Isotropic bands

Bisecting the I band and serving as an anchoring point for the two adjacent actin filaments is the Z disc.

[3] Tropomyosin, a protein, shields the myosin-binding sites, hindering actin from binding to myosin.

During muscle relaxation, the troponin-tropomyosin complex inhibits myosin heads from binding to the active sites on actin microfilaments.

These two regulatory proteins cooperate in response to calcium levels, overseeing sarcomere contraction.

Consequently, myosin heads can attach to these exposed sites, forming cross-bridges and initiating muscle contraction.

The muscle is made up of several myofibrils packed into functional units surrounded by different layers of connective tissues ( epimysium , perimysium , and endomysium ). The main contractile unit is mainly composed of protein filaments (myofilaments), namely myosin (thick filaments) and actin (thin filaments).