Final answer:
Gap junctions are protein-lined pores that allow the passage of water and small molecules. They play an important role in cardiac muscle by facilitating the transmission of electrical signals for synchronized contraction.
Step-by-step explanation:
Gap junctions are protein-lined pores formed by connexins, which are proteins in the plasma membrane of adjacent animal cells. When the connexons align, they create a channel between the cells that allows the passage of water and small molecules, but not proteins or nucleic acids. Gap junctions play a crucial role in cardiac muscle, where they facilitate the efficient transmission of electrical signals for synchronized contraction.