Final answer:
Single-unit smooth muscle fibers are connected by gap junctions and contract as a single unit, allowing them to vary their force of contraction. Multi-unit smooth muscle fibers do not possess gap junctions and have limited ability to vary their force of contraction. However, they can maintain low-level contractions for extended periods.
Step-by-step explanation:
Single-unit and multi-unit smooth muscle fibers have structural and functional differences. Single-unit smooth muscle fibers are connected by gap junctions and contract as a single unit. They are found in the walls of visceral organs and have the ability to vary their force of contraction through a stress-relaxation response. In contrast, multi-unit smooth muscle fibers do not possess gap junctions and are not electrically coupled. Their contraction is confined to the cell that was originally stimulated. They are found around large blood vessels, in the respiratory airways, and in the eyes. Although multi-unit smooth muscle fibers do not have the same ability to vary their force of contraction as single-unit smooth muscle fibers, they can maintain low-level contractions for long periods of time.