2nd lecture dr.hameed

Membrane Potential and Action Potential

• Membrane Potential,• Distribution of ions in body fluids,• Contribution of various ions to membrane potential,• Nernst Equation• Resting Membrane Potential• Action Potential

CHAPTERS 6 & 7

Membrane PotentialMembrane Potential : The difference in ionic distribution between inside and outside of cells which results in electrical potential difference across the cell membrane.

CELL

_+

o _

+

Voltmeter

Units of this potential are volts (millivoltes)

Charged Substances in Body Fluids

Many molecules in our body fluids have electrical charges due to the presenceof negative groups (e.g. phosphate, RCOO-) or positive groups such as RNH3+. Also most mineral elements such as sodium, potassium, chloride, and calcium(Na+, Cl-, K+, Ca++) present in our body fluids as ions.

Positive Charges Negative Charges

Na+

K+

Ca++

Mg++

RNH3+

Cl_

HCO3

-

PO42- & Organic anions

Proteins

Distribution of Main Charged

Substances in Body Fluids

Na+

Na+Na+

Cl_

Cl_ Cl

_

Cl_

Cl_

Cl_

Cl_

Cl_

Na+

Na+ Na+

Na+

Na+

Na+

Na+ Cl_

Cl_ K+ K+

K+

K+

K+

K+

K+K+

K+

K+

K+

K+

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FIXED ___

_ _

FIXED ___

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FIXED ___

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FIXED ___

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FIXED ___

CELLECF

Properties of Charged ChemicalsDistribution of various charges outside and inside the cell play a significant rolein cell function. Excitable cells such as muscle and nerve use this phenomenon togenerate and propagate electrical signals.

Like charges repel each other. + +Electrical

Force

_ _Electrical

Force

Opposite charges attract each other.

+ _Electrical

Force

This creates force at opposite directionof each charge.

The force of attraction between thesecharges has the potential of creatingforce and hence performing work. This iscalled electrical potential.

Distribution of ChargesThe membrane potential is due to small excess of negative ions inside of the cell and small excess of positive ions outside of the cell.

The negative charges are attracted to the positive charges and they forma thin layer of negative inside and positive outside of the cell.

The rest of intracellular and extracellular fluid remain neutral.

Electrical CurrentThe movement of electric charges is called current. The electric force betweencharges makes the charges flow producing current.

Current depends on:

1- Potential difference = E (voltage)

2- The medium in which the charges move = R (resistance)

I = E

R

The relationship is given by Ohm’s law:

- Movement of ions through channels is referred to current (I). - The Voltage (E) is the membrane potential.- The cell membrane shows the highest resistance (R ) to ionic movements.

- Movement of ions through channels is referred to current (I). - The Voltage (E) is the membrane potential.- The cell membrane shows the highest resistance (R ) to ionic movements.

Membrane Potential

The difference in ionic distribution between inside and outside of the cellresults in a negatively charged intracellular compartment compared to the positive extracelluar environment.

Membrane potential is produced by:

1- The action of Na/K pump at the cell membrane.

2- Proteins, ATP and other organic molecules in the cell are negatively charged (anions), and can not cross the cell membrane therefore this makes inside of the cell negative.

3- Ion Channels which cause leakage of ions across the cell membrane.

1- The action of Na/K pump at the cell membrane is essential for the production of membrane potential.

2

3

Na/K pump is electrogenic; this transporter causes a difference in ionicdistribution between inside and outsideof the cell.

An important function of Na/K pump isto control the volume of the cells; thistransporter pumps 3 ions out and 2 ions into the cell. By doing this it keeps a balance in concentration of particles between inside and outside of the cell hence keeping cell volume controlled.

2

3

2- Organic anions are trapped inside the cell due to their largesize and negative charge:

Cell membrane phospholipidbilayer does not allow thesemolecules to pass through therefore preventing these molecules from

exiting out of the cell.

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FIXED ___

High resistanceHigh resistance

3- Ion Channels in The Cell Membrane

Permeability of the phospholipid bilayer to charged ions is very low (zero).But special proteins in cell membrane can allow small ions to pass across it. This is due to the presence of specific ion channels (integral proteins) in the cell membrane.

Properties of ion channels:

- Selective (size, charge)

- Gated / non-gated

Membrane Potential Can be MeasuredThe potential difference across the cell membrane can be measured using microelectrodes and a sensitive voltmeter.

In some cells the membrane potential can change. In some cells the membrane potential can change.

Contribution of Various Ions to Membrane Potential

The unequal distribution of ions across the plasma membrane results in the membrane potential. Each ion has a different contribution to the creation of this membrane potential.

The degree of contribution of each ion to membrane potential depends on:

1) Concentration difference of the ion

2) Membrane permeability of the ion.

Membrane Potential

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FIXED ___

K+

K+

Na+ Na+

ATP

2 K+

3 Na+

Gated Na+ Channel

Gated K+ Channel

Non-gated K+ Channel

Sodium potassium leak channels

Nernst Equation

_

EMF (mV) = + 61 log

- In the body at 37 oC the electrical difference that balance the concentration difference of a univalent ions such as Na+ can be determined by Nernst Equation:

C1

C2

EMF = Electromotive force between side 1 and 2 of a membrane,C1 = concentration on side 1

C2 = concentration on side 2.

140 mM K+

14 mM K+

_+

This means that 61 mV negative charges inside of the cell is required to keep the above concentration difference of K+ between inside and outside of the cell. Assuming that the cell membrane is highly permeable to K+.

Nernst Equation:

EMF (mV) = + 61 log C1

C2

- In excitable cells the potential difference between inside and outside of the cell during rest is called “resting membrane potential” (r.m.p.)

Resting Membrane potential

Excitable cells:- Nerve cells- Muscle cells- Some endocrine cells- Some immune cells- Some reproductive cells

Change in r.m.p occurs in these excitable cells to perform a function. Usually this change indicate a signal. For example nerve cells produce signals and propagate them by undergoing changes in their r.m.p.

- During rest in nerve cell negativity of inside of the cell is maintained at (-60 to -90 mV). - This results from the difference between concentrations of Na+ and K+ inside and outside of the cells which produces a potential difference across the membrane.

Resting Membrane potential (Nerve Cell)

Na+ 145 mM

Membrane potential = -75 mV

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150 mMK+ 5 mM

12 mM

Magnitude of resting membrane potential in Nerve cells

- The magnitude of r.m.p. for large nerve cells is -90 mV negative inside.

The factors that determine this potential are:

1- Na/K pump

2- Leakage of Na and K through the nerve cell membrane:

a- Contribution of the K diffusion

b- Contribution of Na diffusion

In large nerve cells the potential caused only by K+ and Na+ diffusion is approximately -86 mV. The other -4 mV potential is due to the action of Na/K pump.

Diffusion through Na+ and K+ channels contribute to about - 86 mV.Na/K pump contributes about -4 mV.

Na14

Na142

K140

K4

Because there are 75 to 100 times more K leak channels than Na leak channelstherefore the sum of potential is -86 mV. The rest ( -4 mV) is contribution by Na/K pump.

EMF (mV) = + 61 log C1

C2

Action PotentialAction potential is referred to the change in membrane potential inexcitable cells. The change in the membrane potential is due tomovement of ions in and out of the cell.

140 mM

K+ 4 mM

14 mM

Na+ 142 mM

Nerve cell

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Axon ofA nerve cell

Membrane potential = -90 mV

Ionic Movements During Action potential