Читать книгу Musculoskeletal Disorders - Sean Gallagher - Страница 76

The contraction of muscle tissues provides motion, maintenance of posture, and heat production. The mechanisms of contraction of sarcomeres are key to muscle function. The thin and thick filaments only partially overlap in resting sarcomeres (Figure 3.7). However, during contraction, the amount of overlap between these filaments increases due to sliding of the filaments past one another (Figure 3.10), which causes shortening of the sarcomere. This contraction is induced by an action potential produced at the myoneural synapse (the juncture of a somatic motor axon and a muscle fiber). At the sarcomere level, during contraction, a small subset of myosin heads align with available actin‐binding sites. As the bound myosin heads move the actin, they provide for alignment of new actin–myosin cross bridges. The old bridges detach only after the myosin binds a new ATP molecule. This action resets the myosin head and prepares it for another contraction cycle. If no ATP is available, the actin–myosin complex become stable, which accounts for the extreme muscle rigidity associated with death (rigor mortis). A single muscle contraction is the result of hundreds of bridge‐forming and bridge‐breaking cycles. The contraction activity that leads to a complete overlap between thin and thick filaments continues until Ca²⁺ ions are removed, and the troponin–tropomyosin complex again covers the myosin‐binding site.