CHAPTERS 6 & 7 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
Membrane PotentialMembrane Potential : The difference in ionic distribution betweeninside and outside of cells which results in electrical potentialdifference across the cell membrane. Units of this potential are volts (millivoltes) o _ + Voltmeter _ + CELL
Charged Substances in Body FluidsMany 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+ Cl - K+ HCO3 Ca++ - PO42 & Organic anions Mg++ Proteins RNH3+
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. Electrical Like charges repel each other. + Force + This creates force at opposite direction of each charge. _ Electrical _ Force Opposite charges attract each other. Electrical _ The force of attraction between these + Force charges has the potential of creating force and hence performing work. This is called electrical potential.
Distribution of ChargesThe membrane potential is due to small excess of negative ions insideof 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 andextracellular fluid remain neutral.
Electrical Current The movement of electric charges is called current. The electric force between charges makes the charges flow producing current. Current depends on: 1- Potential difference = E (voltage) 2- The medium in which the charges move = R (resistance) The relationship is given by Ohm’s law: E I = R- 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 PotentialThe difference in ionic distribution between inside and outside of the cellresults in a negatively charged intracellular compartment compared to thepositive 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.Na/K pump is electrogenic; thistransporter causes a difference in ionicdistribution between inside and outside 2of the cell. 2An important function of Na/K pumpisto control the volume of the cells; thistransporter pumps 3 ions out and 2 3ions into the cell. By doing this it keeps 3a balance in concentration of particlesbetween inside and outside of the cellhence keeping cell volume controlled.
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 thereforepreventing these molecules fromexiting out of the cell. High resistance _ _ _ _ FIXED _
3- Ion Channels in The Cell MembranePermeability 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 bemeasured using microelectrodes and a sensitive voltmeter. In some cells the membrane potential can change.
Contribution of Various Ions to Membrane PotentialThe unequal distribution of ions across the plasma membrane results in themembrane potential. Each ion has a different contribution to the creation of thismembrane 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.
Nernst Equation _- In the body at 37 oC the electrical difference that balance the concentration difference of aunivalent ions such as Na+ can be determined by Nernst Equation: C1 EMF (mV) = + 61 log C2EMF = Electromotive force between side 1 and 2 of a membrane,C1 = concentration on side 1C2 = concentration on side 2.
Nernst Equation: C1 EMF (mV) = + 61 log C2 + _ 140 mM K+ 14 mM K+This means that 61 mV negative charges inside of the cell is required to keep theabove concentration difference of K+ between inside and outside of the cell. Assumingthat the cell membrane is highly permeable to K+.
Resting Membrane potential- In excitable cells the potential difference between inside and outside of the cellduring rest is called “resting membrane potential” (r.m.p.) Excitable cells: - Nerve cells - Muscle cells - Some endocrine cells - Some immune cells - Some reproductive cellsChange in r.m.p occurs in these excitable cells to perform a function.Usuallythis change indicate a signal. For example nerve cells produce signals andpropagate them by undergoing changes in their r.m.p.
Resting Membrane potential (Nerve Cell)- 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. Na+ 145 mM 12 mM K+ 5 mM 150 mM _ _ _ _ FIXED _ Membrane potential = -75 mV
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 diffusionIn large nerve cells the potential caused only by K+ and Na+ diffusionis approximately -86 mV. The other -4 mV potential is due to the action ofNa/K pump.
Diffusion through Na+ and K+ channels contribute to about - 86 mV. Na/K pump contributes about -4 mV. Na142 K4 C1 EMF (mV) = + 61 log Na14 K140 C2Because 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.
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. Na+ 142 mM K+ 4 mM 14 mM _ _ 140 mM _ _ FIXED _ Nerve cell Membrane potential = -90 mV Axon ofA nerve cell