Membrane biophysics

Question 1

What is current?

Answer 1

Rate of flow of charge

Question 2

What is a capacitor?

Answer 2

A device that stores charge, made of two conductors separated by an insulator
One plate stores negative charge while the other stores positive charge

Question 3

How is a membrane thickened?

Answer 3

Through myelination

Question 4

What is the relationship between capacitance, surface area and insulator thickness?

Answer 4

Capacitance is proportional to surface area and inversely proportional to insulator thickness

Question 5

What factors determine the membrane resistance and capacitance?

Answer 5

Membrane resistance depends on density of open ion channel

Capacitance depends on surface area and membrane thickness

Question 6

What is the time constant and what is it equal to?

Answer 6

The time it takes for a membrane potential to change by 63% of its maximum
TC = Membrane resistance x capacitance

Question 7

What is the length constant?

Answer 7

The distance from injection point where a voltage signal has decreased by 63%
The longer the length constant, the better the conductor

Question 8

What are the three components of resistance?

Answer 8

Membrane resistance
Axial resistance
Extracellular fluid resistance

Question 9

What is excitability equal to?

Answer 9

1/threshold current to initiate an AP

Question 10

What is the local circuit theory?

Answer 10

The depolarisation of a region of an axon membrane (active zone) causes induces depolarisation further down down the axon (resting zone). The current can’t move backwards because the preceding region is in its refractory period.

Question 11

What two factors make a good length constant?

Answer 11

A high membrane resistance coupled with a low axial resistance

Question 12

What is a myelin sheath and what does it do?

Answer 12

A Schwaan cell wapped around a neuron
Increases membrane resistance and decreases capacitance
Also increases the length constant

Question 13

Explain Saltatory conduction

Answer 13

Myelinated sheaths are separated by Nodes of Ranvier which have a high density of Na+ and K+ ion channels. These are put in place to regenerate action potentials and allow for rapid depolarisation
APs are passed between them by electrotonic (passive) conduction

Question 14

What is refractoriness?

Answer 14

A decrease in membrane excitability following an AP

Question 15

What does the Nernst equation measure

Answer 15

Equilibrium potential

Question 16

Give two examples of how sodium current is blocked

Answer 16

Tetrodotoxin

Replacement of Na+ with cations that are too large to pass through the channel

Question 17

Why does the resting membrane potential never reach Ek as expected?

Answer 17

It isn’t only the potassium channels that are open

Question 18

What is the advantage of the voltage clamp technique over radioactive ion flux measurements?

Answer 18

It has a greater time resolution, measuring current events on a microsecond timescale

Question 19

How does the voltage clamp technique work?

Answer 19

Membrane potential is controlled and membrane current is measured
A feedback amplifier measures membrane potential and if it isn’t equal to the voltage command (what you choose) it injects a current to return it to that voltage
If the current is enough to activate voltage gated channels, an ionic current is generated and the amplifier injects an equal and opposite charge which is how the membrane current is measured

Question 20

What determines the resting membrane potential?

Answer 20

The selective permeability of the membrane to K+ coupled with different electrochemical gradients

Question 21

How do voltage gated channels know when to open or close?

Answer 21

They have voltage sensing domains

Question 22

What is conductance?

Answer 22

A measure of ability to pass ionic current

Question 23

How does hyperpolarisation occur?

Answer 23

Permeability to K+ is higher than at rest and conduction is higher for K+ than Na+

Question 24

What does the patch clamp technique enable you to do?

Answer 24

Measure the current across a patch of membrane rather than the whole cell using a fine micropipette

Question 25

What is the mechanism behind the absolute refractory period?

Answer 25

The inactivation gate of the channel is closed which makes the channel non-conducting. The channels need time at negative potentials before they can become active again

Question 26

Why is the open probability of potassium channels never zero?

Answer 26

There are two populations, one that opens at the resting membrane potential and another (VG) that respond to changes in membrane potential

Question 27

What does axial resistance depend on?

Answer 27

Axon diameter, the bigger the axon the lower the resistance