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.
Which of the following statements correctly describes why a series of closely spaced action potentials causes a sustained contraction rather than a series of closely spaced twitches?
A. Ca2+ ions are released quickly from troponin, keeping the Ca2+ concentration in the cytosol high between closely spaced action potentials.
B. Release of Ca2+ from the sarcoplasmic reticulum through channels is slow compared to the uptake of Ca2+ into the SR via ATP-dependent pumps, resulting in Ca2+ slowly trickling into the sarcomeres between closely spaced action potentials.
C. When a series of action potentials is closely spaced, there is not sufficient time for Ca2+ uptake into the sarcoplasmic reticulum between action potentials, and Ca2+ remains bound to troponin throughout the series.
D. Fewer Ca2+ ions are released from the sarcoplasmic reticulum as a result of several closely spaced action potentials than as a result of a single action potential.