cardiac action potentials - ventricular muscle Flashcards
What is the sarcolemma?
it is the cell membrane that surrounds cardiac muscle
What is cardiac muscle contraction triggered by?
- it is triggered by an action potential in the sarcolemma
Where does the electrical excitation orginate and where does it spread to?
- initiates in the SA node
- spreads to the atria and ventricles
- rapid conduction of this electrical signal (ie AP) causes a co-ordinated contractile response
What are the** 3 main ions** that **modulate the membrane potential **(change it) in cardiac myocytes?
- Na+
- K+
- Ca2+
- when their channels are open, ions flow down their concentration gradient (high to low)
Describe the resting membrane potential in cardiac myocytes (in terms of ion channels - open or closed etc)
- at RMP, Na+ and Ca2+ channels are closed
- the RMP is closer to the K+ equilibrium conc as there are K+ leak channels present on the membrane
what causes the ventricular muscle to have an action potential?
- the cardiac contractile muscles in the ventricle are excited by the pacemaker cells in the SA node
- once the** threshold is reached**, an action potential is generated and is propagated through the ventricles
* Na+ influx (upstroke **- depol) and Ca2+ influx at plateau (peak)
What is the cardiac muscle refractory period?
- refractory is defined as electrically inexcitable
- no sustained contractions
- this period allows the** filling of ventricles** between contractions
How many phases are there in the ventricular/ cardiac contractile cells?
5 - 0,1,2,3,4
What is phase 0 of ventricular contraction?
- action potential reaches myocyte and depolarisation begins
- fast voltage gated Na+ channels open and allow a rapid influx of Na+
- this is the rapid depolarisation
what is phase 1 of the ventricular contraction?
- brief period of repolarisation, Na+ channels become inactivated and K+ voltage gated channels open
- outward flow of K+
What is phase 2 of ventricular contraction?
-
plataeu period with an influx of Ca2+
* Ca2+ influx is through L type (long type) Ca2+ channels - there us a reduced K+ efflux
What is phase 3 of ventricular contraction?
- as the plataeu period moves on, there is an increase in the K+ efflux
- this is due to the opening of voltage gated K+ channels, known as delayed rectifier or slow K+ channels
- the Kir (influx) channels reopen to complete the depolarisation
What is **phase 4 **of ventricular contraction?
- RMP is re-established
- the raised Ca2+ in cytosol is pumped back into the sarcoplasmic reticulum via Ca2+-ATPase and via transporters on sarcolemma
What happens if calcium is removed from the extra-cellular fluid (fluid outside cells)?
- there would be a stop of contraction
- there would be no Ca2+ entry to trigger Ca2+ induced Ca2+ release from the SR
- no long plataeu phase in AP
Describe the role of the ANS in relation to the action potenial
- the ANS affects the AP duration
- sympathetic stimulation shortens the duration of the AP due to repolarisation occuring sooner
- the shorter duration allows more AP’s per minute
