The sarcomere is the basic contractile unit of skeletal muscle in vertebrates. Recall that each of the myofibrils in a muscle cell consists of repeating sarcomere units arranged end to end. At the cellular level, an action potential on the plasma membrane of a muscle cell leads to a release of Ca2+ ions from the sarcoplasmic reticulum. These Ca2+ ions interact with protein components of the sarcomere, controlling the extent and duration of muscle contraction. a) The thin filaments and thick filaments and their associated proteins are the key functional components of the sarcomere. It is important to be able to identify these components in order to understand their roles in muscle contraction.
b) Contraction of a sarcomere, and thus contraction of the muscle as a whole, is accomplished by a cyclic sequence of interactions between actin (thin filaments) and myosin (thick filaments) in the sarcomere. Understanding this sequence of events is essential to understanding the mechanism of muscle contraction at the cellular and molecular levels.
Begin by considering a relaxed muscle. In this state, the myosin molecules of the thick filaments are not in contact with the actin of the thin filaments, and ADP and Pi are bound to the myosin heads. An action potential on the muscle cell plasma membrane initiates contraction of the sarcomeres from this relaxed state.
c) In a relaxed muscle, the myosin heads of the sarcomeres' thick filaments are extended and ready to bind to the actin strands of the thin filaments. But this binding does not occur until an action potential is triggered in the muscle cell. An action potential results in the release of Ca2+ ions from the sarcoplasmic reticulum into the cytosol of the muscle cell.
As the concentration of Ca2+ rises in the cytosol, so does the concentration of Ca2+ in the sarcomeres. In response to changes in the Ca2+ concentration in the sarcomeres, two protein components of the thin filaments, troponin and tropomyosin, control access to actin's myosin-binding sites. In this way, Ca2+ concentration in the cytosol and sarcomeres regulates muscle contraction.
Which of the following statements correctly describe(s) the relationship between Ca2+ concentration in the cytosol and the response in the sarcomere?
d) Skeletal muscle contraction is a graded process, meaning that you can voluntarily alter the strength and extent of contraction of your skeletal muscles, such as your biceps. Increasing the strength and extent of contraction occurs by increasing the number of muscle cells that receive action potentials. In addition, increasing the number of action potentials sent to a muscle cell can also increase muscle tension, as shown in the graph.
A tiny muscle "twitch" is caused by a single action potential.
Two or more closely spaced action potentials have an additive effect because the muscle does not have sufficient time to relax between action potentials.
A long series of closely spaced action potentials results in a sustained, maximum contraction, called tetanus.