What are the two types of smooth muscle?

The distinction between the two types of smooth muscle is well-defined in muscle physiology. The correct choice identifies that smooth muscle can be classified into visceral (also known as single-unit) and multiunit types.

Visceral smooth muscle is found in the walls of hollow organs such as the intestines and blood vessels. It operates as a cohesive unit where individual muscle fibers are interconnected through gap junctions, allowing for synchronized contractions. This type of smooth muscle is typically responsible for involuntary movements like peristalsis in the digestive tract.

Multiunit smooth muscle, on the other hand, consists of individual muscle fibers that are not interconnected. This type allows for finer control and is generally found in locations such as the iris of the eye and the arrector pili muscles in the skin. Each muscle fiber can be activated independently, making this type suited for functions requiring precise adjustments.

The other options include muscle types that are not classified as smooth muscle. Cardiac muscle is specific to the heart and has a unique structure and function distinct from smooth muscle. Skeletal muscle is under voluntary control and also does not fall within the classification of smooth muscle. Therefore, the identification of visceral and multiunit as the two types of smooth muscle is the correct classification in muscle physiology.