myofibrilli

Myofibrils are the fundamental contractile units of muscle cells, also known as muscle fibers. These long, cylindrical organelles are packed tightly within the sarcoplasm of a muscle cell and are responsible for muscle contraction. Each myofibril is composed of a repeating arrangement of smaller protein filaments: actin and myosin. These filaments are organized into repeating units called sarcomeres, which are the basic functional units of striated muscle.

The characteristic striped or striated appearance of skeletal and cardiac muscle is due to the highly organized arrangement of these actin and myosin filaments within the myofibrils. The overlapping patterns of these filaments create distinct light and dark bands. During muscle contraction, the myosin filaments slide past the actin filaments, shortening the sarcomere and thus the myofibril, which ultimately leads to the shortening of the entire muscle. This sliding filament theory is the primary mechanism of muscle action.

Myofibrils are crucial for all forms of muscle movement, from voluntary actions like walking and lifting to involuntary processes such as the beating of the heart and the movement of food through the digestive system. The number and arrangement of myofibrils within a muscle fiber can vary depending on the muscle's function, with muscles requiring greater strength or endurance having a higher density of myofibrils.