S5 - Action Potentials Flashcards
What is the all-or-nothing principle?
If the threshold potential is reached, an action potential will occur and won’t stop until the ionic ‘flood’ is complete.
If the threshold is not reached, no action potential occurs.
What factors mean the threshold potential is more likely to be reached?
Lots of Na+ channels and a larger axon diameter
Why does a larger axon diameter increase conduction velocity?
Less resistance for the ion flow
How do you calculate conduction velocity?
Distance traveled (m)/time to complete one reflex arc (s)
What 3 factors need to be considered for an action potential to occur?
- A good ionic gradient?
- Threshold reached?
- Out of refractory period?
What is absolute refractoriness?
All Na+ channels are inactivated, no action potential is produced.
What is relative refractoriness?
Most Na+ channels inactivated, but some are closed/ready to open again so an action potential can occur if a big enough stimulus is applied.
What is the refractory period?
A recovery period, when no/less action potentials can be produced - it allows ion gradients to be re-established.
How many alpha-subunits do the Na+ and K+ channels have?
Na+ has 1
K+ has 4
What do voltage gated ion channels contain?
A pore region, a voltage sensor and inactivation gates that block the poor region after activation.
What is the order at which local anaesthetics affect axons?
- Small myelinated
- Un-myelinated
- Large myelinated
What do local anaesthetics do?
They block Na+ channels, preventing depolarisation and so preventing an action potential in axons responsible for us feeling pain.
What does ‘use-dependent’ block mean in terms of local anaesthetics?
The anaesthetic has a preference for blocking Na+ channels that are open or in the inactivated state (bind more strongly this way)
What is an example of a local anaesthetic?
Lidocaine
What is capacitance?
The ability for a membrane to store charge.
Low capacitance = faster conduction
