Membrane Potential (Em)

1
Membrane Potential (Em)

• What is it?
• How does the membrane potential arise?
• How do you estimate the Membrane Potential with
  the Nernst Equation?
• Why study membrane potential? see next slide
• Page 191, 203 (ion channels), 208-209 (CF),
  365-368 (Understanding mem pot, Nernst equa)

2
Why study the membrane potential Em? (Dont
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)

• http//carbon.cudenver.edu/bstith/membrpotential.
  gif (D\cell biol 3611\ch 7 8 9 membrane
  trans\membrpotential.gif)
• http//carbon.cudenver.edu/bstith/nernst.mov
  (D\cell biol 3611\ch 7 8 9 membrane
  trans\nernst-short.mov)
• http//distance.stcc.edu/AandP/AP/AP1pages/nervssy
  s/unit10/resting.htmwhat20is20RP
• http//www.taumoda.com/web/nernstjava/
• 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 cellthere 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(Cout/Cin)
• (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
• Nao 140 mM
• Ko 5 mM
• Clo 100 mM
  

• inside the cell
• Nain 14 mM
• Kin 124 mM
• Clin 40 mM

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Estimate the Membrane Potential for human cell
(37C)

• Em (RT/ZF) ln (Cout/Cin)
• Plug in what ion concentrations since it is the
  most permeable ion (more these ion channels are
  open so the ions 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 (Cout/Cin)
• (1.987)(27337)/ (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
CHANNELSRESTING 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)23062ln (140/14)

61 mV (make sure that you can perform this
calculation-on exam)
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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)
Na determines membrane potential
S
Na CH. open
Na CH. closes
K CH. open
K CH. open
Fig. 13-12
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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 Cl- movement
• The membrane potential changes from about -40 to
  -20 mV.

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Estimate the Membrane Potential with Chloride
channels open

• Clin 40 mM Clout 100 mM
• Cl has a negative charge (z-1)
• Em (RT/ZF) ln (Cout/Cin)
• 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