ICPP - Action Potential

Question 1

Which type of action potential can be observed to have a plateau in the middle?

Answer 1

Cardiac ventricle

Question 2

What happens if a cell is depolarised to a level higher than the threshold value?

Answer 2

An action potential occurs

Question 3

What does the level of conductance of a membrane to a particular ion depend on?

Answer 3

The number of channels for the ion that are open.

Question 4

How can we prove experimentally that Na+ is responsible for AP depolarisation?

Answer 4

Plot a graph of predicted change in [Na+] against peak of action potential as external [Na+] is reduced.

Question 5

How is change in concentration calculated in an axon of 1 micrometer in diameter?

Answer 5

Change in concentration = mol/volume

Question 6

Which rises first during an action potential, Na+ conductance or K+ conductance?

Answer 6

Na+ conductance rises first, then K+ rises slightly later (and doesn’t rise as high).

Question 7

What sort of mechanism allows the sodium channels to open more and more during the action potential?

Answer 7

Positive feedback

Question 8

Is the Na+K+ATPase pump involved in the repolarisation of the action potential?

Answer 8

Nope. It just returns the concentrations to normal afterwards.

Question 9

What is the difference between the ARP and the RRP?

Answer 9

ARP (absolute refractory period) - nearly all Na+ channels inactivated, shorter, cannot be overcome

RRP (relative refractory period) - Na+ channels recovering from inactivation, excitability returning towards normal, can be overcome by strong enough action potential

Question 10

What is the function of the S4 region in a voltage-gated Na+ or K+ channel?

Answer 10

It has positive amino acid residues, which contribute to voltage sensitivity.

Question 11

What is the function of the P region in a voltage gated Na+ or K+ channel?

Answer 11

Contributes to pore selectivity

Question 12

How many subunits is a functional K+ channel formed from?

Answer 12

Four separate alpha subunits

Question 13

How many subunits is a voltage gated Na+ channel formed from?

Answer 13

One alpha subunit

Question 14

Which type of axons are blocked first by local anaesthetics?

Answer 14

Blocked in following order:

• small myelinated axons
• un-myelinated axons
• large myelinated axons

Question 15

Which type of channels do local anaesthetics block?

Answer 15

Na+ channels

Question 16

What is the “length constant” of the action potential?

Answer 16

The distance it takes for the potential to fall to 37% of its original value.

Question 17

What is “capacitance”?

Answer 17

Ability to store charge, which is a property of the lipid bilayer.

Question 18

What does it mean when a membrane has high capacitance?

Answer 18

Voltage changes more slowly in response to current injection

Question 19

What does it mean when an axon has high resistance?

Answer 19

Change in voltage spreads further along the axon

Question 20

What do the nodes of Ranvier have lots of?

Answer 20

High density of Na+ channels (around 10,000 per node)

Question 21

How are the Na+ channels distributed on an unmyelinated axon?

Answer 21

Evenly across its surface

Question 22

What is saltatory conduction?

Answer 22

Action potential “jumps” from node to node, allowing a much faster conduction velocity.

Question 23

In what ways does the myelin sheath improve conduction?

Answer 23

• large increase in membrane resistance
• large decrease in membrane capacitance
• this means length constant is increased
• slight decrease in time constant

Question 24

Give some examples of disease states affecting conduction of the action potential.

Answer 24

• multiple sclerosis (CNS)
• Devic’s disease (CNS)
• Landry-Guillain-Barre syndrome (PNS)
• Charcot-Marie-Tooth disease (PNS)