/OFF NEURON.
MANY MEDICINES AFFECT ION CHANNELS TO
AFFECT NEURON (ANTIDEPRESSENT).
Typically, there are more K channels open, so K
moves out of the cell and sets the membrane
potential to negative inside.
In an action potential, sodium channels open up
and sodium movement sets the membrane
potential
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In the Xenopus Oocyte
Potassium channels are more open, so K
efflux sets the membrane potential to
about -50 mV
However, if chloride channels open,
chloride moves out of the cell and this
reduces the membrane potential to about
-20 mV.
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HOW DO YOU ESTIMATE THE
MEMBRANE POTENTIAL?
Nernst Equation:
Vm or Em=(RT/ZF)ln([C]out/[C]in)
(EQUATION 13.1 (old 9.1) IN TEXT)
R= 1.987 cal/deg mole; Z is charge of the
ion, F = 23,062 cal/volt equiv; T =temp in Kelvin
(C +273), Ln is natural log base e= 2.718.
[C] = concentration of the ion that is most
permeable (its channels are more open)concentration outside the cell or inside the cell
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Example: Estimate Plasma
Membrane Potential (Em)
-write this down
outside the cell
membrane:
[Na]o =140 mM
[K]o = 5 mM
[Cl]o = 100 mM
inside the cell
[Na]in = 14 mM
[K]in = 124 mM
[Cl]in = 40 mM
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Estimate the Membrane Potential
for human cell (37C)
Em = (RT/ZF) ln ([C]out/[C]in)
Plug in what ion concentrations since it is the most
permeable ion (more these ion channels are
open so the ion’s concentrations determine the
membrane potential)
R= 1.987 cal/deg mole; Z is charge of the ion, F =
23,062 cal/volt equiv; T =temp in Kelvin (deg C
+273), Ln is natural log base e= 2.718.
[C] = concentration of the ion that is most
permeable (its channels are more open)concentration outside the cell or inside the cell
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Do the calculation
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K is most permeable:
Em = (RT/ZF) ln ([C]out/[C]in)
= (1.987)(273+37)/ (+1)(23062)
ln (5 mM/124 mM)
= 0.0267 x -3.21 = -0.0857 Volts
(note the concentrations must be in same units)
Usually, scientists report answer in milliVolts:
= - 85.7 mV is the estimate for the membrane
potential of the liver cell
For practice: questions 13-2, -4, and -5 in book
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ACTION POTENTIAL in nerve cell
IS DUE TO OPENING OF SODIUM
ION CHANNELS, THEN CLOSING
OF THESE ION CHANNELS
RESTING CELL before ActPot: K
IS MOST PERMEABLE (SOME K
CHANNELS OPEN)
PEAK OF ACTION POTENTIAL:
Na IS MOST PERMEABLE
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What if Na channels open? Em
now set by Na Concentrations
Em = [(1.987x310)/(+1)23062]ln (140/14)
= +61 mV
(make sure that you can
perform this calculation-on exam)
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Na determines membrane potential
Na CH.
open
K CH.
open
Na CH.
closes
K CH.
open
Fig. 13-12
Before,
K channels are
more open and
K determines
the Em, then
the Na channel
open to
determine Em
(and closes) to
make the
action potential
(turns on nerve
cell)
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WHAT IF Chloride becomes the
MOST PERMEABLE ION?
DOES THIS HAPPEN WITH ANIMAL
CELLS? Yes.
Chloride channels open up to allow Clmovement
The membrane potential changes from about
-40 to -20 mV.
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Estimate the Membrane Potential
with Chloride channels open
[Cl]in = 40 mM
[Cl]out = 100 mM
Cl has a negative charge (z=-1)
Em = (RT/ZF) ln ([C]out/[C]in)
= HORMONES RELEASE CALCIUM INTO
THE CYTOPLASM AND CALCIUM OPENS
CHLORIDE CHANNELS TO CHANGE THE
Em
WE WILL MEASURE THIS CHANGE IN Em…
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Chloride Channels in human
disease- cystic fibrosis
Chloride channels need to be present and
functional for cells to function
In cystic fibrosis, chloride channels do not
make it to the plasma membrane AND
CHOLORIDE DOES NOT MOVE ACROSS THE
MEMBRANE
So, the symptoms of cystic fibrosis
develop (high salt in sweat, destruction of
organs, thick mucus in lungs that causes
infections).
Pgs. 208-209 in 6th edition of our text
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Chloride Channel
Fig. 13-8a
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We will study the Chloride Channel
in Xenopus frog oocytes
Acetylcholine binds to a membrane
receptor
To increase the concentration of Calcium
in the cytoplasm
Calcium binds to and opens the Chloride
channel
Chloride moves across the membrane and
out of the cell
This Cl movement causes the membrane
potential to change from -50 mV to a lower
value: -25 mV
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PA (new hormone?)
Acetylcholine
Calcium
Ach receptor
Cl-
- - ++++
Note that chloride
Efflux reduces the
Membrane potential
(less negative inside,
Less positive outside)
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Today, we will use the NeuroLab
program
(later, maybe the Neuroscience
Program from HHMI)
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end
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