Myocytes

Myocytes are muscle cells responsible for contraction in vertebrate muscle tissue. They are specialized to convert chemical energy into motion. There are three main types: skeletal, cardiac, and smooth myocytes.

Skeletal myocytes are long, cylindrical cells that are typically multinucleated from fusion of myoblasts. They show cross-striations due to sarcomeres, the contractile units of actin and myosin. The sarcolemma and T-tubules coordinate excitation-contraction coupling, and the sarcoplasmic reticulum releases calcium during contraction. These fibers rely on aerobic and anaerobic metabolism, and regenerative capacity is limited, sustained mainly by satellite cells.

Cardiac myocytes form the heart muscle. They are shorter and branched, typically with a single nucleus, and are joined by intercalated discs containing gap junctions and desmosomes. This arrangement enables rapid electrical coupling and synchronized contraction. Cardiac myocytes rely heavily on oxidative metabolism and have abundant mitochondria. They respond to impulses from the conduction system and show automaticity in pacemaker regions.

Smooth myocytes are spindle-shaped cells in the walls of hollow organs and vessels. They have a single nucleus and lack striations, reflecting non-sarcomeric actin–myosin organization. Contraction is slower and can be sustained, regulated by calmodulin and myosin light chain kinase, and modulated by autonomic nerves and hormones. They control diameter and tone in the gastrointestinal tract, blood vessels, airways, and other structures.

Clinical relevance: Myopathies may involve any myocyte type. Skeletal disorders include muscular dystrophies; cardiac disease includes cardiomyopathies. Diagnosis often relies on histology and biopsy, while treatment targets gene therapy, pharmacology, and rehabilitation.