Which are the two main types of smooth muscle and an example for each?

• A. Single-unit smooth muscle — example: iris; Multi-unit smooth muscle — example: intestinal
• B. Single-unit smooth muscle — example: uterus; Multi-unit smooth muscle — example: iris
• C. Single-unit smooth muscle — example: iris; Multi-unit smooth muscle — example: intestinal
• D. Single-unit smooth muscle — example: iris; Multi-unit smooth muscle — example: uterus

Answer: (B)

Smooth muscle contracts in two organizational patterns. In single-unit (visceral) smooth muscle, many cells are linked by gap junctions so they depolarize together and contract as one coordinated unit. This setup is typical in hollow organs where wave-like, simultaneous contractions move contents, such as the uterus and much of the gastrointestinal tract. In multi-unit smooth muscle, cells contract more independently, each often receiving separate neural input, allowing finer, graded control—this is seen in the iris, which regulates pupil size, and in some other structures like the ciliary muscle.

So the pair matches these patterns: the uterus as a single-unit example and the iris as a multi-unit example.