Which of the following best explains the cellular mechanisms involved in the process of summation in skeletal muscle contraction? a. Increased stimulation frequency results in higher turnover of ATP, which enables the buildup of a greater twitch force. b. Additional motor units are stimulated to contract due to an increase in stimulation frequency, which results in a greater force output from the muscle as a whole. c. Myosin is still bound to actin and the sarcomere is still partially contracted when the muscle is stimulated to contract again. This results in additive levels of force output because the next contraction begins from this state rather than from rest. d. The active transport of calcium ions back into the sarcoplasmic reticulum is accelerated with increased stimulation frequency. This results in longer interactions between actin and myosin, which generates greater muscle tension.
Added by Shannon S.
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Step 1: When skeletal muscle is stimulated, myosin is still bound to actin and the sarcomere is still partially contracted. Show more…
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During contraction of skeletal muscle: a. ATP stimulates $\mathrm{Ca}^{2+}$ to move out of the cytosol, which allows tropomyosin to bind myosin causing contraction of the thin filament. b. myosin crossbridges use ATP to relax the molecular spring in the myosin head, which pulls the thick filaments away from the thin actin filaments. c. actin binds ATP, allowing troponin in the thick filaments to form the myosin crossbridge. d. action potentials cause the release of $\mathrm{Ca}^{2+}$ allowing tropomyosin fibers to uncover the actin binding sites needed for the myosin crossbridge. e. botulinum toxin could increase the release of acetylcholine at the contracting muscle site.
Which statement about skeletal muscle contraction is not true? a. A single action potential at the neuromuscular junction is sufficient to cause a muscle to twitch. b. Once maximum muscle tension is achieved, no ATP is required to maintain that level of tension. c. An action potential in the muscle cell activates contraction by releasing $\mathrm{Ca}^{2+}$ into the sarcoplasm. d. Summation of twitches leads to a graded increase in the tension that can be generated by a single muscle fiber. e. The tension generated by a muscle can be varied by controlling how many of its motor units are active.
Joanna Q.
The function of ATP during muscle contraction is to a. cause an allosteric change in myosin so it detaches from actin. b. provide the energy necessary for the movement of the crossbridge. c. expose the myosin-binding sites on the thin filaments. d. do all of the above. e. do a and b only
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