Final answer:
A gap junction forms from membrane-spanning proteins called connexins that create a channel between two adjacent cells, which is vital for synchronization of contractions in cardiac muscle.
Step-by-step explanation:
A gap junction forms when opposing membrane-spanning proteins called connexins on two adjacent cells unite, creating a protein channel that is capable of opening and closing. Specifically, gap junction develops when six of these proteins arrange themselves in a ring structure, known as a connexon. The connexon then aligns with a corresponding connexon on an adjacent cell to form a channel that allows the passage of water, ions, and small solutes, but not proteins or nucleic acids. These gap junctions play a crucial role in the human body, particularly in cardiac muscle, where they enable the electrical signals that cause muscle contraction to be efficiently passed between heart muscle cells, leading to synchronized contractions.