Action Potentials And Its Properties (4) Flashcards Preview

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0

What is an action potential?

- A change in voltage across a membrane

1

What does an action potential depend on?

- Ionic gradient
- Relative permeability of the membrane

2

What is the rule given to action potentials and why is this the case?

- All or nothing
- An action potential will only occur if the threshold value is met.

3

Give the values of the beginning of depolarisation and the beginning of repolarisation for the following:
- Axon
- Skeletal
- SAN
- Cardiac

- Axon: -70mv, +30mv
- Skeletal: -90mv, +40mv
- SAN: -60mv, +30mv
- Cardiac: -90mv, +30mv

4

With reference to sodium and potassium channels describe how depolarisation and repolarisation occur.

- Depolarisation: Na channels open to allow an influx of Na to increase membrane potential.
K channels remain closed.
- Repolarisation: Na channels become inactive no more Na influx.
K channels open to allow an efflux of K to decrease membrane potential.

5

How can Na channels reopen after repolarisation?

- A negative enough potential much be reached to allow the reformation of the closed Na channels.llll

6

What does the conductance for a particular ion depend on?

- The number of channels for the ion that are open.

7

What sort of percentage change in ion concentration is required for an action potential to 'fire'?

- ~0.4%

8

How can ionic currents be measured?

- Voltage clamping
- Patch clamping enables current flowing through individual ions channels to be measured
- Using different ionic concentrations allows contribution of specific ions to be calculated.

9

What is an axon hillock?

- Point on axon where AP is initiated.

10

How does depolarisation of an AP occur?

- Depolarisation to threshold: Na channels open
- Na influx
- Membrane depolarises
- Positive feedback loop

11

How does repolarisation of an AP occur?

- Depolarisation opens K channels and inactivates Na channels
- K channels open causes K efflux
- Inactivation of Na channels causes Na influx to stop

12

What does ARP stand for and what occurs during this period?

- Absolute refractory period
- Can't fire another AP
- Nearly all Na channels are inactivated

13

What does RRP stand for and what occurs during this period?

- Relative refractory period
- Na channels are recovering from inactivation, excitability returns towards normal as the number of channels in inactivated state decrease.
- Second AP can fire in this period if signal is strong enough.

14

Outline how Na channels move from closed to open to inactivated.

- Na channels have two voltage gates.
- When closed the top gate is close and the bottom gate is open and the top one is closed
- When the membrane potential is at -70mv the bottom gate opens to allow Na to influx causing the potential to increase
- When potential has reached +30mv the Na gates become inactive, this is when the top gate remains open but the bottom gate shuts allowing Na to enter then rebound out.
- When the potential is negative enough the bottom gate reopens and the top gate shuts, so closed.

15

What is hyperpolarisation's role with Na channels?

- Na channels moving from inactive form to closed form.

16

What is accommodation?

- Longer the stimulus, the larger the depolarisation necessary to initiate an AP
- Threshold becomes more positive

17

What effect does accommodation have on the threshold value and the possibility of an AP firing?

- Doesn't change AP
- With accommodation no AP may be fired even if slightly over threshold value.

18

To a subunit level what is a Na channel?

- 4 subunits turn in to face each other.
- The 'inactivation particle' between subunit 3 and 4 acts as a plug for the channel.

19

Give an example of an anaesthetic.

- Procaine

20

How do anaesthetics act on Na channels?

- Bind and block Na channels thereby stopping AP generation

21

In what order does anaesthetics act on axons?

- Small myelinated axons (responsible for pain)
- Non-myelinated axons
- Large myelinated axons

22

For what type of state of Na channel does anaesthetics have the greatest affinity?

- Inactivated as can fit into pore with greatest ease to block.

23

Which diseases are associated with AP?

- CNS: - Multiple sclerosis: all CNS nerves
- Devic's disease: optic and spinal chord nerves only
- Peripheral NS: - Landry-Guillian- Barre
- Charcot-Marie tooth disease
- Disease resulting from breakdown/damage to myelin sheaths.

24

What would a graph of the reading taken from an intact membrane during an AP look like?
What would one of a damaged membrane look like?

- Diphasic
- Monophasic

25

How can conduction velocity be calculated?

Axon length (x) / time (ms)

26

What is local current theory?

- Injection of current into an axon will cause the resulting change to spread along the axon and cause IMMEDIATE LOCAL CHANGE in membrane potential

27

What is capacitance?

- The ability to store charge.

28

What does membrane resistance depend on?

- Depends on number of ion channels open
- The lower the resistance the more ion channels that are open.

29

What does the spread of local current depend on?

- Membrane's resistance
- Membrane's capacitance