14.4: The Sarcomere
A sarcomere is a microscopic segment repeating in a myofibril. The sarcomere fundamentally consists of two main myofilaments: thick filaments called myosin and thin filaments called actin. These filaments interact by sliding past each other in response to stimulus. In addition to myosin and actin, several other proteins, such as tropomyosin, troponin, titin, nebulin, myomesin, α-actinin, and dystrophin, play crucial roles in regulating, structuring, and functioning of the sarcomere.
Each myosin molecule in the thick filament has a head and a tail. The head has a binding site for ATP and actin. It is the engine that powers muscle contraction. The myosin heads reach out and form cross-bridges with the actin filaments. Actin is a globular protein that forms a helical structure of thin filaments. Each actin monomer has a binding site for myosin, making it integral to muscle contraction. Tropomyosin, another protein present in thin filaments, is a regulatory protein controlling the interaction between actin and myosin. In a relaxed muscle fiber, tropomyosin blocks the myosin binding sites on actin. Troponin is a complex of three proteins attached to tropomyosin. It has a binding site for calcium ions. When calcium binds to troponin, it causes a conformational change that moves tropomyosin away from the myosin-binding sites on actin.
Titin runs from the Z disc to the M line. It passively maintains muscle elasticity by preventing the overstretching of the sarcomere and assisting it in returning to rest post-contraction or stretch. Nebulin is a large protein thought to regulate the assembly of the actin filaments and, therefore, the length of thin filaments. Myomesin and α-actinin are also integral to the organization of the sarcomere. Myomesin forms the M line, connecting and anchoring the thick filaments. α-actinin, found in the Z-discs, helps anchor the thin filaments. Lastly, dystrophin is a structural protein that plays a crucial role in connecting thin filaments of the sarcomere to integral membrane proteins in the sarcolemma. These proteins are then attached to the connective tissue matrix surrounding muscle fibers, which helps reinforce the sarcolemma and transmit tension generated by sarcomeres to tendons.
