Seneca
Sign up

Skeletal Muscle

Biology: Cambridge International A Level, topic 15.1.10

Muscle Fibres

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

Muscle fibres

Muscle fibres

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

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.
Sarcoplasmic reticulum

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.
Mitochondria and nuclei

Mitochondria and nuclei

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

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.

Sarcomere

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.
Myofilaments

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.
Myosin filaments

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.
Actin filaments

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.

Test yourself on Skeletal Muscle

  • Which of these are NOT a feature of muscle fibres?

  • Which of the following statements about myofibrils are true?

  • What is the middle of the sarcomere called?

  • Which of the following is NOT a structure in myofibrils?

Practise this topic for free