11

Membrane Potential (Em)Membrane Potential (Em)1.1. What is it?What is it?2.2. How does the membrane How does the membrane

potential arise?potential arise?3.3. How do you estimate the How do you estimate the

Membrane Potential with the Membrane Potential with the Nernst Equation?Nernst Equation?

4.4. Why study membrane potential?Why study membrane potential?= see next slide= see next slide

Page 191, 203 (ion channels), 208-209 (CF), 365-Page 191, 203 (ion channels), 208-209 (CF), 365-368 (Understanding mem pot, Nernst equa)368 (Understanding mem pot, Nernst equa)

22

Why study the membrane potential Em? Why study the membrane potential Em? ((Don’t memorize!!):Don’t memorize!!):

Our cells make ATP (possibly the most important reaction in the Our cells make ATP (possibly the most important reaction in the body) with use of the membrane potential. body) with use of the membrane potential. Epilepsy is thought to be due to bad "voltage-gated" potassium Epilepsy is thought to be due to bad "voltage-gated" potassium channels (voltage-gated means that the channels are opened or channels (voltage-gated means that the channels are opened or closed by the membrane voltage Em). closed by the membrane voltage Em). Cystic fibrosis is due to bad chloride movement across membranes.Cystic fibrosis is due to bad chloride movement across membranes.Heart drugs such as cardiotonic steroids (i.e., cardiac glycosides) Heart drugs such as cardiotonic steroids (i.e., cardiac glycosides) are Na-K pump (active transport) inhibitors- this can effect Emare Na-K pump (active transport) inhibitors- this can effect EmNeurotransmitters act by changing ion fluxes across membranes-Neurotransmitters act by changing ion fluxes across membranes-this changes Em.this changes Em.Ions move across membranes in the eye- changing Em, allowing us Ions move across membranes in the eye- changing Em, allowing us to see.to see.Ion movement across membranes and changes in Em is important Ion movement across membranes and changes in Em is important for each muscle contraction and for each nerve impulse.for each muscle contraction and for each nerve impulse.One of the most deadly poisons known (tetrodotoxin) is from the One of the most deadly poisons known (tetrodotoxin) is from the Puffer fish (a specialty in the Japanese diet); the poison acts by Puffer fish (a specialty in the Japanese diet); the poison acts by blocking sodium channels- this prevents action potentials (which are blocking sodium channels- this prevents action potentials (which are changes in Em).changes in Em).Some anesthetics work by altering Em. Some anesthetics work by altering Em.

33

CHEMICAL WORKCHEMICAL WORK

FIG. 5-2 The cell spends a tremendous FIG. 5-2 The cell spends a tremendous amount of energy maintaining the amount of energy maintaining the

Membrane PotentialMembrane Potential

Note positive ion moving outMakes the membrane potentialWith an excess of + outside, - inside

44

The membrane potential:The membrane potential:VOLTAGE OF A MEMBRANE: Vm OR Em is the VOLTAGE OF A MEMBRANE: Vm OR Em is the symbolsymbol

WHAT IS A VOLTAGE? WHAT IS A VOLTAGE? SINCE MEMBRANES SINCE MEMBRANES ARE HYDROPHOBIC AND PREVENT IONS ARE HYDROPHOBIC AND PREVENT IONS FROM CROSSING, A POTENTIAL DIFFERENCE FROM CROSSING, A POTENTIAL DIFFERENCE (IN UNITS OF VOLTAGE) CAN BUILD UP (IN UNITS OF VOLTAGE) CAN BUILD UP ACROSS THE MEMBRANE. ACROSS THE MEMBRANE.

That is, + charges build up on one side of aThat is, + charges build up on one side of aMembrane, and – charges build up on the otherMembrane, and – charges build up on the other

CELLS ARE REGULATED BY CHANGES IN THE CELLS ARE REGULATED BY CHANGES IN THE MEMBRANE POTENTIAL (ESP. NERVE CELLS)MEMBRANE POTENTIAL (ESP. NERVE CELLS)

55

Membrane Voltage: IS THE INSIDE OF THE Membrane Voltage: IS THE INSIDE OF THE CELL POSITIVE OR NEGATIVE? FIG. 13-11CELL POSITIVE OR NEGATIVE? FIG. 13-11

CELL

CELL HASHI [K]AND LOW[Na] +

-We will measure membrane potential this way

-4O MILLIVOLTS

66

If a Positive ion like K moves out of If a Positive ion like K moves out of the cell, a membrane potential the cell, a membrane potential

develops- where is the plus sign? develops- where is the plus sign? Inside the cell or out?Inside the cell or out?

