Which of the following is NOT found in both skeletal and cardiac muscle?

In the context of muscle tissue and its structure, the correct answer is that gap junctions are not found in both skeletal and cardiac muscle.

Cardiac muscle is characterized by the presence of gap junctions, which facilitate direct electrical coupling between adjacent muscle cells, allowing for synchronized contraction of the heart. These junctions are crucial for the coordinated rhythmic contractions of the heart that pump blood efficiently.

In contrast, skeletal muscle does not have gap junctions between its fibers. Instead, skeletal muscle fibers are innervated by motor neurons, and while they are multinucleated and can communicate via the neuromuscular junction, they do not require the electrical coupling provided by gap junctions as seen in cardiac tissue.

Other structural features, such as troponin, sarcomeres, and being striated, are shared by both skeletal and cardiac muscle. Troponin complexes are integral in the regulation of muscle contraction in both muscle types. Sarcomeres, the basic contractile units, align to give both types of muscle their striated appearance. Thus, gap junctions serve a unique role in cardiac muscle physiology that does not apply to skeletal muscle.