EM 358 Skeletal Muscle

Skeletal Muscle Fibers

Transmission electron microscopy (TEM) of a longitudinal section through portions of three myofibers (tan, green, and blue). Myofibers, also known as muscle fibers, are the individual cells that makeup muscle.

Skeletal muscle cells are long, cylindrical cells with multiple nuclei at their periphery and cytoplasm packed with myofibrils. Myofibrils are composed of repeating units of sarcomeres that appear as alternating dark and light bands.

• A Bands (dark bands) - dark bands in the middle of sarcomeres
  • Thick myosin filaments and parts of thin actin filaments where they overlap
• I Bands (light bands) - light bands on either side of the A bands
  • Only thin actin filaments
• Z Lines (black) - narrow bands at the center of I bands
• H bands - central region of A bands
  • Only myosin filaments without overlapping actin filaments
• M Line - dark band at the center of H bands
  • Holds the myosin filaments together

The openings of the T-tubules can be seen at the junction of the A-and I-bands.

Subcellular Structures

• Nuclei (blue) - flattened nuclei at the periphery of the cell
• Mitochondria (red)
• Cytoplasm (tan/green/blue)
• Sarcolemma - cell membrane of skeletal muscle cells

Courtesy of John H. Venable, Department of Anatomy, College of Veterinary Medicine, Colorado State University, Fort Collins, CO.