K+ K+

- +Voltage becomes very negative== -50 mV (or more)

77

For most cells (cells are “rest” or For most cells (cells are “rest” or resting cells), as K moves out of the resting cells), as K moves out of the

cell, this makes the membrane cell, this makes the membrane potentialpotential

88

Go through these web sites to review Go through these web sites to review (see them on my web site for cell (see them on my web site for cell

biology 3611)biology 3611)1)1) http://http://carbon.cudenver.edu/~bstith/membrpotential.gifcarbon.cudenver.edu/~bstith/membrpotential.gif ((

D:\cell D:\cell biolbiol 3611\ch 7 8 9 membrane trans\ 3611\ch 7 8 9 membrane trans\membrpotential.gifmembrpotential.gif))

2)2) http://carbon.cudenver.edu/~bstith/nernst.movhttp://carbon.cudenver.edu/~bstith/nernst.mov ( (D:\cell D:\cell biolbiol 3611\ch 7 8 9 membrane 3611\ch 7 8 9 membrane trans\nernst-short.movtrans\nernst-short.mov))

3)3) http://distance.stcc.edu/AandP/AP/AP1pages/nervssys/http://distance.stcc.edu/AandP/AP/AP1pages/nervssys/unit10/resting.htm#what%20is%20RPunit10/resting.htm#what%20is%20RP

4)4) http://www.taumoda.com/web/nernstjava/http://www.taumoda.com/web/nernstjava/5)5) For the computational biology students: view this paper:For the computational biology students: view this paper:http://http://carbon.cudenver.edu/~bstith/koch.pdfcarbon.cudenver.edu/~bstith/koch.pdf

99

Then, if a Negative Ion like Chloride Then, if a Negative Ion like Chloride (Cl-) moves out of the cell, the (Cl-) moves out of the cell, the membrane potential decreases membrane potential decreases

toward zerotoward zero

Cl- Cl-

+ -Em changes from -50 mV to a less negative number

(Cl- are leaving)--Such as -20 mV

1010

How do Ions (like How do Ions (like sodium or Na, sodium or Na,

potassium or K, potassium or K, Chloride or Cl) cross Chloride or Cl) cross

membranes?membranes? IONS CROSS IONS CROSS MEMBRANES MEMBRANES THROUGH THROUGH CHANNELS (A CHANNELS (A PROTEIN THAT PROTEIN THAT CROSSES THE CROSSES THE MEMBRANE)MEMBRANE)

FIG 13-8

1111

How does the membrane potential How does the membrane potential develop?develop?

Most cells are negative inside, about – 40 to -60 Most cells are negative inside, about – 40 to -60 mVmVThis potential is typically due to potassium This potential is typically due to potassium moving out of the cell—there are more K moving out of the cell—there are more K channels open than channels for other ionschannels open than channels for other ionsK moves out because K concentration is very K moves out because K concentration is very high in the cell (and low outside)-so K moves high in the cell (and low outside)-so K moves from high to low conc. (due to the NaK pump)from high to low conc. (due to the NaK pump)

What would happen to the membrane potential if What would happen to the membrane potential if negative Cl moved out of the cell? We will see...negative Cl moved out of the cell? We will see...

1212

ION CHANNELS allow ions to ION CHANNELS allow ions to move across the membranemove across the membrane

ION CHANNELS ALLOW ONLY IONS TO CROSS, ION CHANNELS ALLOW ONLY IONS TO CROSS, CHANGING MEMBRANE POTENTIAL OF CELLCHANGING MEMBRANE POTENTIAL OF CELLCHANGE Em, TURN ON/OFF NEURON.CHANGE Em, TURN ON/OFF NEURON.MANY MEDICINES AFFECT ION CHANNELS TO MANY MEDICINES AFFECT ION CHANNELS TO AFFECT NEURON (ANTIDEPRESSENT).AFFECT NEURON (ANTIDEPRESSENT).

