Which tissue type consists of unstriated fibers lining various organs, attach to hair follicles, and line blood vessels?

• A. Skeletal muscle
• B. Smooth muscle
• C. Cardiac muscle
• D. Muscle fibers

Answer: (B)

Smooth muscle tissue is made of unstriated, spindle-shaped cells that contract involuntarily. It forms the walls of hollow organs and blood vessels, regulating the size of lumens and the flow of contents. It also includes the arrector pili muscles that attach to hair follicles, pulling hairs upright in response to autonomic input. This combination of unstriated fibers and its presence in the walls of vessels and in association with hair follicles makes it the tissue described. Skeletal and cardiac muscles are striated and under different control, and “muscle fibers” is a generic term that doesn’t specify this smooth, nonstriated tissue.