Final answer:
Heart muscle cells are connected by structures known as intercalated discs, which contain gap junctions for electrical coupling and desmosomes for mechanical stability, facilitating synchronized contractions.
Step-by-step explanation:
The structures that connect individual heart muscle cells are called intercalated discs. These discs support the synchronized contraction of cardiac muscle by allowing the passage of ions and coordinating the electrical impulses that lead to muscle contraction. Intercalated discs consist mainly of two types of junctions: gap junctions and desmosomes.
Gap junctions create channels for the flow of depolarizing current between cardiac cells, which facilitates the quick transmission of action potentials. Desmosomes, in turn, help to anchor cardiac muscle fibers so they do not pull apart during contraction, ensuring the mechanical integrity of the heart muscle during its forceful contractions. This arrangement allows heart muscle cells to work together as a unit, making the heart an efficient pumping organ.