vöðvaþræðir

Vöðvaþræðir, known in English as muscle fibers, are the fundamental cellular units of muscle tissue. They are elongated, multinucleated cells that are specialized for contraction. These fibers are the building blocks that allow for movement in the body. Each muscle fiber is surrounded by a connective tissue sheath called the endomysium.

There are different types of muscle fibers, primarily categorized as slow-twitch (Type I) and fast-twitch (Type II). Slow-twitch fibers are more efficient at using oxygen and are better suited for endurance activities like long-distance running. They appear red due to a high concentration of myoglobin and mitochondria. Fast-twitch fibers, on the other hand, contract more forcefully and rapidly but fatigue more quickly. They are further divided into Type IIa (intermediate) and Type IIb (fastest), and are more involved in short bursts of power and speed, such as sprinting or weightlifting.

Within each muscle fiber are myofibrils, which are long, cylindrical structures containing the contractile proteins actin and myosin. The organized arrangement of these proteins within myofibrils creates a repeating pattern known as sarcomeres, the functional units of muscle contraction. When a nerve impulse signals a muscle to contract, calcium ions are released, initiating a sliding motion between the actin and myosin filaments, thereby shortening the sarcomere and causing the muscle fiber to contract. The strength and duration of a muscle contraction depend on the number and type of muscle fibers recruited and the frequency of their stimulation.