1.
Describe the method for measurement of membrane
potential
2.
Define resting membrane potential.
3.
Discuss the ionic basis of resting membrane potential.
4.
Describe the role of sodium-potassium pump in
maintenance of resting membrane potential.
----
+
+
+
+
+
-
+
What are the charged things
that run through our body
fluids? Electrolytes?
1. Anions
-Large intracellular proteins
-Chloride Cl2. Cations (+)
+Sodium Na+
+Potassium K+
+Calcium Ca++
1.
Current: the flow of charge
2.
Voltage: separation of opposite
charges (mV)
Resistance: opposition to charge
movement (friction)
Conductance: allowing a charge to
move (permeability)
3.
4.
3
+
+
+
+
_ _ _ _
_ _ _ _
_ _ _ _
+
+
+
+
+
+
+
+
Inside of the cell is negative due
to :
1. Abundance of negatively
charged proteins
+
+
+
+
2. Na+/K+ ATPase (net loss of
positive charges~ 4mV)
3. Membrane is 100x more
permeable (“leaky”) to K+
4
Membrane permeable to water
Osmotic
Pressure
1. Compartments of the body are in a state of osmotic
equilibrium but in a state of chemical and electrical
disequilibrium.
2. The electrical disequilibrium (resulting from separation
of charge across the membrane) is of prime importance
to electrical signalling in nerve and muscle.
Electricity Review
1. 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.
2. Opposite charges attract and like charges repel.
3. Energy is needed to separate charge.
4. If separated charges could move towards one another, the
material through which they are moving is called a conductor.
5. If the material prevents the movement of separate charges,
the material is called an insulator. The cell membrane is a
good insulator.
6. Static electricity arises from the separation of electric charge.
-Ions in solutions are in random motion
1. Concentration gradient
2. Electrostatic gradient
3. Differential Permeability of the Membrane (passive)
4. Leads to osmosis
Resting Membrane Potential
outside
+
Na
K+
Cl
Membrane
inside
A
Na+
+
K
Cl-
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
1) At rest, K+ leak results in a
negative membrane
Na+
Cl -
K+
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
membranes of cells in the resting condition are, polarized
which means that they show an electrical potential
difference,
4 factors
1. Polarity of each ion
2. Membrane permeability of the ions(Na+ and K+)during
the resting state
3. Concentrations of respective ions on both sides: (i=
inside), (o= outside) especially K+ across the membrane
4. Na+-K+ pump
Bioelectric Potential
OUTSIDE
POS
INSIDE
NEG
0 mV
+
+
-
+
-
+
+
-
+
-
+
-
-55 TO -90 mV
+
[K+] = 4
[Na+] = 142
[Cl-] = 103
A+
+
-
+
-
+
+
-
+
-
+
-
-55 TO -90 mV
[K+] = 140
[Na+] = 10
[Cl-] = 4
A-
-
+
Patch clamp recording
Suction
"Giga-seal"
1 µm
Cytoplasm
Glass
microelectrode
Ion channels
Cell Membrane
16
Single channel record
Closed
4 pA
Open
100 ms
18
Resting Membrane Potential
outside
+
+
+
+
+
+
+
+
+
+
+
-
-
-
-
Membrane
inside
-
-
-
-
-
-
Simplest Case Scenario:
inside
outside
If a membrane were permeable
to only K+ then…
+
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).
K+
Resting Membrane Potential
Vm -90 to -70
0 mV
ENa +61
EK -94
Why is Vm so close to EK?
Ans. The membrane is far more
permeable to K than Na..
Na+/K+ ATPase
(Electrogenic pump)
1.
2.
3.
carrier protein located on the plasma membrane of all cells
plays an important role in regulating osmotic balance by maintaining Na+ and
balance e.g (inhibition by ouabain causes cells to swell and burst!)
requires one to two thirds of cells energy!
4.  subunit
1. 100,000 MW
2. binds ATP, 3 Na+, and 2 K+
5.  subunit
1. 55,000 MW
2. function ???
Transport is electrogenic but contributes
less than 10% to the membrane potential(-4mv)
Inside
Outside
Na+
Na+
Na+
K+
K+
ATP
Inside
Outside
K+
K+
1.
2.
Regulation of cell volume
1. “fixed anions” attract cations causing osmosis
2. cell swelling stimulates the Na+- K+ pump to
 ion concentration,  osmolarity and cell swelling
Heat production (thyroid hormone increase # of pumps; heat a byproduct)
3.
Maintenance of a membrane potential in all cells
1. pump keeps inside negative, outside positive
4.
Secondary active transport (No ATP used)
1. steep concentration gradient of Na+ and K+ maintained across the cell
membrane
2. carriers move Na+ with 2nd solute easily into cell
1. SGLT saves glucose in kidney
Measurements of Vm have shown that many types of cells are
electrically excitable. Examples of excitable cells
1.
2.
3.
Neurons,
Skeleton and smooth muscle fibers, heart muscle cells,
Secretory cells of the pancreas
4.
Macrophages
5.
Ciliated epithelial cells