Bioelectricity

Question 1

How is the measurement of the membrane potential (Vm) different to the patch clamp technique?

Answer 1

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

Question 2

What is membrane potential (Vm) determined by?

Answer 2

Unequal distribution and SELECTIVE MOVEMENT of ions:
K+
Na+
A-

Question 3

What is A-?

Answer 3

Large organic anions which DONT pass through the membrane and are present ENTIRELY INTRACELLULAR

Question 4

What does Na/K ATPase prevent?

How?

Answer 4

Loss of negative membrane potential caused by Na+ leaking into the cell

By transporting Na+ OUT

Question 5

Which channel is elecrogenic?

Answer 5

Na/K ATPase

Question 6

What percentage does the Na/K ATPase contribute to in regulation of membrane potential

Answer 6

20%

Question 7

What does the NERST EQUATION describe?

Answer 7

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)

Question 8

Describe the model of K+ with semi-permeable membrane

Answer 8

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

Question 9

What is the equation for equilibrium potential of an ion?

Answer 9

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

Question 10

What is the valance of the ion?

Answer 10

The charge:

eg. Na+ = +1
K+ = +1
Ca+ = +2
Cl- = -1

Question 11

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?

Answer 11

-90mV

Actually, resting membrane potential is -70mV

Means that at rest, many ions are travelling across the membrane

Question 12

What is the conc of Na+ inside/outside the cell?

Answer 12

Inside: 15mM

Outside: 150mM

Question 13

What is the conc of K+ inside/outside the cell?

Answer 13

Inside: 150mM

Outside: 5mM

Question 14

What is the conc of A- inside/outside the cell?

Answer 14

Inside: 65mM

Outside: 0mM

Question 15

Describe the contribution of Na+ to the resting membrane potential

Answer 15

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

Question 16

What is the potential gradient?

Answer 16

Gradient where -ve ions attract +ve ions

Question 17

What is the Eion of Na+?

Answer 17

+61mV

Question 18

As the Vm is closer to Ek than Ena, what does this show?

Answer 18

The membrane is more permeable to K than Na at rest

Question 19

What is the permeability of the membrane at rest to K+ if to Na+ it is 1?

Answer 19

50-75

Question 20

What can permeability ratios be used for?

Answer 20

Goldmann equation - to determine a more accurate membrane potential

Question 21

What is the permeability of the membrane at rest to A- if to Na+ it is 1?

Answer 21

0

Question 22

What is the Goldmann eqn?

Answer 22

Vm = (RT/zF) log (Pna [Na]out + Pk [K]out / Pna [Na]in + Pk [K]in)

NOTE: at 37º (RT/zF) = 61.2

Question 23

When does the membrane potential change?

Answer 23

Action potential

Question 24

What channels make changes to the membrane potential?

Answer 24

Ion channels

Question 25

Describe the process of an action potential (ions channels)

Answer 25

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

Question 26

Describe the action of the Na/AA cotransporter

Answer 26

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

Question 27

What can help to sustain Na+ coupled transport at a high level whilst maintaining a NORMAL membrane potential?

Answer 27

Activating K+ channels, which drive the Vm towards Ek

Question 28

When the temperature is 37º, what should be used in the Goldman and Nerst equations?

Answer 28

61.2 x log (…….)