Which ions directly control the contraction of muscles?

a. Sodium
b. Potassium
c. Calcium
d. Magnesium


ANS: C
Contraction begins as the calcium ions combine with troponin, a reaction that overcomes the in-hibitory function of the troponin-tropomyosin system. The release of intracellular calcium ions is the critical link between a nerve impulse (electrical excitation) and muscle contraction. The criti-cal link that allows for the contraction of muscles is not reliant upon sodium, potassium, or mag-nesium.

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