Lecture 9: Electrical Excitability - The Action Potential

Question 1

What are the 5 properties of action potentials?

Answer 1

1. Change in voltage across membrane
2. Depend on ionic gradients and relative permeability
3. Only occur if a threshold level is reached
4. All or nothing
5. Propagate without loss of amplitude

Question 2

What is the channel activity during an action potential?

Answer 2

Na channels open, leading to influx of Na+, depolarizing membrane, initiating positive feedback loop

Na+ channels inactivate, stopping influx of Na+, during depolarization, K+ channels open and K+ efflux = membrane depolarizes

Question 3

What is the basic structure of a voltage gated Na+ channel?

Answer 3

One alpha subunit consisting of 4 similar sections

Question 4

What is the basic structure of a voltage gated K+ channel?

Answer 4

4 individual alpha subunits

Question 5

How is the Na+ channel activity like after an action potential?

Answer 5

It goes into absolute refractory period where nearly all Na+ channels are in the inactivated state. Then it goes into relative refractory period where Na+ channels are recovering from inactivation, becoming closed and then being able to open again

Question 6

What are the 2 forms of local anaesthetics?

Answer 6

Proton aged and unprotonated

Question 7

What is an example of a local anaesthetic?

Answer 7

Lidocaine

Question 8

What are the 3 ways that local anaesthetics block Na+ channels?

Answer 8

1. Hydrophilic block pathway - when channel is open charged anaesthetic comes in and block it
2. Hydrophobic block pathway - uncharged local anaesthetic comes in through the membrane and causes blockage

Question 9

How can conduction velocity be measured?

Answer 9

Measure time needed for axons for arrive at different points

Question 10

What is the local circuit theory of propagation?

Answer 10

Local depolarization ahead of action potential maintains propagation, bc its past the absolute refractory period, the membrane potential is above threshold and so it moves the action potential along the axon

Question 11

How is conduction velocity linked to fibre diameter?

Answer 11

As diameter decreases, conduction velocity decreases.

Question 12

How does myelination impact conduction?

Answer 12

Acts as good insulator, increasing length constant, enabling the local circuit currents to depolarize next node above threshold and initiate an action potential, so action potential jumps from node to node allowing a much faster conduction velocity

Question 13

What are the 4 ways myelination improves conduction?

Answer 13

1. Large increase in membrane resistance - thickness
2. Large decrease in membrane capacitance - charge density
3. Increase length constant - increase the distance where potential decreased to 37% of its original amplitude
4. Slight decrease in time constant - time taken to fully charge

Question 14

What are some consequences of demyelination?

Answer 14

Density of action current reduced, failure to reach threshold, stops saltatory conduction

Question 15

How is action potential generated?

Answer 15

Depolarization to threshold triggers opening of many voltage-gated Na+ channels, Na+ influx produces upstroke of action potential, depolarizing membrane, causing inactivation of Na+ channels and opening of voltage-gated K+ channels, Na+ influx stops and K+ efflux leads to depolarization

Question 16

How is action potential propagated?

Answer 16

Action potential causes local current flow, leading to an immediate depolarization of adjacent sections of the axon, where this local depolarization reaches threshold, action potential initiated

Question 17

What are nodes of Ranvier?

Answer 17

Areas between myelin sheaths where axon is bare and have a high concentration of Na+ channels

Question 18

What is saltatory conduction?

Answer 18

Action potential jumping from node to node, faster than in unmyelinated axons