Myofibrils, Actin and Myosin (concise) (copy)

Structure of skeletal muscle fibres

  • Skeletal muscle is made of long fibres - formed when embryonic muscle cells fuse together. Therefore, junctions between adjacent cells eliminated, so less weak points within the muscle.

  • Muscle cells longer than normal cells.

  • Muscle fibres contain sarcoplasm (essentially cytoplasm) and the sarcoplasm contains multiple nuclei, mitochondria and the sarcoplasmic reticulum (essentially a modified form of endoplasmic reticulum), from which calcium ions are released during muscle contraction.

  • Muscle fibre surrounded by membrane called a sarcolemma.

  • Sarcolemma contains T tubules (folds in the membrane). Facilitates transport of calcium ions for muscle contraction.

  • One muscle fibre is composed of many myofibrils (where the contraction occurs).

Structure of myofibrils

  • Cylindrical and parallel so all contract in the same direction.

  • Mainly composed of myosin and actin

  • Myosin and actin arranged to form filaments.

  • Myosin - rod-like fibres with heads that point outwards. Forms thick filaments.

  • Actin - 2 strands wrapped around each other to form a helix. Forms thin filaments.

Arrangement of actin and myosin

  • Z line - fine line running down the centre of each light band. The distance between two Z lines is a sarcomere.

  • I bands/light bands - made only from thin filaments, so only contain actin. Appears light on a microscope image.

  • A bands/dark bands - made from thick and thin filaments, so contains actin and myosin overlapping. Appears dark on a microscope image.

  • H zone - slightly lighter section at centre of dark band where only thick filaments are present and no thin filaments (so only myosin and no actin).

    Contraction of myofibril

  • Sarcomere shortens ( Z lines move closer together).

  • H zone shortens - thin filaments slide over thick filaments during contraction, so region of only thick filaments decreases.

  • I band shortens - as thin filaments slide over thick filaments, so region of only thin filaments decreases.

  • A band does not shorten - thick filament length has not changed during contraction.