1. Electrical properties of cell membrane II (Diffusion & Equilibrium Potential) DR QAZI IMTIAZ RASOOL

2. OBJECTIVES Describe the method for measurement of membrane potential
Define resting membrane potential.
Discuss the ionic basis of resting membrane potential.
Describe the role of sodium-potassium pump in maintenance of resting membrane potential.

3. Electricity - Current: the flow of charge - - - - - - - +
Voltage: separation of opposite charges (mV)
Resistance: opposition to charge movement (friction)
Conductance: allowing a charge to move (permeability)
What are the charged things that run through our body fluids? Electrolytes?
1. Anions
-Large intracellular proteins
-Chloride Cl-
2. Cations (+)
+Sodium Na+
+Potassium K+
+Calcium Ca++

4. Not just separation of solutes, but charges, too!+
_ _ _ _ + +
Inside of the cell is negative due to :
Abundance of negatively charged proteins
Na+/K+ ATPase (net loss of positive charges~ 4mV)
Membrane is 100x more permeable (“leaky”) to K+ +

5. Membrane permeable to water
Osmotic Pressure
Compartments of the body are in a state of osmotic equilibrium but in a state of chemical and electrical disequilibrium.
The electrical disequilibrium (resulting from separation of charge across the membrane) is of prime importance to electrical signalling in nerve and muscle.

6. Electricity Review
Law of conservation of charge: the net amount of electric charge produced in a system is zero. ie. for every +ve charge on an ion, there is an electron on another ion. Overall, the body is electrically neutral.
Opposite charges attract and like charges repel.
Energy is needed to separate charge.
If separated charges could move towards one another, the material through which they are moving is called a conductor.
If the material prevents the movement of separate charges, the material is called an insulator. The cell membrane is a good insulator.
Static electricity arises from the separation of electric charge.

7. Random Motion (passive)
-Ions in solutions are in random motion
Concentration gradient
Electrostatic gradient
Differential Permeability of the Membrane (passive)
Leads to osmosis
thus, any time that there is an accumulation of a particular class of ions in one area, the probability is increased that random motion will move ions out of this area (because there are more ions available to leave) and the probability is decreased that random motion will move more ions into the area (because there are fewer ions available to come in)
like charges repel and opposite charges attract; therefore electrostatic pressure disperses any accumulation of positive or negative charges in an area
ions pass through membrane at special pores (made of proteins) called ion channels
when neurons are at rest, the membrane is:
totally resistant to the passage of protein ions,
extremely resistant to the passage of Na+ ions
and only slightly resistant to the passage of K+ ions and Cl- ions

8. Cl- Na+ K+ A- Resting Membrane Potential Membrane K+ Cl- Na+ outside
inside

9. Membrane voltage = Membrane potential
Resting membrane potential – RMP
Potential difference between a microelectrode inside the cell (-ve potential) and a surface electrode outside the cell (zero potential)
Membrane voltage = Membrane potential
RMP is the electrical gradient across the cell membrane.
Resting: the membrane potential has reached a steady state and is not changing.
Extracellular space
Intracellular space
Extracellular space

10. Resting Membrane Potential
1) At rest, K+ leak results in a negative membrane K+
Na+
Cl -
Why? Positive Ions moving OUT of a cell
result in fewer positive ions inside the cell
This results in a MORE NEGATIVE ICF
2) Chloride leak ensures stabilization of resting potential
Neg. ions moving out make membrane a little more positive

11. Features of RMP 4 factors
membranes of cells in the resting condition are, polarized which means that they show an electrical potential difference,
4 factors
Polarity of each ion
Membrane permeability of the ions(Na+ and K+)during the resting state
Concentrations of respective ions on both sides: (i= inside), (o= outside) especially K+ across the membrane
4. Na+-K+ pump
Membrane potential refers to a separation of charges across the membrane or a difference in the relative number of cations and anions in the ICF and ECF.

12. Bioelectric Potential
OUTSIDE
POS
NEG
INSIDE

13. 0 mV

14. + + + + + - - - + - + - + - -
-55 TO -90 mV

15. [K+] = 4
[Na+] = 142
[Cl-] = 103 A- + + + + + - - - + - + - + - -
[K+] = 140
[Na+] = 10
[Cl-] = 4 A-
-55 TO -90 mV

16. Patch clamp recording Cell Membrane Glass microelectrode Suction 1 µm
"Giga-seal"
Cell Membrane
Cytoplasm
Ion channels

18. Single channel record
Closed
4 pA
Open
100 ms

19. + + + + + + + + + + + - - - - - - - - - - -
Resting Membrane Potential
outside + + + + + + + + + + +
Membrane - - - - - - - - - - -
inside

20. Simplest Case Scenario:
inside
outside
If a membrane were permeable to only K+ then… K+ K+
K+ would diffuse down its concentration gradient until the electrical potential across the membrane countered diffusion.
The electrical potential that counters net diffusion of K+ is called the K+ equilibrium potential (EK).

21. Resting Membrane Potential
Vm -90 to -70
EK -94
ENa +61
0 mV
Vm for warm blooded animals in skeletal muscle and nerve is between -55 and -100 mV. Vm is between -55 and -30 for smooth muscle fibers. Most textbooks give a value of -70 mV for nerve and skm.
Why is Vm so close to EK?
Ans. The membrane is far more permeable to K than Na..

22. Na+/K+ ATPase (Electrogenic pump)  subunit binds ATP, 3 Na+, and 2 K+
carrier protein located on the plasma membrane of all cells
plays an important role in regulating osmotic balance by maintaining Na+ and K+ balance e.g (inhibition by ouabain causes cells to swell and burst!)
requires one to two thirds of cells energy!
 subunit
100,000 MW
binds ATP, 3 Na+, and 2 K+
 subunit
55,000 MW
function ???
Transport is electrogenic but contributes
less than 10% to the membrane potential(-4mv)

23. Inside
Outside
Na+
Na+
Na+
Na+ K+ K+ K+
ATP

24. Inside
Outside
Na+ K+ K+ K+ K+

26. Functions of Na+ -K+ Pump
Regulation of cell volume
“fixed anions” attract cations causing osmosis
cell swelling stimulates the Na+- K+ pump to  ion concentration,  osmolarity and cell swelling
Heat production (thyroid hormone increase # of pumps; heat a by- product)
Maintenance of a membrane potential in all cells
pump keeps inside negative, outside positive
Secondary active transport (No ATP used)
steep concentration gradient of Na+ and K+ maintained across the cell membrane
carriers move Na+ with 2nd solute easily into cell
SGLT saves glucose in kidney

27. SUMMARY
Measurements of Vm have shown that many types of cells are electrically excitable. Examples of excitable cells
Neurons,
Skeleton and smooth muscle fibers, heart muscle cells,
Secretory cells of the pancreas
Macrophages
Ciliated epithelial cells