Describe the ultrastructure of striated muscle with reference to sarcomere structure using electron micrographs and diagrams

Sourav PanNovember 1, 2024

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The ultrastructure of striated muscle, particularly skeletal muscle, is characterized by its highly organized arrangement of muscle fibers, which allows for efficient contraction. The basic functional unit of striated muscle is the sarcomere, which is a repeating structural unit within the myofibrils of the muscle fibers. Below, we’ll describe the ultrastructure of striated muscle with specific reference to sarcomere structure, including key components and their arrangement, as visualized by electron micrographs and diagrams.

1. Overall Structure of Striated Muscle

Striated muscle fibers are long, cylindrical cells (muscle fibers) that contain numerous myofibrils. Each myofibril consists of a series of sarcomeres arranged end to end. The striated appearance of skeletal muscle arises from the alternating light and dark bands produced by the arrangement of actin and myosin filaments within the sarcomeres.

2. Ultrastructure of the Sarcomere

A sarcomere is the fundamental contractile unit of striated muscle and can be defined from one Z line to the next. It has a highly organized structure, which can be broken down into several key components, visualized using electron micrographs:

A. Key Components of Sarcomere Structure

• Z Line (Z Disc): The Z line defines the boundaries of each sarcomere and is where actin filaments anchor. It appears as a dense line in electron micrographs. The Z lines connect adjacent sarcomeres and help maintain the alignment of the myofibrils.
• I Band: This is the light band that contains only thin filaments (actin) and is located on either side of the Z line. The I band appears light in color on electron micrographs and is bisected by the Z line.
• A Band: The A band is the dark band that contains both thick (myosin) and thin (actin) filaments. It appears dark under electron microscopy. The length of the A band corresponds to the length of the myosin filaments, and it does not change during muscle contraction.
• H Zone: The H zone is the central region of the A band where there is only myosin (thick filaments), and no overlapping actin (thin filaments). This zone is light in comparison to the surrounding A band.
• M Line: The M line is a thin line in the middle of the H zone that serves as an anchor for the thick filaments. It contains proteins that help maintain the alignment of myosin filaments.

B. Filament Arrangement

• Thin Filaments: Composed primarily of actin, thin filaments also include regulatory proteins troponin and tropomyosin. In the sarcomere, these filaments are anchored to the Z line and extend toward the center of the sarcomere, overlapping with thick filaments in the A band.
• Thick Filaments: Composed mainly of myosin molecules, thick filaments are located in the center of the sarcomere, spanning the A band. Each myosin filament consists of a long tail and globular heads, which can interact with actin during muscle contraction.

3. Visualization Using Electron Micrographs

Electron micrographs of striated muscle fibers provide a detailed view of the sarcomere structure. The alternating light (I band) and dark (A band) bands are clearly visible, along with the Z lines and M line. The myofibrils, with their organized arrangement of sarcomeres, appear as a series of repeating units along the length of the muscle fiber.

• Electron Micrograph Features:
  • I Band: Appears as a lighter region containing thin filaments.
  • A Band: Appears darker due to the presence of both thick and thin filaments.
  • Z Line: Appears as a dense line that bisects the I band.
  • M Line and H Zone: Located within the A band, the H zone can be seen as a lighter region flanked by the dense A band.