PH1124 - Cardiac potentials Flashcards
Describe the process of Nodal/Pacemaker/ Slow action potentials.
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.
Summarise the process of Nodal/pacemaker/slow action potentials.
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
Describe the process of Non-nodal/fast action potentials
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
Describe the absolute refractory period
Once an action potential is initiated, there is a period of time that a new action potential cannot be initiated
what happens with action potentials in the heart? (6)
- 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
what is the net charge inside a cell?
- negative
what is membrane potential determined by? (3)
- 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
what are the two forms of cardiac action potentials? (2)
- pacemaker (nodal) slow response
- non-nodal fast response
what is automaticity?
- the ability of the heart to start and maintain rhythmic activity without the use of the nervous system
what is phase 0 of non-nodal APs?
- fast Na2+ influx
what is phase 1 of non-nodal APs? (2)
- transient outward K+ current
- activation of inward Ca2+ current
what is phase 2 of non-nodal APs?
- plateau maintained by Ca2+ influc and K+ effluc (‘Ikr’)
what is phase 3 of non-nodal APs? (2)
- K+ efflux (‘Ikr’)
- Ca2+ channels inactivate
what is phase 4 of non-nodal APs? (2)
- ‘Ik1’ outward K+ current maintains resting potential
- Na+ channels transitions to closed state
what is Na+ channel activation/inactivation like in non-nodal APs?
- fast
what is K+ channel activation like in non-nodal APs?
- delayed and takes a while to build up
what is Ca2+ channel activation like in non-nodal APs?
- relatively prolonged
what is the ion that causes steep polarisation in nodal APs and non-nodal APs? (2)
- calcium in nodal
- sodium in non-nodal
what is the importance of Ca2+ in APs and muscle contraction? (5)
- 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
what is the relationship between contractile force and amount of Ca2+?
- contractile force increases with the amount of free Ca2+ available
how does muscle relaxation occur?
- Ca2+ needs to removed by pumping it back into the SR or leaving the cell via the calcium pump or Na+/Ca2+ exchanger
what does CICR stand for?
- calcium induced calcium release
what is inotropy? (2)
- refers to the force of myocardial contraction
positive inotropy is increasing the contractile force
what is lusitropy? (2)
- referring to the rate of myocardial relaxation
- positive lusitropy is faster relaxation.
what is chronotropy? (2)
- refers to the rate of myocardial contraction
- positive chronotropy is faster rate of contraction.
how do inotrophy lusitrophy and chronotrophy have an effect on APs? (4)
- 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
how does increase funny current cause positive chronotropy?
- causes increased gradient in pacemaker potential
- faster depolarisation
