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
What is membrane resistance?
The number of ion channels open
Higher resistance = faster conduction
Why is a low capacitance and a high resistance good?
Low capacitance means the voltage changes quicker
High resistance means the change in voltage spreads further (less ions likely to leave cell)
What is a myelinated axon and what does it mean for the conduction?
An axon with areas of ‘insulation’ - insulation by Schwann cells (PNS) or ogliodendroctyes (CNS).
Means faster conduction (saltatory conduction) - only Na+ channels at nodes of Ranvier so signal jumps from node to node meaning further spread of local current.
What disease is a result of demyelination?
Multiple Sclerosis (MS)
What does demyelination mean?
Delayed or blocked signals as signal can’t get to next node. Leads to a low resistance and high capacitance as the local current doesn’t spread.
What is the local circuit theory of propagation?
An immediate local change in membrane potential occurs - causing depolarisation of next section (local current spreads)
What does the spread of local current depend on?
Resistance and capacitance
