Final answer:
The opening in the center of a connexon allowing passage of solutes between cells is known as a gap junction channel, a crucial component for cell-to-cell communication and coordinated function in tissues such as cardiac muscle.
Step-by-step explanation:
The opening in the center of a connexon that allows the passage of solutes between cells is called a gap junction channel. Gap junctions develop when a set of six proteins, known as connexins, in the plasma membrane arrange themselves in an elongated donut-like configuration, forming a connexon. When these connexons' pores, or "doughnut holes," of adjacent animal cells align, a direct channel between the two cells is established. This channel permits the passage of water and small solutes, but not larger molecules like proteins or nucleic acids, and plays a crucial role in cell-to-cell communication, particularly in cardiac muscle where it allows the electrical signal for the muscle to contract to pass efficiently, enabling the heart muscle cells to contract in unison.