Which characteristic best describes smooth muscle as non-striated and involuntary?

• A. Striated, voluntary.
• B. Attached to bones.
• C. Non-striated, involuntary, and visceral muscle.
• D. Lacks actin and myosin.

Answer: (C)

Muscle types are distinguished by how their contractile proteins are organized and how they’re controlled. Smooth muscle is non-striated and under involuntary control, and it mainly forms the walls of hollow organs—the visceral muscles. The lack of striations comes from the contractile proteins (actin and myosin) being arranged differently than in skeletal muscle, without obvious sarcomeres, so light microscopy doesn’t show stripes. This arrangement lets smooth muscle contract slowly and sustain contractions, which is essential for moving contents through the intestines, constricting blood vessels, and other functions in the body’s internal organs. It’s controlled by the autonomic nervous system and hormones, not by conscious effort. It’s not attached to bones (that would be skeletal muscle, which is striated and voluntary), and while it does contain actin and myosin, they’re organized without forming the banded pattern seen in skeletal muscle.