Lecture 4.1 Flashcards
What are the relative durations of the SA node, cardiac ventricle, skeletal muscle and neurone axon action potentials?
SA node and cardiac ventricle - long
Skeletal muscle and neuronal axon - short
What are the 5 properties of an action potential?
- All or nothing potential
- Propagated wit loss of amplitude
- Change in voltage across membranes
- Depends on ionic gradients and relative permeability
- Only occurs if a threshold is reached
How are action potentials generated?
Sodium hypothesis of an AP: an increase in permeability to sodium ions, bringing the membrane potential closer to ENa.
What is the sodium hypothesis of the action potential?
Sodium channels open - membrane becomes less negative
After action potential - sodium channels close and potassium channels open, membrane becomes more negative
What is the voltage clamp? What does it measure?
Measures the current of the membrane at a specific voltage (membrane potential) and therefore shows the effect of voltage on huge number of sodium and potassium channels open at different membrane potentials
What is the effect of voltage (membrane potential) on sodium channels?
The more depolarised the membrane potential is, the quicker the inflow of sodium but the quicker the sodium channels become inactivated
What is the effect of voltage on potassium channels?
As the membrane potential becomes more depolarised, the activation speed of potassium ion channels increases. Potassium channels take longer than sodium channels to activate. Potassium channels don’t inactivate until the membrane potential is back to normal.
Why does hyper polarisation occur with an action potential?
Potassium conductance remains higher than normal even when the membrane potential has returned to resting as potassium channels take longer to close than sodium channels. This causes hyperpolarisation.
What is the upstroke of an action potential known as?
Depolarisation - only occurs if a threshold is reached. After which, a system of positive feedback occurs.
How are stronger signals achieved to generate an AP?
Presynaptic neurones release more NT
What is the downstroke of an action potential?
Repolarisation
Is the Na-K-ATPase pump involved in AP repolarisation?
No
How is repolarisation achieved?
Inactivation of sodium channels so sodium influx stops and reflux of potassium channels cause repolarisation.
What is excitability?
A cell’s ability to fire an action potential. 0-1 < can only fire at 1
What do depolarisation and repolarisation do to sodium channels?
Depolaristion opens closed sodium channels and inactivated opened sodium channels.
Repolarisation closes inactivated sodium channels.
After inactivation, how can sodium channels open again?
After inactivation, they must recover before reopening and can only achieve this when the membrane potential has returned to rest
What’s the absolute refractory period?
Nearly all sodium channels are inactive and the cell can not fire a stimulus no matter how much it’s stimulated
What is the relative refractory period?
Period where sodium channels are recovering from inactivation and excitability is returning to normal as number of inactive channels decrease. The axon can be stimulated to generate an AP but a stronger stimulus is needed so synapse needs to fire more transmitter or closer to axon hillock.
How are sodium channels inactivated?
Once open, they we susceptible to inactivation by an inactivating molecule
How does accommodation occur?
- Continual small depolarisation which opens sodium channels but depolarisation is not large enough to surpass threshold
- More and more sodium channels are inactivated so less can be worked with
- Threshold becomes more positive
- So the longer the stimulus, the larger the depolarisation needed to fire the AP because of accommodation, a depolarisation that was large enough to surpass threshold before will no longer pass threshold
Chemically, what are local anaesthetics?
They are weak bases that cross the membrane in an ionised form.
Name a type of local anaesthetics. How does it work?
Procaine. Works by blocking Na+ channels (easier to block open channels but also has a high affinity for inactive channels) and therefore blocks AP from firing
What is the order for which local anaesthetics block conduction in nerve fibre types?
Small myelinated axons
Non-myelinated axons
Large myelinated axons
