Question

which of the following is preventing the myosin from accessing the actin so calcium is needed to override these two items?
nitrogen
tropomyosin
troponin
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Explanation:

In muscle contraction, tropomyosin covers the myosin - binding sites on actin. Troponin is involved in the regulation of this process. When calcium binds to troponin, it causes a conformational change in troponin, which then moves tropomyosin away from the myosin - binding sites on actin. Nitrogen is not directly involved in this mechanism of preventing myosin from accessing actin.

Answer:

Tropomyosin, Troponin