The myocyte is a specialized cardiac muscle cell that is approximately 25 µ in diameter and about 100 µ in length. The myocyte is composed of bundles of myofibrils that contain myofilaments. The myofibrils have distinct, repeating microanatomical units, termed sarcomeres, which represent the basic contractile units of the myocyte. The sarcomere is defined as the region of myofilament structures between two Z-lines.
Chemical and physical interactions between the actin and myosin cause the sarcomere length to shorten, and therefore the myocyte to contract during the process of excitation-contraction coupling. The interactions between actin and myosin serve as the basis for the sliding filament theory of muscle contraction.