Muscle fiber

Muscle fiber or muscle cell refers to a muscle tissue's individual cells. They are formed from the fusion of developmental myoblasts (a type of embryonic progenitor cell) in a process known as myogenesis. Muscle fibers are cylindrical and have more than one nucleus. They are also multi-mitochondrial. The three types of muscle fibers are: Type I fibers, Type IIa fibers, and Type IIx fibers.

Structure

Muscle fibers are long, cylindrical structures that are bound by a plasma membrane (the sarcolemma). Each muscle fiber contains many myofibrils, which are strands of proteins (actin and myosin) that are used for muscle contraction. The region of the muscle fiber where the actin and myosin filaments overlap is very dense, leading to a dark appearance under the microscope, and is known as the A-band. The lighter I-band is the region where the actin and myosin filaments do not overlap.

Types

There are three types of muscle fibers: Type I fibers, Type IIa fibers, and Type IIx fibers. Type I fibers are slow-twitch, fatigue-resistant fibers used for endurance and postural maintenance. They have a high myoglobin content, which makes them appear red. Type IIa fibers are intermediate fibers that are also known as fast oxidative-glycolytic fibers. Type IIx fibers are fast-twitch, glycolytic fibers that are used for short, rapid bursts of power.

Function

Muscle fibers are responsible for the contraction of muscles. This is achieved through the sliding filament model of muscle contraction. In this model, the myosin heads of the thick filaments bind to the actin of the thin filaments, forming cross-bridges. The myosin heads then pull the thin filaments towards the center of the sarcomere, shortening the muscle and causing contraction.

See also

• Myocyte
• Myofibril
• Sarcomere
• Muscle contraction
• Sliding filament model

References

Muscle fiber Resources
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