Action Potential Flashcards
What is action potential?
A change in voltage across a membrane.
Only occurs if the threshold level is reached, an ‘all or nothing’ process.
During an action potential, when do the Na+ channels open and close?
They open just before the threshold point, when a certain level of depolarisation is met in the cell, it initiates the opening of Na+ channels to go over the threshold and reach an action potential.
After that potential is met, the Na+ channels close, causing rapid depolarisation.
When do K+ channels open in the action potential cycle?
K+ channels open soon after the Na+ channels close. Hyperpolarisation ‘overshoots’ the resting membrane potential, so leaky K+ channels opens to bring the cell back to equilibrium, from about -90mv to -70mv.
What is the absolute refractory period?
The time between the opening and closing of Na+ channels. Low excitability.
What is the relative refractory period?
The recovery period after Na+ channels close, cell returns to normal resting potential.
Excitability returns to high.
How to local anaesthetics work in regard to action potential?
They bind to Na+ channels and block them, stoping action potential from occurring.
They are generally weak bases that can cross a membrane in an un-ionised form, and can get in more easily when Na+ channels are open.
What is conduction velocity and what does it determine?
The speed at which an electrochemical impulse propagates down a neural pathway.
It is determined by how far along a nerve axon the current spreads.
How do you calculate conduction velocity?
Distance (to jump between nodes) divided by time (to cross to next node).
How does conduction velocity affect local current spread?
The FURTHER the local current spread down the axon the FASTER the conduction velocity will be.
What is capacitance?
The ability to store charge.
This is a property of the lipid bilayer.
If a cell has a high capacitance, then it holds in the charge more therefore cannot spread as far
HIGH CAPACITANCE = LOW CONDUCTION VELOCITY
What is membrane resistance?
How many ion channels are open.
Low membrane resistance = lots of open channels, therefore current is lost and there is limited local spread.
HIGH MEMBRANE RESISTANCE = HIGH CONDUCTION VELOCITY
What is the structure of a myelin sheath?
A Schwann cell (specific to myelin)
Axon in the middle followed by a spiral folds surrounding it.
Also contains a nucleus.
How does a myelin sheath impact on conduction velocity, capacitance and membrane resistance?
Increases conductance velocity, reduces capacitance, increase membrane resistance.
What lies in between the myelin sheaths?
Nodes of Ranvier.
How does an unmyelinated axon compare to a myelinated axon is term of Na+ channel spread?
Unmyelinated - even spread of channels.
Myelinated - High concentration of Na+ channels are nodes of Ranvier, less in the internode.
