Skeletal Muscle

Describe the anatomy and physiology of skeletal , smooth, and cardiac muscle

Describe the mechanism of excitation-contraction coupling

Skeletal muscle has a number of functions:

• Facilitate movement
• Posture
  Via tonic contraction of antagonistic muscle groups.
• Soft tissue support
  Abdominal wall and pelvic floor support viscera.
• Voluntary sphincter control
• Heat production

Structure and Contents

Skeletal muscle consists of long tubular cells, known as muscle fibres, which run the length of the muscle. Skeletal muscle cells:

• Are under voluntary control from the somatic nervous system via α-motor fibres
  α-motor fibres may control multiple myofibres, forming a motor unit.
• Are 10-100μm in diameter
• Contain several hundred peripheral nuclei
• Contain multiple mitochondria
  • Slow oxidative fibres (red fibres)
    Contain multiple mitochondria, produce sustained contraction, and are resistant to fatigue.
  • Fast glycolytic fibres (white fibres)
    Contain low numbers of mitochondria and large amounts of glycogen, and produce strong contractions but are more easily fatigued.
• Contain sarcoplasmic reticulum
• Contain large amounts of glycogen
  ~200g total.
• Contain myoglobin
• Appear striated microscopically due to the arrangement of myofibrils
  • Myofibrils are multiple myofilaments arranged in parallel
  • Myofilaments are formed from multiple sarcomeres arranged in series
  • A sarcomere is the functional unit of muscle

Muscle fibres are surrounded by layers of connective tissue:

• Endomysium
  Thin layer which surrounds each muscle fibre.
• Perimysium
  Surrounds bundles of muscle fibres.
• Epimysium
  Thick layer which surrounds an entire muscle.

These layers of connective tissue join at the end of a muscle to form a tendon or aponeurosis.

Sarcomere

The sarcomere is the functional contractile unit of muscle. Average sarcomere length is 2.5μm.

The sarcomere contains two main proteins:

• Myosin (thick) filaments
  Myosin is a large protein with two heads, which bind actin and ATP. The myosin head flexes on its neck during contraction.
• Actin (thin) filaments
  Actin is a smaller protein than myosin, and potentiates the ATPase of myosin. Actin filaments have a groove which contains another protein called tropomyosin, to which troponin attaches to.
  • Troponin has three subunits:
    • Troponin T - binds troponin to tropomyosin
    • Troponin I - prevents myosin binding to actin by physically obstructing the binding site
    • Troponin C - Binds Ca²⁺ which initiates contraction

These proteins are arranged to form three bands and two lines:

• A-band
  The myosin filaments.
• H-band
  The section of myosin filaments not overlapping with actin filaments.
• I-band
  The section of actin filaments not overlapping with myosin filaments.
• Z-line
  Each end of the sarcomere. Actin from adjacent sarcomeres are connected at the Z line.
• M-line
  Band of connections between myosin filaments.

Excitation-Contraction Coupling

Muscle contraction normally requires the coordination of electrical (signaling) events with mechanical events.

• In response to ACh stimulating nicotinic receptors, the Na⁺ and K⁺ conductance of the end-plate increases and an end-plate potential is generated
• Muscle fibres undergo successive depolarisation and an action potential is generated along T tubules
  These deliver the AP deep into the cell, and close to the sarcoplasmic reticulum.
• Ca²⁺ is released from sarcoplasmic reticulum
  This process involves:
  • Dihydropyridine Receptor
    Specialised voltage-gated L-type Ca²⁺ channel, activated by T-tubular depolarisation. Responsible for a small amount of Ca²⁺ transport.
  • Ryanodine Receptor
    A second Ca²⁺ channel which is attached to, and activated by, the dihydropyridine receptor, causing a much larger release of Ca²⁺.

• Ca²⁺ is released from the SR (increasing intracellular Ca²⁺ 2000x) and binds to troponin C, weakening the troponin I - actin link and uncovering myosin-binding sites on actin
• Cross-linkages form between actin and myosin, which releases ADP
• The release of ADP triggers a power stroke, which is a process of attachment, pulling, and detachment
  Each cycle shortens the sarcomere by ~10nm:
  • The myosin head rotates on its 'neck', moving to a new actin binding site
  • ATP binds to the (now free) binding site on the myosin
  • ATP is hydrolysed to ADP, in the process "re-cocking" the myosin head
    This process causes the thick and think filaments to slide on each other, with the myosin heads pulling the actin filaments to the centre of the sarcomere. Therefore, over the course of a power stroke:
    • The A-band is unchanged
    • The H-band shortens
    • The I-band shortens

• Power strokes continue as long as there is ATP and Ca²⁺ available

• In relaxation:

  • Ca²⁺ is pumped back into the sarcoplasmic reticulum
    This is an ATP-dependent process, and is why muscle relaxation is active.
  • Troponin releases Ca²⁺
  • Binding sites are occluded by troponin, and no further contraction occurs

References

1. Kam P, Power I. Principles of Physiology for the Anaesthetist. 3rd Ed. Hodder Education. 2012.
2. Barrett KE, Barman SM, Boitano S, Brooks HL. Ganong's Review of Medical Physiology. 24th Ed. McGraw Hill. 2012.
3. Slomianka, L. Muscle. University of Western Australia - School of Anatomy and Human Biology.