Each myofibril contains interdigitating thick and thin filaments arranged longitudinally in sarcomeres. Repeating units of sarcomeres account for the unique banding pattern in striated muscle. A sarcomere runs from Z line to Z line.

Troponin is a complex of three globular regulatory proteins (T, C, and I). Troponin is essential to the contraction of skeletal and cardiac muscle, but is not involved in the contraction of smooth muscle. Troponin T (think of: T for tropomyosin) binds to tropomyosin creating the troponin-tropomyosin complex, which helps to position tropomyosin onto actin. Troponin I (think of I for inhibition) binds actin, holding the actin-tropomyosin complex in place. This inhibits the myosin heads from binding their binding sites on actin. Lastly, troponin C (think of C for calcium) is responsible for binding calcium. This step is allows for the interaction of actin and myosin, thus activating the muscle for contraction.

The ryanodine receptor is a calcium channel that is responsible for the release of calcium ions. It is located in the sarcoplasmic reticulum. The release of calcium is a required step in muscle contraction. The ryanodine receptor pumps calcium ions from the intracellular fluid into the interior of the sarcoplasmic reticulum, this process keeps the intracellular calcium ions low and creates a concentration gradient.

The H-zone is an area made up of only thick filaments (myosin). The I-band is thin filaments (actin) only, and the A-band is where there are thick and thin filaments. The Z-disc is dividing feature between sarcomeres and appears as dark lines in electron micrographs.

Muscle contraction results in both the H-band and I-bands shortening, but the A-band remains the same length (A band is Always the same). The Z-band is a static structure and doesn't change size.

ATP binds to myosin head regions and is hydrolyzed to ADP and inorganic phosphate when muscle relaxes. The release of these products allows the contraction to occur as the myosin head changes position. The binding of new ATP releases the myosin head from actin, and allows the muscle to relax prior to another round of hydrolysis. This explains why, in the absence of adequate ATP, muscle can remain in a contracted state. This phenomenon, when seen in the deceased, is called rigor mortis.

Thick filaments contain myosin. Thick filaments are present in the A band in the center of the sarcomere. Myosin has six polypeptide chains, including one pair of heavy chains and two pair if light chains. Each myosin molecule has two “heads” attached to a single “tail.” The myosin heads are capable of ATP hydrolysis and bind ATP and actin, and are involved in cross bridge formation.

Troponin is the regulatory protein that permits cross bridge formation when it binds . Troponin is a complex of three globular proteins. Troponin C () is the binding protein, that when bound to , permits the interaction of actin and myosin via conformational change that reveals the myosin binding site on actin, and thus muscular contraction.

During muscular contraction, the myosin heads pull the actin filaments toward one another resulting in a shortened sarcomere. While the I band and H zone will disappear or shorten, the A band length will remain unchanged. This is because the A band corresponds to the full length of the myosin filament, or thick filament. Since the myosin filament does not actually change length, the A band remains constant.

The I band corresponds to the region of action that does not overlap with myosin. The length of the actin filament does not change during contraction, but the region of overlap increases. This results in a decrease of the non-overlapped I band.

The H zone refers to the region of myosin that is not overlapped by action. As the region of overlap grows, the H zone shrinks.

Troponin (also called troponin C) is the most direct structure that interacts with ions involved in initiating muscle contractions. Once bound to calcium ions, troponin facilitates the movement of tropomyosin away from actin binding sites, allowing myosin to bind and, ultimately, contract. Without the binding of calcium ions to troponin, the myosin-binding site on actin remains obscured by tropomyosin and contraction cannot occur.