14.18: Structure of Cardiac Muscles
Cardiac muscle, or myocardium, is a specialized type of muscle found exclusively in the heart. Its unique structural and functional characteristics enable the heart to perform its vital role of pumping blood throughout the body continuously and rhythmically. The cardiac muscle cells, or cardiomyocytes, possess an endomysium and perimysium but do not have an epimysium.
Compared to skeletal muscles, cardiac muscle cells are small and mostly have a single nucleus. Additionally, they are usually branched and composed of regularly arranged sarcomeres in the myofibrils, giving them a striated appearance. The T-tubules, which are short but broad, arise from the sarcolemma at the sarcomere boundaries and are closely associated with the sarcoplasmic reticulum. To sustain their high energy requirements, cells possess a multitude of mitochondria, as well as reserves of both glycogen and lipids. These energy stores are crucial for proper cellular function and enable cells to carry out their various metabolic processes with optimal efficiency.
Adjacent cardiac muscle cells are connected by intercalated discs, regions of thickened sarcolemma that interlock. These regions contain gap junctions that control the passage of ions and help propagate electrical signals during muscle contraction. The intercalated discs also have an abundance of desmosomes that anchor the cells to each other and prevent cell separation during contraction. This way, the cardiac cells can coordinate to produce the heart's rhythmic contractions.
Similar to skeletal muscle, cardiac muscle fibers can experience hypertrophy when there is an increase in workload. This results in a naturally enlarged heart, which is why many athletes have a larger heart. However, if the heart enlarges due to significant heart disease, it is considered pathological.
