American Academy of Professional Coders (AAPC) Practice Exam

What type of muscle contains interlocking involuntary fibers?

1. Cardiac muscle

2. Skeletal muscle

3. Smooth muscle

4. Striated muscle

The correct answer is: Cardiac muscle

Cardiac muscle is characterized by its interlocking involuntary fibers, which are essential for the heart's function. This muscle type is unique because it contains cells that are connected by intercalated discs, allowing for rapid communication between muscle cells. This interconnection facilitates synchronized contractions, which are crucial for pumping blood effectively throughout the body. In contrast, skeletal muscle is under voluntary control, and its fibers are long, cylindrical, and striated but do not interlock. Smooth muscle, found in the walls of internal organs, is also involuntary but consists of non-striated fibers that are not interlocking. The term "striated muscle" refers collectively to both skeletal and cardiac muscle types, but only cardiac muscle has the unique interlocking fibers specific to the heart. Thus, the defining characteristic of cardiac muscle is its interlocking fibers and involuntary nature, making it the correct answer.