Final answer:
The correct answer to the student's question is that the low-resistance pathways that enable the spread of action potentials between myocardial cells are the gap junctions (Option A).
Step-by-step explanation:
The low-resistance pathways between myocardial cells that allow for the quick spread of action potentials and the coordinated contraction of the entire heart are the gap junctions. Gap junctions form channels that permit the depolarizing current produced by cations to flow from one cardiac muscle cell to another. This process, known as electric coupling, is essential for the synchronized contraction of the heart muscle. Although T tubules, sarcoplasmic reticulum (SR), and intercalated disks play critical roles in muscle cell function, it is specifically the gap junctions within the intercalated disks that facilitate this rapid and direct spread of electrical impulses.
The remaining component of intercalated disks, the desmosomes, serves a structural purpose by anchoring the ends of the cardiac muscle fibers together.