Final answer:
The claim that gap junctions are present only in cardiac muscle is false; gap junctions are found in both cardiac and single-unit smooth muscle cells and are vital for the transmission of electrical signals for coordinated contraction.
Step-by-step explanation:
The statement that gap junctions have only been identified in cardiac muscle is false. Gap junctions are essential components of both cardiac and single-unit smooth muscle cells. These junctions arise from connexins in the plasma membrane forming a structure known as a connexon. When adjacent cells align their connexons, a channel is created allowing for communication between the cells.
In cardiac muscle, this communication is critical for the efficient transmission of electrical signals that enable the heart muscle cells to contract simultaneously, a process vital for proper heart function. However, gap junctions are not exclusive to cardiac muscle and are also found in smooth muscle tissues.