15.1.11—Striated muscle ultrastructure

A striated muscle fibre contains sarcoplasm, mitochondria and myofibrils. Each myofibril is a bundle of thin actin and thick myosin protein filaments arranged in repeating units called sarcomeres.

• A sarcomere extends from one Z line to the next; Z lines anchor thin actin filaments.
• The M line anchors thick myosin filaments at the centre.
• The A band contains the full length of the myosin region, including overlap with actin.
• The I band contains actin without myosin, while the H band contains myosin without actin.

The sarcolemma surrounds the muscle fibre, and the sarcoplasm contains the contractile myofibrils and mitochondria that provide ATP for contraction. T-tubules and the sarcoplasmic reticulum are structural components associated with excitation and calcium handling; their activation sequence is covered in the preceding card.

The bands and lines describe the arrangement of filaments; actin and myosin are the filaments, not separate bands. This card identifies static ultrastructure and stops before explaining how cross-bridges make the filaments slide.