M6 L2: Action potentials Flashcards
What is an action potential
a brief fluctuation in (ms in axons, slightly longer in soma) membrane potential due to the transient opening of voltage gated ion channels which spreads like a wave along axon.
What is the difference between hyperpolarisation and depolarisation
Hyperpolarisation is when mp becomes more negative- moves closer to Ek, away from Ena
Depolarisation is when mp becomes less negative - closer to Ena than Ek
What is the significance of action potentials
help to start signal transmission along axons, and codes information
What initial * stage before an action potential, the event and relative permeability to ions
Pre-potential initiation stage: physical (current, light, stretch) or chemical (drug or neurotransmitter) stimulus acts on membrane channels and triggers a slow and graded depolarisation from -65 to -55 mV. The voltage gated Na+ channels start to open near threshold Pk>Pna
Name the three stages of action potentials and the * stage
RMP to threshold in pre-potential initiation stage.
- fast depolarisation
- Repolarisation
- After-hyperpolarisation
What is the first step of action potential, the event and relative permeability to ions
Fast depolarisation
After the MP reaches threshold there is a fast depolarisation to +30mV (overshoot=over 0). because of voltage gated Na+ channels opening very fast. PNa»>Pk
What is the second step of action potential, the event and relative permeability to ions
Repolarisation
Na+ channels inactivate and voltage gated K+ channels open, bringing the mp closer to eq.potential for K+ so depolarising. Pk»Pna
What is the third step of action potential, the event and relative permeability to ions
After-hyperpolarisation
Voltage gated K+ channels remain open for a while then close and this causes the MP to go below RMP and then back up. PK»>Pna and then Pk»Pna
What is the absolute refractory period
Period of time (during fast depolarisation) and repolarisation) when even if you apply a strong stimulus it will not trigger another AP
What is the relative refractory period
During after hyperpolarisation, a 2nd stimulus can trigger another action potential but it has to be much stronger as it needs to depolarise even more to reach threshold
What is the change in PK: PNa ratio when the 1. voltage gated Na+ channels open
2. Voltage gated K+ channels open, Na+ channels close
It goes from 40:1 to 1: 20
- 100:1
What is the key role of voltage gated Na+ channels in AP
Na+ channels activate for a fraction of a millisecond causing fast influx of Na+ down both conc and electrical gradient. It stops because inside potential becomes more positive and quick inactivation.
What is the difference between action potentials and subthreshold depolarisation
Action potential is all or nothing 100mV constant amplitude and doesn’t depend on stimulus provided its suprathreshold. Whereas subthreshold is graded.
When action potentials are evoked experimentally by electrical stimulation (battery) how does it work
Current generated by battery flows through 2 main paths : 1, outside from + to - and 2ndly across membrane and inside the axon. As current follows the path of least resistance, most of the current goes through path1. but path 2 can change the RMP-> action potential in both directions
When the current generated by an outside source flows through the cell membrane from outside to inside what is happening to MP
The MP becomes more negative (hyperpolarisation)
