American Academy of Professional Coders (AAPC) Practice Exam

What distinguishes cardiac muscle from skeletal muscle?

1. It is voluntary

2. It contains smooth muscle fibers

3. It attaches to tendons

4. It is never involuntary

The correct answer is: It contains smooth muscle fibers

Cardiac muscle is distinct in several ways that highlight its specialized function in the heart. One of the most significant differences is that cardiac muscle is an involuntary muscle, meaning it is not under conscious control, unlike skeletal muscle, which is voluntary and can be controlled consciously. This involuntary nature is critical, as the heart must continuously pump blood without conscious thought. Cardiac muscle fibers are also structurally unique, featuring intercalated discs that facilitate rapid communication and synchronized contraction between muscle cells. This structural adaptation is essential for the efficient pumping action of the heart and is not found in skeletal muscle, which connects to bones via tendons. Additionally, while cardiac muscle shares some characteristics with smooth muscle, it does not actually contain smooth muscle fibers. Rather, it has its own distinct type of muscle fibers that are striated, similar to skeletal muscle, but function involuntarily like smooth muscle. Therefore, the unique characteristics that set cardiac muscle apart are its involuntary function and specialized tissue structure, making it critical for the heart's consistent performance.