ICPP - Action Potential Flashcards
Which type of action potential can be observed to have a plateau in the middle?
Cardiac ventricle
What happens if a cell is depolarised to a level higher than the threshold value?
An action potential occurs
What does the level of conductance of a membrane to a particular ion depend on?
The number of channels for the ion that are open.
How can we prove experimentally that Na+ is responsible for AP depolarisation?
Plot a graph of predicted change in [Na+] against peak of action potential as external [Na+] is reduced.
How is change in concentration calculated in an axon of 1 micrometer in diameter?
Change in concentration = mol/volume
Which rises first during an action potential, Na+ conductance or K+ conductance?
Na+ conductance rises first, then K+ rises slightly later (and doesn’t rise as high).
What sort of mechanism allows the sodium channels to open more and more during the action potential?
Positive feedback
Is the Na+K+ATPase pump involved in the repolarisation of the action potential?
Nope. It just returns the concentrations to normal afterwards.
What is the difference between the ARP and the RRP?
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
What is the function of the S4 region in a voltage-gated Na+ or K+ channel?
It has positive amino acid residues, which contribute to voltage sensitivity.
What is the function of the P region in a voltage gated Na+ or K+ channel?
Contributes to pore selectivity
How many subunits is a functional K+ channel formed from?
Four separate alpha subunits
How many subunits is a voltage gated Na+ channel formed from?
One alpha subunit
Which type of axons are blocked first by local anaesthetics?
Blocked in following order:
- small myelinated axons
- un-myelinated axons
- large myelinated axons
Which type of channels do local anaesthetics block?
Na+ channels
What is the “length constant” of the action potential?
The distance it takes for the potential to fall to 37% of its original value.
What is “capacitance”?
Ability to store charge, which is a property of the lipid bilayer.
What does it mean when a membrane has high capacitance?
Voltage changes more slowly in response to current injection
What does it mean when an axon has high resistance?
Change in voltage spreads further along the axon
What do the nodes of Ranvier have lots of?
High density of Na+ channels (around 10,000 per node)
How are the Na+ channels distributed on an unmyelinated axon?
Evenly across its surface
What is saltatory conduction?
Action potential “jumps” from node to node, allowing a much faster conduction velocity.
In what ways does the myelin sheath improve conduction?
- large increase in membrane resistance
- large decrease in membrane capacitance
- this means length constant is increased
- slight decrease in time constant
Give some examples of disease states affecting conduction of the action potential.
- multiple sclerosis (CNS)
- Devic’s disease (CNS)
- Landry-Guillain-Barre syndrome (PNS)
- Charcot-Marie-Tooth disease (PNS)
