Muscular tissue

1
2. Sliding filament theory of muscle contraction

• Muscular tissue
• Muscular tissue is composed of many elongated
  cells called muscle fibres (or myoneme) which
  consist of many myofibril. Each myofibril is
  composed of two types of proteinaceous
  myofilaments actin (thin filament) and myosin
  (thick filament).
• Under the electronic microscope, light and dark
  bands can be observed in every muscle fibre

2

• The centre of myosin is located at M line while
  that of actin is located at Z line. Light band is
  composed of actins while dark band is composed of
  both actins and myosins. H zone is composed of
  myosins only.

3

• Myosins are composed of many rod-shaped
  myoglobins which can be divided into the myosin
  rod and myosin head. Myosin rods are collected
  together to comprise the main branch of myosin
  while myosin heads project out to form the
  cross-bridges.

• Cross-bridges can twist and contain ATPase which
  hydrolyze ATP to generate energy

4

• Each actin filament (A) is made up of two helical
  strands of globular actin molecules (G-actin)
  which twist round each other. It also consists of
  two accessory proteins called tropomyosin (T) and
  troponin.

• Actins and myosin heads can be linked to form the
  cross-bridges. However, at resting stage, the
  attachment site is covered by tropomyosin to
  prevent the formation of cross-bridges. Troponin
  has high infinity to Ca2 and can reversibly bind
  with Ca2 .

5

• Sarcolemma
• The structure of sarcolemma is similar to a
  typical cell membrane but it is invaginated and
  surrounds each muscle fibre. Fluid can pass
  through the membrane and external stimuli can be
  transmitted through it also.

6

• Sarcoplasm
• The cytoplasm of the myofibril is called
  sarcoplasm and contains a network of internal
  membranes termed the sarcoplasmic reticulum.
  Running transverely across the fibre and between
  fibrils is a system of tubules known as the T
  system. A T tubule together with a pair of
  vesicles is called a triad which is involved in
  the uptake and release of Ca2 ions and holds 80
  of Ca2 in a muscle cell.

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• Mechanism of muscle contraction

• Nerve impulse is transmitted to sarcolemma
• Sarcolemma is depolarized
• Action potential spreads out and reaches the
  triad through T system
• Impulse stimulates the release of Ca2 into the
  sarcoplasm
• Ca2 binds with cross-bridge and changes the
  conformation of troponin

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• Attachment site is exposed, ATPase in myosin head
  is activated to hydrolyze ATP to generate energy
• Release of energy accompanies cross-bridge
  formation and pulls the actins towards M line
• Length of sarcomere is shortened and resulted in
  muscle contraction
• Ca2 is actively pumped back into the triad after
  excitation, myosin head combines with another ATP
• Cross-bridge is broken, actins and myosins return
  to original conformation as in resting stage