During muscle contraction, (a) sodium ions are released by the muscle cell’s endoplasmic reticulum. (b) calcium ions allow myosin heads to bind to actin. (c) ATP causes the myosin heads to pivot,...

During muscle contraction,

(a) sodium ions are released by the muscle cell’s endoplasmic reticulum.

(b) calcium ions allow myosin heads to bind to actin.

(c) ATP causes the myosin heads to pivot, pulling on actin.

(d) actin heads repeatedly bind to myosin, pivot, release, extend, attach, and pivot again.