7. Cellular and Molecular Events in the CVS

Question 1

Talk through the opening and closing of relevant channels in a ventricular or cardiac action potential.

Answer 1

There is opening of voltage gated Na+ channels that cause the rapid depolarisation. There is a transient outward K+ current. Then voltage gated Ca2+ channels open to stop the repolarisation. Ca2+ channels inactivate and the voltage gated K+ channels open to return the membrane potential to resting.

Question 2

What do the numbers: 0, 1, 2, 3, and 4, correspond to on ventricular and cardiac action potentials?

Answer 2

0 = depolarisation
1 = transient repolarisation
2 = plateau 
3 = repolarisation
4 = resting potential.

Question 3

Talk through the SA node action potential.

Answer 3

Pacemaker potential gives an initial slope to the threshold by the funny current of Na+ influx. When the threshold, -50mV, is met, voltage gated Ca2+ channels open and there is the upstroke. The downstroke is caused by opening of V-gated K+ channels.

Question 4

What is the structure of cardiac muscle?

Answer 4

Signal central nucleus, cells joined at intercalated disks, gap junction permit movement of ions and electrically couple cells, desmosomes rivet cell together.

Question 5

How does depolarisation cause increased cytosolic calcium?

Answer 5

It opens L-type Ca2+ channels in T-tubule system (25% of Ca2+ entry). Localised Ca2+ entry opens calcium induced calcium release channels in the SR (75% of Ca2+ entry).

Question 6

How are cardiac myocytes relaxed after excitation?

Answer 6

Return of [Ca2+]i to normal, resting levels. Most Ca2+ is pumped back into SR via the SERCA (stimulated by raised Ca2+), some exits across the cell membrane - sarcolemmal Ca2+ATPase and Na+/Ca2+ exchanger.

Question 7

Why does the myosin light chain need activating for cardiac muscle contraction?

Answer 7

So they can interact with actin. Only activated, phosphorylate myosin light chains can bind to actin for contraction.

Question 8

How is contraction of vascular smooth muscle regulated?

Answer 8

Ca2+ binds to calmodulin and activated myosin light chain kinase, this phosphorylated the myosin light chain to permit interaction with actin. Ca2+ levels decline and there is relaxation as the myosin light chains dephosphorylate. If MLCK is phosphorylate by PKA, its own action is inhibited.

Question 9

Talk through shape of ventricular and cardiac action potentials.

Answer 9

There is a rapid upstroke and depolarisation. There is then a transient repolarisation before an extended plateau. Finally there is repolarisation to the resting potential.