Skeletal muscles maintain posture, cause movement, and produce heat. Skeletal muscles are named according to their size, shape, location, direction of fibers, number of attachments, or action. The plasma membrane of a muscle fiber is its sarcolemma. Its endoplasmic reticulum is called the sarcoplasmic reticulum, which stores calcium. The sarcolemma invaginates into T tubules that contact the sarcophasmic reticulum.
Myofibrils are cylindrical structures within muscle fibers.
Myofilaments that make up myofibrils are arranged such that they exhibit striations. Striations are grouped into sarcomeres. Within sarcomeres, thick filaments are made up of myosin, and thin filaments are made up of actin. When a nervous impulse reaches a muscle fiber, the sarcoplasmic reticulum releases its stored calcium, and the fiber contracts. The myosin filaments have cross-bridges that pull on the actin myofilaments, causing them to slide past each other.