Skeletal muscles stimulated to contract and act as effectors

• Describe skeletal muscles

  • Act in antagonistic pairs against an incompressible skeleton
  • As one muscle in the pair contracts, the other relaxes
• Describe features of the gross structure of the skeletal muscle

  1. Each fibre shares nuclei, sarcoplasmic reticulum, mitochondria and sarcoplasm
  2. Each fibre is wrapped around a fused cell membrane called the sarcolemma
  3. The sarcolemma has many inward folds called T tubules
  4. Each fibre contains lots of long protein filaments called myofibrils
• Describe the microscopic structure of the skeletal muscle

  Myofibrils are composed of two long proteins

  1. Actin - a thinner filament associated with tropomyosin, which wraps around it
  2. Myosin - a thicker filament made of long strands with heads that point outward

  Myofibrils are made of sarcomeres
• Describe the ultrastructure of a sacromere and during contraction

  A band = overlap between actin and myosin

  I band = only actin

  H zone = only myosin

  During contraction:

  A band = stays the same length

  I band = shortens

  H zone = disappears or shortens
• How is a muscle fibre stimulated to contract?

  • When the sarcolemma is depolarised, action potentials are transmitted down the T tubules
  • This opens the voltage gated Ca2+ channels on the sarcoplasmic reticulum and Ca2+ enters the sarcoplasm
• Describe the role of Ca2+ in muscle contraction

  1. Ca2+ bind to tropomyosin, changing its tertiary structure
  2. Tropomyosin pulls away from actin binding sites, exposing them
• Describe the role of ATP in muscle contraction

  1. Myosin heads with ADP attached bind to the actin - this forms actinomyosin bridges
  2. The myosin heads change angle and pull the actin along in a power stroke movement. This releases the ADP
  3. A new ATP attaches to the myosin environment
  4. Myosin heads detach from the actin - this breajs the actinomyosin bridges
  5. Ca2+ activates ATP hydrolase on the myosin heads to hydrolyse ATP, this allows the myosin heads to recock and bind further along actin
• Describe muscle relaxation

  1. When depolarisation stops, Ca2+ are actively transported back into the sarcoplasmic reticulum
  2. Tropomyosin mow blocks the actin again, so the myosin heads are unable to bind
  3. Contraction of the antagonistic muscle pulls the actin back to its original position
• Describe phosphocreatine during exercise

  Phosphocreatine releases its phosphate to combine with ADP. This maintains the supply of ATP so muscle contraction occurs for longer
• Describe phosphocreatine at rest

  ATP hydrolysis releases phosphate groups to combine with creatine. This replenishes stores of phosphocreatine
• Describe myoglobin

  • A protein in muscle fibres that binds with oxygen to store it
  • Oxygen can then be released when the ppO2 decreases to allow aerobic respiration to continue for longer
• Describe slow twitch muscle fibres

  • Thin myosin filaments
  • Aerobic respiration
  • Long term, endurance exercise
  • Darker red as lots of myoglobin
  • Lots of mitochondria
  • Fatigues slowly
  • Small store of phosphocreatine
  • Lots of capillaries
  • Lots of glycogen
  • Abundant in muscles that control posture
• Describe fast twitch muscle fibres

  • Thick myosin filaments
  • Anaerobic respiration
  • Short term, powerful exercise
  • Lactic acid builds up
  • Lighter colour due to less myoglobin
  • Fatigues quickly
  • Few mitochondria
  • Large store of phosphocreatine
  • Few glycogen
  • Few capillaries
  • Abundant in muscles that contract rapidly