All muscle tissue is formed from myocytes, whereby a distinction is made between cardiac, skeletal and smooth muscle cells. Skeletal muscle cells are elongated, multinuclieat, cylindrical and striated, hence the muscles they contain are also called striated muscles. A sarcomere is the basic unit of striated muscle, comprising actin (light bands) and myosin (dark bands) filaments. These myofilaments assemble into myofibrils.
Unlike skeletal muscle cells, cardiac muscle cells (cardiomyocytes) are short, narrow and have a rectangular shape, and contain many sarcosomes, which provide the energy needed for contraction. Cardio myocytes are structurally connected via striations, which have gap junctions for diffusion and communication. These serve to transmit contractile force between cells as electrical depolarization can spread directly from cell to cell. Thus, the key role of cardiac muscle cells throughout the body is to generate sufficient contractile force to allow the heart to beat effectively and independently.
Smooth muscle cells are spindle-shaped and contain a single central nucleus and are the smallest muscle cells with a length of 10 to 600 μm. Because the myofibrils of smooth muscle cells are not aligned as in cardiac and skeletal muscle, they appear smooth rather than striated. Smooth muscle cells are arranged in layers, which allows them to contract simultaneously. As they are responsible for involuntary contractions and are found in blood vessel walls and hollow organs such as the gastrointestinal tract, uterus and bladder.