Action Potentials Flashcards
What membrane is impermeable/ permeable to Na+ and K+?
‘Resting’ membrane is impermeable to Na+
‘Resting’ membrane is very permeable to K+
How is the RMP generated?
The RMP arises from the separation of charges on either side of the membrane
The RMP is due mainly to diffusion of K+ from cell interior through K+ channels
- The pump contributes by moving 3 Na+ outwards and 2 K+ inwards
What are the values of the Na+/K+ pump?
The pump moves 3 Na+ outwards and 2 K+ inwards
The Na+/K+ pump is ‘electrogenic’ (generates an electrochemical potential across a cell membrane)
What value is the threshold of an action potential?
-55mV
What is the rising phase of AP due to?
due to Na+ influx from voltage gated Na+ channels (causes overshoot)
What is the falling phase due to?
K+ efflux from voltage gated k-channels
What is the value of the overshoot?
+35mV/+30mv
What are the ways ion channels can be gated?
ligand
voltage
In voltage-gated sodium channels, what are the conditions for the channel being open, closed, and closed (refractory)?
m-gate, h-gate
Channel closed
m-gate closed; h-gate open
Channel open
m-gate open; h-gate open
Channel closed (refractory)
m-gate open; h-gate closed
Describe events of an AP?
- Stimulus applied
- Depolarisation
- MP moves towards ‘threshold’
- Gated ion channels closed
- MP reaches ’threshold’ (–55mV)
- Na+ channels start opening
- Na+ influx
- More depolarisation
- K+ channels remain closed
- All Na+ channels open
- Maximum Na+ influx
- MP overshoots 0mV
- When MP reaches +35mV:
- Na+ channels shut Inactivation (‘h’) gate closes
- K+ channels open
- K+ efflux begins
- AP down stroke (‘recovery’ phase)
- Na+ channels shut (refractory period)
- K+ channels open
- K+ efflux continues
- MP returns to ‘resting’ level
- Ion channels return to resting state
- Excitability restored
What happens at threshold in a AP?
- voltage-gated Na+ channels open
- Na+ diffuse in; > further depolarisation
- Positive feedback involved here
What happens at the peak of a AP?
- Na+ channels close; voltage-gated K+ channels open;
- K+ diffuse out; > repolarisation
What is the refractory period?
After an AP is initiated, the neuron cannot generate another AP until the first one has ended
This period of inexcitability is called the
refractory period
It is due to the inactivation of voltage-gated sodium channels (h gates are shut)
What are the consequences of refractory period?
- Limits maximum firing frequency of action potentials in axons
- Ensures unidirectional propagation of action potentials
- Prevents summation of action potentials
- Prevents summation of contractions in cardiac muscle – the cardiac AP lasts as long as the ventricular contraction
How is the AP propagated?
Current flow sets up depolarization zones ahead of AP
AP is regenerated down the axon
Myelin sheath insulates axons, allowing saltatory propagation between nodes
