In cardiac muscle, calcium ions do not bind to troponin molecules. calcium ions are not released from the sarcoplasmic reticulum. calcium ions play an important role in repolarizing the membrane after the depolarization phase. about 20 percent of the calcium ion required for contraction comes from outside the cell. calcium ions play no role in the process of contraction.

Option). about 20 percent of the calcium ion required for contraction comes from outside the cell.

Explanation:

Calcium ions play an important role in cardiac muscle physiology. Calcium ions influx is responsible for plateau phase after depolarization. Calcium ions also play role in muscle contraction with the help of troponin (regulatory protein).

For contraction, about 20 percent of calcium ions are supplied from outside of the cell, while rest of the calcium ions are released from SR (sarcoplasmic reticulum).

Thus, the correct answer is 'forth option.'