Which is an example of multi-unit smooth muscle?

• A. Intestinal wall
• B. Uterus
• C. Esophagus
• D. Iris

Answer: (D)

Multi-unit smooth muscle is made up of individual smooth muscle fibers that contract independently, with limited electrical coupling between cells. This allows fine, graded control by nerves for precise actions, rather than a tissue-wide wave of contraction. The iris fits this pattern because its two smooth muscle components—the sphincter and the dilator—consist of separate motor units that can be activated independently by autonomic nerves to adjust pupil size.

In contrast, tissues like the intestinal wall and much of the esophagus are organized as single-unit (unitary) smooth muscle. Their cells are linked by many gap junctions, so they function as a coordinated sheet, with contractions spreading like a wave. The uterus is primarily single-unit smooth muscle as well, coordinating contractions across a large area. Therefore, the iris is the example of multi-unit smooth muscle.