Which statement best summarizes the role of calcium in skeletal muscle contraction?
(A) It prevents muscle contractions during action potentials.
(B) It changes the hypertonic nature of the cytosol to allow action potentials.
(C) It amplifies the physical contraction of the sarcomere.
(D) It connects the electrical neuronal signal to the actual physical contraction.

The extracellular environment of the human body is typically abundant in sodium and calcium. In skeletal muscle cells, the sarcoplasmic reticulum is a specialized organelle that actively sequesters calcium from the cytosol. It stockpiles the calcium until a motor neuron triggers its release through an action potential, which opens voltagegated calcium channels in the sarcoplasmic reticulum membrane. The calcium is necessary for the calciummediated functions of the protein troponin. In the presence of calcium, troponin removes tropomyosin from the myosin binding sites on actin filaments. This attachment is essential for sarcomere and muscle contraction.