15.1.10

Skeletal Muscle

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Muscle Fibres

The structure of skeletal muscles is specialised for contraction. Skeletal muscle is a type of striated muscle.

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Muscle fibres

  • Skeletal muscle consists of many bundles of muscle fibres.
  • Muscle fibres are long, specialised cells.
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Sarcolemma

  • The membrane of muscle fibres is called the sarcolemma.
  • The sarcolemma folds inwards to the sarcoplasm (muscle fibre cytoplasm) at certain points.
  • The inwards folds are called transverse (T) tubules. The tubules are very important in initiating muscle contraction.
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Sarcoplasmic reticulum

  • The sarcoplasmic reticulum (SR) is an organelle in the sarcoplasm.
  • The SR is a store for calcium (Ca2+) ions. This is important in muscle contraction.
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Mitochondria and nuclei

  • Muscle fibres also have many mitochondria and nuclei.
  • The mitochondria provide lots of ATP to power muscle contraction.
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Myofibrils

  • Myofibrils are cylindrical organelles that run along the length of muscle fibres.
  • Myofibrils are the site of muscle contraction.

Myofibrils

Myofibrils are cylindrical organelles in muscle fibres. Myofibril structure is highly specialised for muscle contraction.

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Sarcomere

  • Myofibrils are made of multiple units that run end-to-end along the myofibril. These units are called sarcomeres.
  • The end of a sarcomere is called the Z-line.
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Myofilaments

  • Sarcomeres are made from two types of myofilaments.
  • The two myofilaments slide past each other. This movement is what makes muscles contract.
  • The two types of myofilaments are:
    • Thick myofilaments - made of myosin protein.
    • Thin myofilaments - made of actin protein.
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Myosin filaments

  • Myosin and actin filaments are arranged in an alternating pattern in sarcomeres.
  • Thick myosin filaments overlap with the thin actin filaments at each end.
    • The overlapping region is called the A-band.
    • The region with only myosin filament is called the H-zone.
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Actin filaments

  • Thin actin filaments only overlap with myosin filaments in the middle of the sarcomere. The middle is called the M-line.
  • The region with only actin filament is called the I-band.

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1Cell Structure

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