Contractile filaments of muscle – Physiology MCQ

Regarding contractile filaments, which is true?
A. Each successive pair of cross bridges of myosin filaments is axially displaced from the previous pair by 140 degrees.
B. The backbone of the actin filament is G-actin molecule.
C. ADP molecules are the active sites on the actin filament.
D. The strong affinity of tropomyosin for calcium ions initiates the contraction process.

Correct answer : C. ADP molecules are the active sites on the actin filament.

Sarcomere

• Each successive pair of cross bridges of myosin filaments is axially displaced from the previous pair by 120 degrees.
• The backbone of the actin filament is the double stranded F-actin protein molecule.
• Each strand of the double F-actin helix is composed of polymerised G-actin molecules.
• The strong affinity of troponin for calcium ions initiates the contraction process.

Ref: Textbook of Medical Physiology, Guyton and Hall, 11th edition, page 76.
Image credits: Richfield, David. “Medical gallery of David Richfield 2014” ( https://en.wikiversity.org/wiki/Medical_gallery_of_David_Richfield_2014 ). Wikiversity Journal of Medicine 1 (2). DOI:10.15347/wjm/2014.009. ISSN 2001-8762.