Despite its complexity, the human body is made of only four types of tissue: epithelial, connective (CT), muscle, and nerve. The cells of these tissues associate with one another to form organs. The study of tissues is called histology.

Muscle tissue is responsible for locomotion and for the movements of the various parts of the body with respect to one another. There are three categories of muscle tissue: skeletal, cardiac, and smooth muscle. Another term for skeletal muscle is striated muscle. Visceral is a synonym for smooth muscle.

Cardiac muscle is associated only with the heart. It forms the bulk of the heart wall. The muscle of the heart wall is referred to as the myocardium of the heart. This tissue has striations like skeletal muscle, but also has a distinct specialization: the presence of clusters of gap_junctions and desmosomes referred to as the intercalated_discs. These discs help regulate the passage of energy throughout the heart. Cardiac muscle is "ropy" in appearance; whereas skeletal muscle is tightly organized. Cardiac muscle is involuntary.