Typically, there are more K channels open, so K Typically, there are more K channels open, so K moves out of the cell and sets the membrane moves out of the cell and sets the membrane potential to negative inside.potential to negative inside.In an action potential, sodium channels open up In an action potential, sodium channels open up and sodium movement sets the membrane and sodium movement sets the membrane potentialpotential

1313

In the Xenopus OocyteIn the Xenopus Oocyte

Potassium channels are more open, so K Potassium channels are more open, so K efflux sets the membrane potential to efflux sets the membrane potential to about -50 mVabout -50 mV

However, if chloride channels open, However, if chloride channels open, chloride moves out of the cell and this chloride moves out of the cell and this reduces the membrane potential to about reduces the membrane potential to about

-20 mV.-20 mV.

1414

HOW DO YOU ESTIMATE THE HOW DO YOU ESTIMATE THE MEMBRANE POTENTIAL?MEMBRANE POTENTIAL?

Nernst Equation:Nernst Equation:

VVm or m or EEmm=(RT/ZF)ln([C]out/[C]in)=(RT/ZF)ln([C]out/[C]in) (EQUATION 13.1 (old 9.1) IN TEXT)(EQUATION 13.1 (old 9.1) IN TEXT)

R= 1.987 cal/deg mole; Z is charge of the R= 1.987 cal/deg mole; Z is charge of the ion, F = 23,062 cal/volt equiv; T =temp in Kelvin ion, F = 23,062 cal/volt equiv; T =temp in Kelvin (C +273), Ln is natural log base e= 2.718. (C +273), Ln is natural log base e= 2.718.

[C] = concentration of the ion that is most [C] = concentration of the ion that is most permeable (its channels are more open)- permeable (its channels are more open)- concentration outside the cell or inside the cellconcentration outside the cell or inside the cell

1515

Example: Estimate Plasma Example: Estimate Plasma Membrane Potential (Em)Membrane Potential (Em)

-write this down-write this down

outside the cell outside the cell membrane: membrane:

[Na]o =140 mM [Na]o =140 mM

[K]o = 5 mM [K]o = 5 mM

[Cl]o = 100 mM [Cl]o = 100 mM

inside the cellinside the cell

[Na][Na]inin = 14 mM = 14 mM

[K][K]inin = 124 mM = 124 mM

[Cl][Cl]inin = 40 mM = 40 mM

1616

Estimate the Membrane Potential Estimate the Membrane Potential for human cell (37C)for human cell (37C)

EEmm = (RT/ZF) ln ([C]out/[C]in) = (RT/ZF) ln ([C]out/[C]in)Plug in what ion concentrations since it is the most Plug in what ion concentrations since it is the most

permeable ion (more these ion channels are permeable ion (more these ion channels are open so the ion’s concentrations determine the open so the ion’s concentrations determine the membrane potential)membrane potential)

R= 1.987 cal/deg mole; Z is charge of the ion, F = R= 1.987 cal/deg mole; Z is charge of the ion, F = 23,062 cal/volt equiv; T =temp in Kelvin (deg C 23,062 cal/volt equiv; T =temp in Kelvin (deg C +273), Ln is natural log base e= 2.718. +273), Ln is natural log base e= 2.718. [C] = concentration of the ion that is most [C] = concentration of the ion that is most permeable (its channels are more open)- permeable (its channels are more open)- concentration outside the cell or inside the cellconcentration outside the cell or inside the cell

1717

Do the calculationDo the calculation

1818

K is most permeable:K is most permeable:EEmm = (RT/ZF) ln ([C]out/[C]in) = (RT/ZF) ln ([C]out/[C]in)= (1.987)(273+37)/ (+1)(23062)= (1.987)(273+37)/ (+1)(23062)

ln (5 mM/124 mM)ln (5 mM/124 mM)= 0.0267 x -3.21 = -0.0857 Volts= 0.0267 x -3.21 = -0.0857 Volts(note the concentrations must be in same units) (note the concentrations must be in same units)

Usually, scientists report answer in milliVolts:Usually, scientists report answer in milliVolts:= - 85.7 mV is the estimate for the membrane = - 85.7 mV is the estimate for the membrane

potential of the liver cell potential of the liver cell

For practice: questions 13-2, -4, and -5 in bookFor practice: questions 13-2, -4, and -5 in book

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ACTION POTENTIAL in nerve cell ACTION POTENTIAL in nerve cell IS DUE TO OPENING OF SODIUM IS DUE TO OPENING OF SODIUM ION CHANNELS, THEN CLOSING ION CHANNELS, THEN CLOSING

OF THESE ION CHANNELSOF THESE ION CHANNELS

RESTING CELL before ActPotRESTING CELL before ActPot: K : K IS MOST PERMEABLE (SOME K IS MOST PERMEABLE (SOME K

