MICROANATOMY

Detailed explanation-1: -Most of the cytoplasm consists of myofibrils, which are cylindrical bundles of two types of filaments: thick filaments of myosin (about 15 nm in diameter) and thin filaments of actin (about 7 nm in diameter).

Detailed explanation-2: -Muscles contract via interaction between thick (myosin-containing) and thin (actin-containing) filaments in response to increased levels of calcium.

Detailed explanation-3: -An important feature of myosin polymerization is that myosin-II molecules are oriented in opposite directions in each half of the sarcomere. This orientation results in polarized thick filaments that help in directing force toward the center of the sarcomere.

Detailed explanation-4: -d) Myosin filaments are about twice as thick as actin filaments.