Which muscle type is involuntary and lines hollow organs?

• A. Smooth muscle
• B. Skeletal muscle
• C. Cardiac muscle
• D. Striated muscle

Answer: (A)

Smooth muscle is the type that is involuntary and lines hollow organs. Its fibers are spindle-shaped and non-striated, so you don’t see the banded look of skeletal muscle. It forms the walls of many hollow structures—blood vessels, airways, and the digestive tract, as well as organs like the bladder and uterus. Because it’s controlled by the autonomic nervous system and hormones, its contractions are slow and sustained, perfect for propelling contents through a tube system (peristalsis) and regulating lumen diameter. In contrast, skeletal muscle is under voluntary control and attaches to bones, while cardiac muscle is also involuntary but lines the heart itself and is striated. Striated muscle isn’t the correct category for lining hollow organs.