Skeletal muscle

The contractile mechanism in skeletal muscle largely depends on the proteins myosin-II, actin, tropomyosin, and troponin. Troponin is made up of three subunits: 1. troponin I, 2. troponin T, and 3. troponin C.

I band - ActinH band - MyosinA band - Alternating Actin and MyosinZ lineM line

The light I band is divided by the dark Z line, and the dark A band has the lighter H band in its center. A transverse M line is seen in the middle of the H band, and this line plus the narrow light areas on either side of it are sometimes called the pseudo-H zone. The area between two adjacent Z lines is called a sarcomere.

The thick filaments, are made up of 1. myosin; the thin filaments are made up of 1. actin, 2. tropomyosin, 3. troponin. If a transverse section through the A band is examined under the electron microscope, each thick filament is seen to be surrounded by six thin filaments in a regular hexagonal pattern.

The form of myosin found in muscle is myosin-II, with two globular heads and a long tail. The heads of the myosin molecules form cross-bridges with actin. Myosin contains heavy chains and light chains, and its heads are made up of the light chains and the amino terminal portions of the heavy chains. These heads contain an actin-binding site and a catalytic site that hydrolyzes ATP. The myosin molecules are arranged symmetrically on either side of the center of the sarcomereThe M line is the site of the reversal of polarity of the myosin molecules in each of the thick filaments.

The thin filaments are polymers made up of two chains of actin that form a long double helix.

Tropomyosin molecules are long filaments located in the groove between the two chains in the actin . Each thin filament contains 300 to 400 actin molecules and 40 to 60 tropomyosin molecules.

Troponin molecules are small globular units located at intervals along the tropomyosin molecules. Each of the three troponin subunits has a unique function:

Troponin T binds the troponin components to tropomyosin; troponin I inhibits the interaction of myosin with actin; Troponin C contains the binding sites for the Ca2+ that helps to initiate contraction.

Some additional structural proteins that are important in skeletal muscle function include 1. actinin, 2. titin, 3. desmin. Actinin binds actin to the Z lines. Titin, the largest known proteinconnects the Z lines to the M lines and provides scaffolding for the sarcomere. Desmin adds structure to the Z lines in part by binding theZ lines to the plasma membrane.