Final answer:
Intercalated discs in cardiac muscle cells function to electrically couple myocardial cells via gap junctions and mechanically anchor them with desmosomes, facilitating the synchronized and involuntary contractions of the heart.
Step-by-step explanation:
The correct function of intercalated discs is to both couple myocardial cells electrically and attach myocardial cells mechanically. Intercalated discs comprise gap junctions and desmosomes. Gap junctions form channels allowing the flow of depolarizing current from one cardiac muscle cell to the other, facilitating electric coupling, which is essential for quick transmission of action potentials and coordinated cardiac contractions. On the other hand, desmosomes anchor the ends of cardiac muscle fibers together, ensuring the cells do not pull apart during the stress of contractions. Contrary to one of the possible choices, the function of intercalated discs is not responsible for voluntary contraction, as cardiac muscle activity is involuntary.