Lec 6 Cardiac Cellular Electrophysiology Flashcards
Difference in phase 0 between SA and ventricular AP?
Much faster in ventricle [300 V/s vs 20 V/s]
Difference in phase 1 between SA and ventricular AP?
absent in SA AP
Difference in phase 2 between SA and ventricular AP?
abbreviated in SA
Difference in phase 3 between SA and ventricular AP?
comparable
Difference in phase 4 between SA and ventricular AP?
- flatter, more negative in ventricle
- increasing [not flat] in SA
Sequence of cardiac electrical activation [6 things]
- SA node
- Atrium
- AV node
- bundle of His/bundle branches
- purkinje fibers
- ventricles
What AP is similar to SA node AP?
AV node AP
What AP is similar to ventricular AP?
purkinje
What AP is similar to atria AP?
none
How does conduction speed change in different regions [where is it noticeabley slow/fast]?
- slow through AV node
- fast through his/purkinje
How does voltage change in depolarization?
Goes from -85 mV up to 0
How does voltage change in repolarization?
Goes down back to -85 mV
What does phase 0 represent in ventricular AP?
Na coming in
What does phase 1 represent in ventricular AP? [lease important to remember]
- Na coming in and K out [K dominates]
- short burst of repolarization
What does phase 2 represent in ventricular AP?
- Ca coming in and K out
- balance out but K winning slightly so flat [gradual repolarization]
What does phase 3 represent in ventricular AP?
- K+ out
- strong repolarization
What does phase 4 represent in ventricular AP?
- K out dominates, a little NCX in
Do SA node cells need stimulus to get AP?
NO!
Do ventricular cells need stimulus to get AP?
Yes
What is absolute refractory period [ARP]?
Period during action potential when its impossible to trigger a second AP
What is relative refractory period [RRP]?
Period just after action potential when you need stronger stimulus to get a weak AP
What does phase 0 represent in SA AP?
Ca influx [strong depolarization]
What does phase 2 represent in SA AP?
Ca in and K out [K very much dominates]
What does phase 3 represent in SA AP?
K out
What does phase 4 represent in SA AP?
NCX and I[f] in
What is definition of negative current?
positive charge flowing inward
What type of current is positive charge flowing out?
positive current
What is driving force?
V-Ex where V is voltage and Ex is the equilibrium voltage for that ion
What happens to ion if V=Ex? What type of current?
Ion does not move in or out of cell? No current!
What happens to ion if [V-Ex] > 0? What type of current?
- Ion moves out of cell
- positive current
What happens to ion if [V-Ex] < 0? What type of current?
- ion moves into cell
- negative current
At what voltages is K current positive?
Above -90 mV
At what voltages is Na current positive?
above +70 mV
At what voltages is Na current positive?
above +130 mV
At what voltages is Na current negative?
below + 70 mV
At what voltages is Ca current negative?
below +130 mV
At what voltages is K current negative?
below -90 mV
At what voltages does Na channel open?
at voltage > -60 mV
How does Na current stop? Why?
- Na channel automatically closes after opening
- Opens then get current and approaches nernst equilibrium so have less driving force so current stops
How fast does Na channel open and close?
4 ms
How does Ap initiation feedback in I[NA]
- as you increase V [with V still below -40 mV] you get more Ina
- with more current you get more positive V which provides more feedback
- inward Na current depolarizes the membrane
At what voltage does L-type Ca channels open?
-40 mV
Does Na or L-type Ca open at a higher [less negative] voltage?
L-type Ca [thats why Na opens first]
How long does it take for L-type Ca channel to open and close?
50 ms
Does Na or L-type Ca channel open/close faster?
Na 10x faster
2 mains differences Na and Ltype Ca current?
- Ca opens at higher [more positive] voltage
2. Ca opens and inactivates slower
What role do L-type Ca channels play in ventricular myocytes?
- action potential plateau [phase 2]
- initiate SR Ca release
What role do L-type Ca channels play in vSA and AV nodal myocytes?
- action potential upstroke [phase 0]
What role do L-type Ca channels play in vascular smooth muscle?
- supply Ca for constraction
2 types of drugs that are Ca channel blockers
- type IV anti-arrhythmics
- antihypertensives
Inwardly-rectifying K channels [Ik1] - at what voltages are they open?
- open at -90 mV
- closes during action potential plateau [at higher voltages]
What is function of Ik1 in ventricular myocytes?
- stabilizes resting potential in ventricular myocytes
- keeps voltage at -85 mV
Delayed rectifying K channels - at what voltages are they open? function in ventricular myocytes?
- closed at resting potential
- open with delay during plateau of AP
- two type: rapid [Ikr] and slow [Iks]
- repolarize membrane at end of AP plateau
How do three K types act in ventricular AP?
- at begining of AP: Ik1 shuts off to allow plateau
- later: Ikr and Iks open and repolarize membrane
Why does SA node have slow depolarization rather than flat phase 4 [2 channels involved]?
- inward current through NCX [3 NA in, 1 Ca out]
- inward current through “funny current”
What is funny current I[f] in SA node?
- mix of Na and K but mostly Na
- inward current in negative voltages, it is a depolarizing force
- closed at positive voltages [during depolarization]
What 3 things does funny current oppose
- normal Na current
- normal Ca current
- delayed rectifier K current
Are Na channels present in SA and AV nodes? Where else are they present [2 places]
- No not present in SA/AV
- present in: ventricles, purkinje
What 2 channels explain why SA nodes get spontaneous APs [automaticity] and ventricular do not?
- funny current present in SA not ventricle [open at negative potentials
- Inward rectifier [Ik1] present in ventricle not SA [if it were present in SA would prevent spontaneous depolarization since maintains resting potential]
How does phase 0 differ in ventricular and SA?
- Na and Ca in ventricle
- only Ca [no Na] in Sa
- Na more abundant and open faster so get much faster depolarization in ventricle compared to SA
How does phase 3 differ in ventricle and SA?
Its comparable
both have lots of delayed rectifier K channels
How does phase 4 differ in ventricle and SA?
- ventricle has Ik1 so stays at -85 mV so flat
- presence I[f] and absence I[k1] in SA so get slow depolarization
