4 - Action Potentials II

Question 1

What is a signal?

Answer 1

Change in voltage (deviation from RMP)

Question 2

What are the 2 methods of transporting an electrical signal?

Answer 2

• passive

- active

Question 3

How does passive transport of a signal work?

Answer 3

Diffusion of ions through cytoskeleton. Doesn’t need additional energy.

Question 4

Why is passive ion diffusion unreliable for transferring signals between neurons?

Answer 4

Slow and breaks down over long distances

-> signal gets smaller as it propagates across distance because of K2p leak channels (K+ always leaking & pushing mempot back to RMP)

Question 5

Are action potentials passive or active?

Answer 5

Active - require energy

Question 6

Are APs reliable?

Answer 6

Yes - doesn’t break down over long distances

Question 7

Why is the equilibrium pot for Na+ not reached during an AP?

Answer 7

Because K2p channels always leaking, Nav channels start closing and Kv start opening so overshoot doesn’t reach ENa+

Question 8

What are the 6 steps in an AP?

Answer 8

1. Depolarisation to threshold
2. Nav channels open - rapid depolarisation
3. Nav channels close - repolarisation
4. Kv delayed rectifier K+ channels open - hyperpolarisation
5. Nav channels inactivate - absolute refractory period
6. Kv channels close - relative refractory period

Question 9

1. Depolarisation to threshold

Answer 9

AP begins when membrane depolarised past threshold, allowing Nav channels to open. Mempot rapidly heads towards ENa

Question 10

What are causes of initial membrane depolarisation?

Answer 10

Physically-gated Na channels open or depolarisation may be ‘inherited’ from somewhere else in neuron

Question 11

2. Nav channels open - rapid depolarisation

Answer 11

Vm moves towards ENa and as membrane continues to depolarise, more Nav channels open (positive feedback)

Question 12

3. Nav channels close - repolarisation

Answer 12

Nav channels only open for 1ms and then close and inactivate.

As Nav channels close, return to PNa < Pk and Vm moves towards Ek

Question 13

Can Vm still move towards EK if there are no Kv channels?

Answer 13

Yes - but if we do have Kv channels, hyperpolarisation can occur

Question 14

4. Kv delayed rectifier K+ channels open - hyperpolarisation

Answer 14

K2p channels stay open. Kv channels open at high voltages but they’re delayed. Mempot hyperpolarises below RMP as Vm approaches Ek

Question 15

5. Nav channels inactivate - absolute refractory period

Answer 15

Nav channels inactive until Vm hyperpolarised. Can’t have 2nd AP for at least 1ms.

Question 16

6. Kv channels close - relative refractory period

Answer 16

Delayed rectifier Kv channels take sometime to close — prolong hyperpolarisation and makes it more difficult to indicate further APs

Question 17

Channel opening are stochastic

Answer 17

AP depends on thousands of channels working together. There’s variability between each Nav channel or each Kv.

Question 18

T/F - opening of Nav channels is delayed relative to Kv channels

Answer 18

False - other way round

Question 19

Does AP propagate away or towards spike initiation zone?

Answer 19

Away from spike initiation zone and towards axon terminal

Question 20

What happens to Nav channels in adjacent part of axon if sufficient Na+ diffuses?

Answer 20

AP can be triggered by Na+ entry which causes depolarisation

Question 21

What prevents back-propagation of an AP?

Answer 21

Inactivated Nav channels

Question 22

Where are ion channels concentrated in an axon? (Hint talking about myelination)

Answer 22

Nodes of ranvier - only places where Nav and Kv channels are & only place where APs can occur

Question 23

Where does passive diffusion occur in an axon?

Answer 23

Myelinated areas

Question 24

What axons will have faster conduction velocity?

Answer 24

Thicker, more myelinated and at colder temp

Question 25

How does lethal injection KCl work?

Answer 25

Large increase in extracellular K+ depolarises RMP of excitable cells like neurons and cardiac cells (stops heartbeat)