13. The functional unit of contractile system in striated muscle is(A) myofibril(B) cross bridges(C) Z band(D) Sarcomere

18. During the propagate ion of a nerve ion of a nerve impulse, the action potential results from the movement of(A) K+ ions from intracellular fluid to extracellular fluid(B) K+ ios from extracellular fluid to intracellular fluid(C) Na+ ions from extracellular fluid to intracellular fluid(D) Na+ ions from intracellular fluid to extracellular fluid