Lecture 4.1

Question 0

What are the relative durations of the SA node, cardiac ventricle, skeletal muscle and neurone axon action potentials?

Answer 0

SA node and cardiac ventricle - long

Skeletal muscle and neuronal axon - short

Question 1

What are the 5 properties of an action potential?

Answer 1

1. All or nothing potential
2. Propagated wit loss of amplitude
3. Change in voltage across membranes
4. Depends on ionic gradients and relative permeability
5. Only occurs if a threshold is reached

Question 2

How are action potentials generated?

Answer 2

Sodium hypothesis of an AP: an increase in permeability to sodium ions, bringing the membrane potential closer to ENa.

Question 3

What is the sodium hypothesis of the action potential?

Answer 3

Sodium channels open - membrane becomes less negative

After action potential - sodium channels close and potassium channels open, membrane becomes more negative

Question 4

What is the voltage clamp? What does it measure?

Answer 4

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

Question 5

What is the effect of voltage (membrane potential) on sodium channels?

Answer 5

The more depolarised the membrane potential is, the quicker the inflow of sodium but the quicker the sodium channels become inactivated

Question 6

What is the effect of voltage on potassium channels?

Answer 6

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.

Question 7

Why does hyper polarisation occur with an action potential?

Answer 7

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.

Question 8

What is the upstroke of an action potential known as?

Answer 8

Depolarisation - only occurs if a threshold is reached. After which, a system of positive feedback occurs.

Question 9

How are stronger signals achieved to generate an AP?

Answer 9

Presynaptic neurones release more NT

Question 10

What is the downstroke of an action potential?

Answer 10

Repolarisation

Question 11

Is the Na-K-ATPase pump involved in AP repolarisation?

Answer 11

No

Question 12

How is repolarisation achieved?

Answer 12

Inactivation of sodium channels so sodium influx stops and reflux of potassium channels cause repolarisation.

Question 13

What is excitability?

Answer 13

A cell’s ability to fire an action potential. 0-1 < can only fire at 1

Question 14

What do depolarisation and repolarisation do to sodium channels?

Answer 14

Depolaristion opens closed sodium channels and inactivated opened sodium channels.
Repolarisation closes inactivated sodium channels.

Question 15

After inactivation, how can sodium channels open again?

Answer 15

After inactivation, they must recover before reopening and can only achieve this when the membrane potential has returned to rest

Question 16

What’s the absolute refractory period?

Answer 16

Nearly all sodium channels are inactive and the cell can not fire a stimulus no matter how much it’s stimulated

Question 17

What is the relative refractory period?

Answer 17

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.

Question 18

How are sodium channels inactivated?

Answer 18

Once open, they we susceptible to inactivation by an inactivating molecule

Question 19

How does accommodation occur?

Answer 19

1. Continual small depolarisation which opens sodium channels but depolarisation is not large enough to surpass threshold
2. More and more sodium channels are inactivated so less can be worked with
3. Threshold becomes more positive
4. 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

Question 20

Chemically, what are local anaesthetics?

Answer 20

They are weak bases that cross the membrane in an ionised form.

Question 21

Name a type of local anaesthetics. How does it work?

Answer 21

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

Question 22

What is the order for which local anaesthetics block conduction in nerve fibre types?

Answer 22

Small myelinated axons
Non-myelinated axons
Large myelinated axons