sites for actin to myosin during a muscle contraction (Fig. 3.3).
Noncontractile Fibers (Structural Proteins)
Within each muscle fi ber and between fi bers are struc- tural proteins that do not participate in active muscle contractions but contribute to the transmission of forces generated by the active proteins. Two of these proteins are titin and desmin. Titin plays a role in developing passive tension within the muscle fi ber and assists with restoring a muscle to its original resting length. Research indicates that desmin may be involved with transmission of tension between sarcomeres, but its function is uncertain. These
Thick (myosin) filament
MUSCLE CONTRACTION (ACTIVATION)
CROSS-BRIDGE LINKS
Figure 3.4A depicts a contractile unit of the myofi bril. Between the two Z discs are the sarcomeres. The Z discs defi ne the boundary for each sarcomere and link the thin actin fi laments together. The thin actin fi laments
Z line H zone
Thin (actin) filament
53
structural proteins provide internal support and produce passive tension when the muscle fi ber is stretched.
I band A A Resting
A band Sarcomere Z line
I band
B Coupling
C Contraction
D Recharging B
Figure 3.4 (A) A sarcomere, showing the myofilaments. (B) Cross-bridge formation between actin and myosin. (From Levangie, P, Norkin, C. Joint Structure and Function: A Comprehensive Analysis, 5th ed. Philadelphia, PA: F.A. Davis Company, 2011: p. 110, with permission.)
