Myofibril

Definition

A myofibril is a long, thread-like structure within muscle fibers, composed of repeating units called sarcomeres, which are the basic functional units of muscle contraction. These structures are essential for the contraction mechanism in both skeletal and cardiac muscle, as they contain the actin and myosin filaments responsible for muscle movement. In smooth muscle, while myofibrils are not organized in the same way, the principles of contraction involving these protein filaments still apply.

5 Must Know Facts For Your Next Test

1. Myofibrils are organized into bundles within muscle fibers, contributing to the striated appearance of skeletal and cardiac muscles.
2. The arrangement of myofibrils allows for coordinated contraction across the entire muscle fiber, resulting in powerful movements.
3. In skeletal muscle, each myofibril can be up to several centimeters long and is made up of thousands of sarcomeres lined up end to end.
4. Calcium ions play a critical role in the contraction process by binding to troponin on the actin filaments, which exposes binding sites for myosin.
5. Smooth muscle lacks the organized structure of myofibrils found in skeletal and cardiac muscle, but still utilizes actin and myosin filaments for contraction.

Review Questions

• How do myofibrils contribute to the contraction mechanism in skeletal muscle?
  • Myofibrils are crucial for skeletal muscle contraction as they are made up of sarcomeres, which contain the actin and myosin filaments. When a muscle fiber is stimulated, calcium ions are released, allowing myosin heads to bind to actin and initiate a sliding motion between the filaments. This sliding filament mechanism shortens the sarcomeres, causing the entire muscle fiber to contract. The coordinated action of numerous myofibrils working together results in significant force generation.
• Discuss the differences in myofibril organization between skeletal and cardiac muscles and how these differences affect contraction.
  • Skeletal muscle has myofibrils organized into long parallel bundles that are clearly striated due to the regular arrangement of sarcomeres. In contrast, cardiac muscle also contains myofibrils but they are arranged in a more branched structure with intercalated discs connecting adjacent fibers. This allows for synchronized contractions throughout the heart muscle. The presence of gap junctions in cardiac muscle facilitates rapid electrical communication, ensuring that all cells contract in unison for effective pumping action.
• Evaluate the role of calcium ions in regulating myofibril activity during contraction across different types of muscle tissue.
  • Calcium ions are vital for initiating contraction in both skeletal and cardiac muscles by triggering interactions between actin and myosin within myofibrils. In skeletal muscle, calcium binds to troponin on the thin filaments, leading to a conformational change that exposes binding sites for myosin. Similarly, in cardiac muscle, calcium influx from the extracellular space prompts further release from the sarcoplasmic reticulum, enhancing contractility. In smooth muscle, while there are no organized myofibrils like in skeletal and cardiac muscles, calcium still activates contraction through different pathways involving calmodulin and myosin light chain kinase.

Related terms

Sarcomere: The smallest functional unit of a myofibril, defined by the area between two Z-discs, containing organized arrangements of actin and myosin filaments.

Actin:

A protein that forms thin filaments in myofibrils and is essential for muscle contraction by interacting with myosin.

Myosin: A motor protein that forms thick filaments in myofibrils and interacts with actin to facilitate muscle contraction through a cross-bridge cycle.