PH1124 - Cardiac potentials Flashcards

1
Q

Describe the process of Nodal/Pacemaker/ Slow action potentials.

A

where the membrane potential is at its most negative in these cells, at -60mV.
ion channels open that conduct a slow inward movement of mainly Na+. This current is called If – the funny current – The funny current starts to slowly depolarise the membrane as +ive charges move into the cell, this is the initiation of phase 4.

As the membrane potential reaches about -50 mV, another type of channel opens. This channel is called transient or T-type Ca++ channel.
As Ca++ enters the cell, it causes further depolarisation and at -40 mV, a second type of Ca++ channel opens: long-lasting, or L-type Ca++ channels. Opening of these channels causes the cell to reach the threshold potential, causing the depolarisation of Phase 0.

Unlike in neurones and in fast response cardiac action potentials – this depolarisation spike is not caused by fast acting voltage gated Na+ channels – rather by Ca2+channels, which are much slower.

Repolarization occurs in phase 3 as Ca2+ channels inactivate and K+ channels open, thereby increasing outward currents which repolarise and hyperpolarise.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Summarise the process of Nodal/pacemaker/slow action potentials.

A

Summary – PHASE 4 –> 0 –> 3 –> 4 ETC
Slow movement of Na+ ions which slowly depolarises membrane as positive charges move into cell.

At -50mV T-type Ca++ channel opens and causes further depolarisation
At -40mV L-type ca++ channel opens causing the cell to reach threshold potential, causing depolarisation of phase 0 making the cell positive

Repolarisation in phase 3 as ca++ channels inactivate and k+ channels open making the cell negative

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Describe the process of Non-nodal/fast action potentials

A

These are activated by neighbouring cells and are present in all other areas of the heart apart from the sinoatrial node where nodal action potentials take place.

Phases 4 –> 0 –> 1 –> 2 –> 3 –> 4

Phase 0 - there is a rapid depolarization (phase 0) that is caused by the opening of voltage-gated Na+ channels

Phase 1 - depolarisation - Na+ channel inactivation
Transient outward K+ current
Activation of inward Ca2+ due to L-type Ca2+ channels at about -40 mV

Phase 2 - Platuea caused by the K+ efflux and Ca2+ influx of the previous phase

Phase 3 as repolarization starts the Ca2+ channels inactivate, with K+ efflux bringing the membrane potential back down.

Phase 4 - The maintained resting potential is held at about -90mV

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Describe the absolute refractory period

A

Once an action potential is initiated, there is a period of time that a new action potential cannot be initiated

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

what happens with action potentials in the heart? (6)

A
  • AP generated in the SA node
  • atrial depolarisation
  • AP delayed at the AVN
  • AP travels down the septum and into the ventricles
  • wave of AP causes wave of contraction
  • wave of repolarisation rollows wave of depolarisation allowing relaxation
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

what is the net charge inside a cell?

A
  • negative
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

what is membrane potential determined by? (3)

A
  • concentration gradients for K+, Na2+ and Ca2+ across the membrane
  • the relative permeability (electrical conductance) of the membrane to each of those ions (regulated by ion channels)
  • electrogenic ion pumps
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

what are the two forms of cardiac action potentials? (2)

A
  • pacemaker (nodal) slow response

- non-nodal fast response

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

what is automaticity?

A
  • the ability of the heart to start and maintain rhythmic activity without the use of the nervous system
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

what is phase 0 of non-nodal APs?

A
  • fast Na2+ influx
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

what is phase 1 of non-nodal APs? (2)

A
  • transient outward K+ current

- activation of inward Ca2+ current

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

what is phase 2 of non-nodal APs?

A
  • plateau maintained by Ca2+ influc and K+ effluc (‘Ikr’)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

what is phase 3 of non-nodal APs? (2)

A
  • K+ efflux (‘Ikr’)

- Ca2+ channels inactivate

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

what is phase 4 of non-nodal APs? (2)

A
  • ‘Ik1’ outward K+ current maintains resting potential

- Na+ channels transitions to closed state

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

what is Na+ channel activation/inactivation like in non-nodal APs?

A
  • fast
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

what is K+ channel activation like in non-nodal APs?

A
  • delayed and takes a while to build up
17
Q

what is Ca2+ channel activation like in non-nodal APs?

A
  • relatively prolonged
18
Q

what is the ion that causes steep polarisation in nodal APs and non-nodal APs? (2)

A
  • calcium in nodal

- sodium in non-nodal

19
Q

what is the importance of Ca2+ in APs and muscle contraction? (5)

A
  • transverse tubules bring sarcolemma (plasma membrane) and SR membrane together to form diads
  • L-type calcium channels are in close proximity to sarcoplasmic reticulum channels
  • in influx of calcium causes an explosive release of calcium from the SR by CICR
  • bind to contractile proteins to initiate a contraction
20
Q

what is the relationship between contractile force and amount of Ca2+?

A
  • contractile force increases with the amount of free Ca2+ available
21
Q

how does muscle relaxation occur?

A
  • Ca2+ needs to removed by pumping it back into the SR or leaving the cell via the calcium pump or Na+/Ca2+ exchanger
22
Q

what does CICR stand for?

A
  • calcium induced calcium release
23
Q

what is inotropy? (2)

A
  • refers to the force of myocardial contraction

positive inotropy is increasing the contractile force

24
Q

what is lusitropy? (2)

A
  • referring to the rate of myocardial relaxation

- positive lusitropy is faster relaxation.

25
Q

what is chronotropy? (2)

A
  • refers to the rate of myocardial contraction

- positive chronotropy is faster rate of contraction.

26
Q

how do inotrophy lusitrophy and chronotrophy have an effect on APs? (4)

A
  • shorter duration of action potential and the plateau is more positive
  • increased [Ca2+] increases the force of contraction
  • increased removal of calcium increases relaxation
  • faster repolarisation due to K+ channel activation
27
Q

how does increase funny current cause positive chronotropy?

A
  • causes increased gradient in pacemaker potential

- faster depolarisation