Cardiac muscle myofibrils
As in skeletal muscle, cardiac muscle contraction requires binding of the thin and thick filaments and cross-bridge cycling to shorten the sarcomere and create muscle tension.
Because cardiac and skeletal muscle myofibrils are so similar, we recommend reviewing skeletal muscle organization for more details.
- Each myofibril comprises proteins, notably the thick and thin myofilaments, which overlap to form contractile units called sarcomeres.
— Z-discs anchor the thin filaments.
— Thick filaments extend from the perpendicular M line in the spaces between thin filaments.
- The I band, aka, light band, of the myofibril comprises the area where there is no overlap between thick and thin filaments (think I for lIght).
- The area where thick and thin filaments do overlap creates the A band, aka, dark band (think A for dArk).
- The Sarcomere spans from z-disc to z-disc; the sarcomere is the functional contractile unit of the myofibrils.
Basics of the thin and thick filaments
— Feature the protein myosin; the head has the actin binding site, the tails intertwine.
— During contraction, the myosin heads bind with actin to form cross bridges (for more on cross bridge cycling, see "skeletal muscle contraction").
— Actin is the primary protein
Comprises multiple polypeptide subunits (called globular actin) arranged in a double helix.
Has myosin binding sites.
— Tropomyosin strands wrap around the actin molecules and, in a relaxed state, cover their myosin binding sites.
— Troponin is a three-polypeptide complex; one of the polypeptides serves as a calcium binding site.
- To summarize the relationship between calcium and cross-bridge cycling:
1) Troponin binds intracellular calcium, causing
2) Tropomyosin movement and exposure of the myosin-binding sites, allowing
3) Myosin-actin binding and cross bridge cycling to shorten the sarcomeres. Contraction via the sliding filament mechanism occurs as in skeletal muscle, which is discussed in detail, elsewhere.
