Myofibril

Definition

A myofibril is a long, cylindrical organelle found in muscle cells that is responsible for muscle contraction. Composed of repeating units called sarcomeres, myofibrils contain the contractile proteins actin and myosin, which interact to produce muscle tension. The arrangement and organization of myofibrils contribute to the striated appearance of skeletal muscle and are essential for the overall function of muscle tissue.

5 Must Know Facts For Your Next Test

1. Myofibrils make up about 80% of the volume of a skeletal muscle fiber and are organized into a highly structured arrangement to optimize contraction efficiency.
2. Each myofibril is surrounded by a membrane called the sarcoplasmic reticulum, which stores calcium ions necessary for triggering contraction.
3. The striations seen in skeletal muscle are a result of the alternating light and dark bands created by the arrangement of myofibrils and their sarcomeres.
4. During muscle contraction, myofibrils shorten as the actin filaments slide over the myosin filaments in a process known as the sliding filament theory.
5. Myofibril density can increase with resistance training, leading to hypertrophy or growth of muscle fibers due to the addition of more contractile proteins.

Review Questions

• How do myofibrils contribute to the structure and function of skeletal muscle cells?
  • Myofibrils are essential components of skeletal muscle cells, providing both structural integrity and functional capacity for contraction. They are organized into repeating units called sarcomeres, which contain the contractile proteins actin and myosin. This arrangement allows for efficient force generation during muscle contractions, with myofibrils facilitating the sliding filament mechanism. The overall structure and alignment of myofibrils contribute to the striated appearance of skeletal muscles, crucial for their role in movement.
• Discuss the role of calcium ions in the interaction between myofibrils and muscle contraction.
  • Calcium ions play a critical role in muscle contraction by regulating the interaction between actin and myosin within the myofibrils. When a muscle cell is stimulated, calcium is released from the sarcoplasmic reticulum into the cytoplasm. This increase in calcium concentration allows calcium to bind to troponin, a regulatory protein on the actin filament. This binding causes a conformational change that moves tropomyosin away from actin's binding sites, enabling myosin heads to attach and initiate contraction through cross-bridge cycling.
• Evaluate how changes in myofibril structure can affect overall muscle performance and adaptation to training.
  • Changes in myofibril structure significantly influence muscle performance and adaptability to training regimens. For instance, resistance training leads to an increase in myofibril size and number, enhancing force production capabilities through greater cross-sectional area. Additionally, adaptations may include changes in fiber type composition or improved efficiency of energy use during contractions. Understanding these structural changes helps in designing effective training programs tailored to specific performance goals, as well as promoting recovery strategies that support optimal adaptations in muscular strength and endurance.

Related terms

sarcomere: The basic functional unit of a myofibril, defined by the area between two Z discs, containing overlapping actin and myosin filaments that facilitate muscle contraction.

actin: A protein that forms thin filaments in muscle fibers, interacting with myosin during contraction to enable muscle movement.

myosin: A motor protein that forms thick filaments in muscle fibers, working with actin to generate force and facilitate muscle contraction.