Ventricular and Atrial Action Potential Flashcards
What are the K+ and Na+ equilibrium potentials?
K+ : -90mV
Na+ : +60mV
Why is the resting cardiac myocyte membrane potential much closer to the K+ equilibrium potential?
because the sarcolemma is much more permeable to K+ since K+ channels are open
(resting potential is maintained by Na+ and K+ ATPase pumps, pumping 3 NA+ ions out for every 2 K+ ions pumped in)
What happens to Na+ voltage gated channels when an action potential arrives?
They open, allowing Na+ to enter the cell and rapidly depolarise the cell.
This triggers more Na+ channels to open, creating a positive feedback loop.
Ca2+ voltage gated channels also open, but these open much more slowly than Na+ voltage gated channels
What happens when cell potential is positive (+52mV)
voltage gated Na+ channels close, and voltage gated K+ channels open, allowing for partial repolarisation
How does the membrane remain depolarised at a plateau value of roughly 0mV?
The inflow of Ca2+ into T-tubules balances the outflow of K+
the K+ channels open at the start also close, maintaining depolarisation
How does repolarisation eventually occur?
Closure of the L-type Ca2+ channels
Reopening of the K+ channels
What are the 5 phases?
Phase 0: rapid depolarisation (inflow of Na+)
Phase 1: partial repolarisation (Na+ inflow stops, outflow of K+)
Phase 2: plateau (slow inflow of Ca2+)
Phase 3: repolarisation (inflow of Ca2+ stops, K+ outflow)
Phase 4: pacemaker potential (Na+ inflow, slowing of outflow of K+)
What is the cross-bridge cycle?
cardiac muscle contraction (as initiated by Ca2+ binding to ryanodine receptors)
What is the cross-bridge formation?
the formation in which myosin drops its inorganic phosphate molecule in order to bind to actin at its actin binding sitte
What is the power stroke?
the action by which a myosin head drops its ADP to pull actin over myosin, reducing the distance between Z-lines and causing the muscle to contract
What causes rigour mortis (stiffness of muscles in the dead)?
lack of ATP –> myosin head does not detach from actin –> muscle remains contracted
when does muscular contraction stop?
when cytosolic Ca2+ is restored to its original low resting value
what is the refractory period?
it is the period after the action potential where a second impulse cannot cause a second contraction of the cardiac muscle; this is to prevent excessive frequent contractions and to allow time for filling
