How does cardiac muscle differ structurally and functionally from skeletal muscle?

• A. Cardiac muscle is non-striated and involuntary.
• B. Cardiac muscle is striated and involuntary with intercalated discs.
• C. Cardiac muscle is striated and voluntary with long, cylindrical fibers.
• D. Cardiac muscle is striated and involuntary with intercalated discs that contain gap junctions and desmosomes.

Answer: (D)

The important idea is that cardiac muscle is built to contract as a single, coordinated unit. This comes from intercalated discs connecting cardiac muscle cells; these discs house gap junctions that let ions flow directly between cells so a single electrical impulse can travel quickly through the heart, and desmosomes that mechanically bind the cells so they don’t pull apart during strong heart contractions. Cardiac muscle is striated like skeletal muscle, but it is involuntary, meaning it is controlled by the autonomic nervous system and the heart’s own pacemaker activity rather than conscious control. The cells themselves are shorter and branched, forming a network that supports rhythmic, synchronized pumping. In contrast, skeletal muscle fibers are long, cylindrical, and usually require voluntary neural input to contract, and they aren’t joined by intercalated discs. This combination of electrical and mechanical connections in the heart enables the heart to function as one integrated organ.