Ans. C. Titin. (Ref. Guyton & Hall's Textbook of Medical Physiology 11th/pg. 463)SarcomereThe portion of the myofibril (or of the whole muscle fiber) that lies between two successive Z discs is called a sarcomere. When the muscle fiber is contracted, the length of the sarcomere is about 2 micrometers. At this length, the actin filaments completely overlap the myosin filaments, and the tips of the actin filaments are just beginning to overlap one another.Titin# The side-by-side relationship between the myosin and actin filaments is difficult to maintain. This is achieved by a large number of filamentous molecules of a protein called titin. Each titin molecule has a molecular weight of about 3 million, which makes it one of the largest protein molecules in the body.Myosin Filament.# The myosin filament is composed of multiple myosin molecules, each having a molecular weight of about 480,000.# The myosin molecule is composed of six polypeptide chains--two heavy chains, each with a molecular wt of about 200,000, and four light chains with molecular weights of about 20,000 each.# The myosin filament is made up of 200 or more individual myosin molecules. The protruding arms and heads together are calledcross-bridges. Each cross-bridge is flexible at two points called hinges--one where the arm leaves the body of the myosin filament, and the other where the head attaches to the arm.# The total length of each myosin filament is uniform, almost exactly 1.6 micrometers. Note, however, that there are no cross-bridge heads in the very center of the myosin filament for a distance of about 0.2 micrometer because the hinged arms extend away from the center.# ATPase Activity of the Myosin Head. Another feature of the myosin head that is essential for muscle contraction is that it functions as an ATPase enzyme. As explained later, this property allows the head to cleave ATP and to use the energy derived from the ATP's high-energy phosphate bond to energize the contraction process.Actin Filament.# The actin filament is complex, composed of three protein components: actin, tropomyosin, and troponin.# The backbone of the actin filament is a double- stranded F-actin protein molecule. The two strands are wound in a helix in the same manner as the myosin molecule.# Each actin filament is about 1 micrometer long. The bases of the actin filaments are inserted strongly into the Z discs; Tropomyosin Molecules.# The actin filament also contains another protein, tropomyosin. Each molecule of tropomyosin has a molecular weight of 70,000 and a length of 40 nanometers. These molecules are wrapped spirally around the sides of the F-actin helix. In the resting state, the tropomyosin molecules lie on top of the active sites of the actin strands, so that attraction cannot occur between the actin and myosin filaments to cause contraction.Troponin and Its Role in Muscle Contraction.# Attached intermittently along the sides of the tropomyosin molecules are still other protein molecules called troponin.# One of the subunits (troponin I) has a strong affinity for actin, another (troponin T) for tropomyosin, and a third (troponin C) for calcium ions. This complex is believed to attach tropomyosin to the actin.# The strong affinity of the troponin for calcium ions is believed to initiate the contraction process.