What type of muscle is a sphincter?

• A. A smooth muscle that surrounds a body opening
• B. A cardiac muscle found in the heart
• C. A skeletal muscle attached to bones
• D. A striated muscle involved in voluntary movement

Answer: (A)

A sphincter is classified as a type of smooth muscle that functions to surround and control the opening and closing of various body passages or openings. This muscle type is involuntary, meaning it operates without conscious control, which is crucial for the functions of various systems such as the digestive and urinary systems. Smooth muscles, including sphincters, are not striated; instead, they have a smooth appearance under a microscope.

The other types of muscles listed do not fit the definition of a sphincter. Cardiac muscle, for instance, is specialized to make up the heart and is also involuntary but is distinctly different in function and structure. Skeletal muscles are striated and are usually under voluntary control, allowing for movement of bones. Striated muscles are primarily involved in voluntary movement and are found attached to bones, which again differentiates them from sphincters. Therefore, the characterization of sphincters as smooth muscles surrounding a body opening is accurate and highlights their essential role in bodily functions.