Which statement about skeletal muscle contraction is true? A) A single action potential at the neuromuscular junction is not sufficient to cause a muscle to twitch. B) An action potential in the muscle cell activates contraction by releasing Na⁺ into the sarcoplasm. C) Twitches are able to sum when Ca²⁺ is quickly removed from the sarcoplasm between action potentials. 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 is invariable.

Question

Quizplus · Accepted Answer

Summation of twitches leads to a graded increase in the tension that can be generated by a single muscle fiber. This is due to the increased frequency and amplitude of action potentials, which lead to more Ca2+ being released into the sarcoplasm and a stronger contraction. A single action potential at the neuromuscular junction can indeed cause a muscle to twitch, but sustained contractions require summation of multiple twitches. The action potential in the muscle cell does not release Na+, but instead causes voltage-gated Ca2+ channels to open, leading to Ca2+ release from the sarcoplasmic reticulum. The removal of Ca2+ from the sarcoplasm actually leads to relaxation, not further contraction. Finally, the tension generated by a muscle can vary based on factors such as muscle length, muscle fiber recruitment, and fatigue.

Which Statement About Skeletal Muscle Contraction Is True