Cardiac Contraction

Question 1

Briefly describe the structure of a cardiomyocyte

Answer 1

• 60–140 μm in length + 17–25 μm
  diameter make up the branching myofibres.
• Each myocyte contains multiple, rod-like cross-banded strands (myofibrils) that run the length of the cell and are composed of repeating sarcomeres.

Question 2

What are T tubules ?

Answer 2

They are invaginations of the muscle cell membrane (sarcolemma) that penetrate into the centre of cardiac muscle cells.

Question 3

What is the function of a Sarcomere ?

Answer 3

• Sarcomeres cause muscle contraction when their
  component actin and myosin filaments move relative to
  each other.
• The varying actin myosin overlap is shown
  for systole (contraction) and diastole (relaxation).

Question 4

Describe the function of a cardiomyocyte

Answer 4

• T tubules have calcium channels and ensure calcium is delivered deep into the cell close to the sarcomere.
• Ca2+ enters via calcium channel that open in response to the wave of depolarization that travels along the sarcolemma where they trigger the release of more calcium from the sarcoplasmic reticulum and initiate contraction.
• The varying actin-myosin overlap is shown for systole,
  when [Ca2+] is maximal, and diastole, when [Ca2+] is
  minimal.
• Eventually the Ca2+ that has entered the cell leaves
  predominantly through an Na+/Ca2+ exchanger.

Question 5

How is the rise in intracellular calcium related to contraction ?

Answer 5

• Force of contraction proportional to intracellular calcium [Ca2+]
• Higher increases in Ca2+ → increased force of contraction
• Intracellular Ca2+ levels increase from 0.1 μM to about 10 μM

Question 6

Describe how does the rise in calcium initiate contraction ?

Answer 6

• Action potential (Na+ ions) depolarises T-tubules & activates VGCCs (Voltage gated calcium channel ) causing Ca2+ influx
• Ca2+ binds to Ryanodine receptor located on Sarcoplasmic reticulum (SR) - close association with T-tubules
• Release of Ca2+ from SR - Ca induced Ca release (CICR)
• Calcium binds to troponin C which changes conformation of tropomyosin exposing actin binding sites.
• Hydrolysis of ATP causes myosin to extend and bind head to actin forming cross bridges.
• Power stroke moves actin filament relative to myosin.
  ADP + Pi released from the myosin heads.
• Myosin remains attached to actin until a new
  molecule of ATP binds.
• Cycle continues until cellular calcium levels decrease allowing calcium to dissociate from troponin, which returns to original conformation which blocks actin
  binding site.

Question 7

Describe the

Troponin-Tropomyosin complex

Answer 7

Troponin T 
Troponin I
Troponin C
(Tn T)
(Tn I)
(Tn C)
binds to tropomyosin
binds to actin filaments
binds Ca2+