vöðvatrefjur

Vöðvatrefjur, also known as muscle fibers, are the fundamental cellular units of muscle tissue in vertebrates and invertebrates. These elongated cells are specialized for contraction, enabling movement. In skeletal muscle, vöðvatrefjur are long, multinucleated cells that appear striated under a microscope due to the organized arrangement of contractile proteins actin and myosin. Smooth muscle fibers are typically spindle-shaped, uninucleated, and lack striations, found in the walls of internal organs. Cardiac muscle fibers, found in the heart, are branched, striated, and generally uninucleated, connected by intercalated discs.

The contraction of vöðvatrefjur is triggered by a nerve impulse or hormonal signals. This signal initiates a cascade of biochemical events leading to the sliding of actin and myosin filaments past each other, shortening the fiber. This process, known as the sliding filament theory, is fueled by adenosine triphosphate (ATP). Different types of vöðvatrefjur exist, varying in their speed of contraction, fatigue resistance, and metabolic pathways. For example, slow-twitch fibers are efficient for endurance activities, while fast-twitch fibers are suited for explosive movements. The strength and function of muscles depend on the number and type of vöðvatrefjur they contain.