Bioelectricity Flashcards
How is the measurement of the membrane potential (Vm) different to the patch clamp technique?
Patch clamp technique measures the current across the membrane
In patch clamp - the electrode only TOUCHES the membrane
In the measurement of Vm - the electrode is NARROWER and goes THROUGH the membrane
What is membrane potential (Vm) determined by?
Unequal distribution and SELECTIVE MOVEMENT of ions:
K+
Na+
A-
What is A-?
Large organic anions which DONT pass through the membrane and are present ENTIRELY INTRACELLULAR
What does Na/K ATPase prevent?
How?
Loss of negative membrane potential caused by Na+ leaking into the cell
By transporting Na+ OUT
Which channel is elecrogenic?
Na/K ATPase
What percentage does the Na/K ATPase contribute to in regulation of membrane potential
20%
What does the NERST EQUATION describe?
The equilibrium potential of an ion
THEORETICAL value for the BALANCE between the concentration and potential gradient of an ion
Where there is NO NET MOVEMENT of the ion (NO CURRENT flow)
Describe the model of K+ with semi-permeable membrane
1) High conc. of K+ and A- INSIDE the cell
(A- CANNOT pass through the membrane)
2) Conc gradient forces K+ OUT of the cell
3) Cause a negative INTERCELLULAR charge - attracts POSITIVE K+ ions
4) Eventually there is a balance between K+ in and K+ out
What is the equation for equilibrium potential of an ion?
Eion = (RT/zF) x log ([ion]out / [ion]in)
R = gas CONSTANT T = temperature in KELVIN F = faradays CONSTANT
z =VALANCE ion (charge)
NOTE: at 37º (RT/zF) = 61.2
What is the valance of the ion?
The charge:
eg. Na+ = +1
K+ = +1
Ca+ = +2
Cl- = -1
What would the resting membrane potential be if the membrane was ONLY permeable to K+?
What is the resting membrane potential?
What does this mean?
-90mV
Actually, resting membrane potential is -70mV
Means that at rest, many ions are travelling across the membrane
What is the conc of Na+ inside/outside the cell?
Inside: 15mM
Outside: 150mM
What is the conc of K+ inside/outside the cell?
Inside: 150mM
Outside: 5mM
What is the conc of A- inside/outside the cell?
Inside: 65mM
Outside: 0mM
Describe the contribution of Na+ to the resting membrane potential
1) Large conc of Na+ OUTSIDE of the cell
2) Na+ moves DOWN conc gradient (into the cell)
3) Generates POSITIVE charge inside the cell
4) This +ve charge repels more Na+ coming into the cell through the potential gradient
5) Eventually balance
What is the potential gradient?
Gradient where -ve ions attract +ve ions
What is the Eion of Na+?
+61mV
As the Vm is closer to Ek than Ena, what does this show?
The membrane is more permeable to K than Na at rest
What is the permeability of the membrane at rest to K+ if to Na+ it is 1?
50-75
What can permeability ratios be used for?
Goldmann equation - to determine a more accurate membrane potential
What is the permeability of the membrane at rest to A- if to Na+ it is 1?
0
What is the Goldmann eqn?
Vm = (RT/zF) log (Pna [Na]out + Pk [K]out / Pna [Na]in + Pk [K]in)
NOTE: at 37º (RT/zF) = 61.2
When does the membrane potential change?
Action potential
What channels make changes to the membrane potential?
Ion channels
Describe the process of an action potential (ions channels)
1) Resting potential = -70mV
(close to Ek - many K channel open) (far from Ena - not many Na channels open)
2) Triggering event - depolarisation above threshold
3) Membrane permeability to Na increase (opening of Na channels)
4) Potential closer to Ena
5) Na channels close and K channels open
6) Potential back towards Ek
7) Undershoot
8) Back to resting potential
Describe the action of the Na/AA cotransporter
Binds to Na and AA to bring them both INTO the cell
Using the Na+ gradient which is set up by the Na/K ATPase to do so
What can help to sustain Na+ coupled transport at a high level whilst maintaining a NORMAL membrane potential?
Activating K+ channels, which drive the Vm towards Ek
When the temperature is 37º, what should be used in the Goldman and Nerst equations?
61.2 x log (…….)
