Structure of skeletal, cardiac, and smooth muscle

muscle structure summary

Skeletal muscle structure

  • muscle fibres are highly specialised cells

    • multinucleate (syncytium)

    • many mitochondria

    • surrounded by a sarcolemma

  • sarcoplasm contains myofibrils

    • bundles of actin and myosin myofilaments

    • the arrangement of the actin and myosin myofilaments results in the striated appearance of skeletal muscle

  • the sarcolemma contains folds (invaginations) called T-tubules

    • link up to a specialised endoplasmic reticulum called the sarcoplasmic reticulum

    • calcium ions are stored here

single muscle fibre structure

Actin

  • globular protein

  • many actin molecules link together in a long chain

  • two chains twist together to form an actin filament

  • a fibrous protein (tropomyosin) is twisted around the actin filament

  • another protein (troponin) is attached at regular intervals

actin structure

Myosin

  • fibrous protein

  • tail of the protein is attached to the M line

  • several molecules lie in a bundle together

  • the heads stick out to form cross bridges

myosin structureeach myosin filament is surrounded by 6 actin filaments

Sarcomere structure

  • Light bands (I band)

    • areas appear light as they are the region where the actin and myosin filaments do not overlap

  • dark bands (A band)

    • appear dark because of the presence of thick myosin filaments

    • edges are particularly dark as the myosin is overlapped with actin

  • Z lines

    • mark the boundaries of each sarcomere unit

  • M line

    • central line of sarcomere

  • H zone

    • area with only myosin

    • central lighter region within the A band

sarcomere structure