CHANNELS OPEN)CHANNELS OPEN)PEAK OF ACTION POTENTIALPEAK OF ACTION POTENTIAL: :

Na IS MOST PERMEABLENa IS MOST PERMEABLE

2020

What if Na channels open? Em What if Na channels open? Em now set by Na Concentrationsnow set by Na Concentrations

Em = Em = [(1.987x310)/(+1)23062]ln (140/14)[(1.987x310)/(+1)23062]ln (140/14)

= +61 mV= +61 mV(make sure that you can (make sure that you can perform this calculation-on exam)perform this calculation-on exam)

2121

Before,Before,K channels are K channels are more open and more open and K determines K determines the Em, then the Em, then

the Na channel the Na channel open to open to

determine Em determine Em (and closes) to (and closes) to

make the make the action potential action potential (turns on nerve (turns on nerve

cell)cell)

K CH.open

Na CH. open

Na CH. closes

K CH.open

Na determines membrane potential

S

Fig. 13-12

2222

WHAT IF Chloride becomes the WHAT IF Chloride becomes the MOST PERMEABLE ION?MOST PERMEABLE ION?

DOES THIS HAPPEN WITH ANIMAL DOES THIS HAPPEN WITH ANIMAL CELLS? Yes.CELLS? Yes.

Chloride channels open up to allow Cl- Chloride channels open up to allow Cl- movementmovement

The membrane potential changes from about The membrane potential changes from about -40 to -20 mV.-40 to -20 mV.

2323

Estimate the Membrane Potential Estimate the Membrane Potential with Chloride channels openwith Chloride channels open

[Cl]in = 40 mM [Cl]in = 40 mM [Cl]out = 100 mM[Cl]out = 100 mMCl has a negative charge (z=-1)Cl has a negative charge (z=-1)

EEmm = (RT/ZF) ln ([C]out/[C]in) = (RT/ZF) ln ([C]out/[C]in)

= = HORMONES RELEASE CALCIUM INTO HORMONES RELEASE CALCIUM INTO THE CYTOPLASM AND CALCIUM OPENS THE CYTOPLASM AND CALCIUM OPENS CHLORIDE CHANNELS TO CHANGE THE CHLORIDE CHANNELS TO CHANGE THE EmEmWE WILL MEASURE THIS CHANGE IN Em…WE WILL MEASURE THIS CHANGE IN Em…

2424

Chloride Channels in human Chloride Channels in human disease- cystic fibrosisdisease- cystic fibrosis

Chloride channels need to be present and Chloride channels need to be present and functional for cells to functionfunctional for cells to function

In cystic fibrosis, chloride channels do not In cystic fibrosis, chloride channels do not make it to the plasma membranemake it to the plasma membrane AND AND CHOLORIDE DOES NOT MOVE ACROSS THE CHOLORIDE DOES NOT MOVE ACROSS THE MEMBRANEMEMBRANE

So, the symptoms of cystic fibrosis develop So, the symptoms of cystic fibrosis develop (high salt in sweat, destruction of organs, (high salt in sweat, destruction of organs, thick mucus in lungs that causes infections).thick mucus in lungs that causes infections).

Pgs. 208-209 in 6Pgs. 208-209 in 6thth edition of our text edition of our text

2525

Chloride Channel Fig. 13-8a

2626

We will study the Chloride Channel We will study the Chloride Channel in Xenopus frog oocytesin Xenopus frog oocytes

Acetylcholine binds to a membrane receptorAcetylcholine binds to a membrane receptorTo increase the concentration of Calcium in To increase the concentration of Calcium in the cytoplasmthe cytoplasmCalcium binds to and opens the Chloride Calcium binds to and opens the Chloride channelchannelChloride moves across the membrane and out Chloride moves across the membrane and out of the cellof the cellThis Cl movement causes the membrane This Cl movement causes the membrane potential to change from -50 mV to a lower potential to change from -50 mV to a lower value: -25 mVvalue: -25 mV

2727

Acetylcholine

Ach receptor

Calcium

Cl-

- - -+ + + +

Note that chloride Efflux reduces the Membrane potential (less negative inside,Less positive outside)

PA (new hormone?)

2828

Today, we will use the NeuroLab Today, we will use the NeuroLab program program

(later, maybe the Neuroscience (later, maybe the Neuroscience Program from HHMI)Program from HHMI)

2929

endend