Sliding Filament Theory
The sarcomere is the functional unit of skeletal and cardiac muscle contraction, and it is organized in a highly structured manner. The first component of the sarcomere is actin filaments, which are thin filaments primarily comprising the protein actin. They are anchored at the Z line and extend towards the center of the sarcomere but do not reach the M line. Actin filaments contain binding sites for myosin heads (Saladin, 2021). The second component is myosin filaments, which are thick filaments with the protein myosin. The myosin heads extend towards the actin filaments and contain binding sites for actin and ATP. The myosin tails form the central portion of the filament and are arranged in a staggered manner. The third component is the Z line, which is a protein structure that serves as the boundary of the sarcomere. The fourth component is the M line, a protein at the center of the sarcomere, where myosin filaments attach (Tamborrini et al., 2023). It helps stabilize the myosin filaments during muscle contraction. The last component is titin filaments, a large protein that spans from the Z line to the M line, running alongside the myosin filaments. It provides elasticity and helps maintain the alignment of the sarcomere during contraction and relaxation.
During muscle contraction, the sarcomere undergoes several changes, including the sliding of filaments, whereby the interaction between actin and myosin filaments causes them to slide past each other, making the sarcomere shorten and causing overall muscle contraction (Saladin, 2021). Second, filaments overlap, whereby as the sarcomere contracts, the actin and myosin filaments overlap to a greater extent. This overlap increases the number of cross-bridges formed between actin and myosin, leading to stronger contraction forces (Herzog & Schappacher-Tilp, 2023). Third, the sarcomere length decreases, whereby the Z lines move closer together as the sarcomere contracts, resulting in a decrease in sarcomere length. This shortening of sarcomeres contributes to muscle shortening and overall muscle contraction. Finally, the last change in sarcomere is an increase in calcium concentration.
References
Herzog, W., & Schappacher-Tilp, G. (2023). Molecular mechanisms of muscle contraction: A historical perspective. Journal of Biomechanics, 111659.
Saladin, K. S. (2021). Anatomy & physiology: The unity of form and function (9th ed.). McGraw-Hill Education.
Tamborrini, D., Wang, Z., Wagner, T., Tacke, S., Stabrin, M., Grange, M., & Raunser, S. (2023). Structure of the native myosin filament in the relaxed cardiac sarcomere. Nature, 623(7988), 863-871.
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Question 
Skeletal and cardiac muscle tissues contract via a process known as the sliding filament theory. To create these contractions, the muscle must be arranged in a specific pattern known as a sarcomere.
Sliding Filament Theory
Discuss the different components of the sarcomere, how they are arranged, and what changes (if any) happen to them when the muscle is contracted.