3.6.3 Skeletal muscles

• Muscles act in antagonistic pairs, with one contracting as the other relaxes
• Muscle fibres have:

  • a cell surface membrane called the sarcolemma
  • endoplasmic reticulum called sarcoplasmic reticulum
  • Cytoplasm called sarcoplasm
  • Many myofibrils (measured in sacromeres)
• I band - Actin filaments

  H zone - myosin filaments

  A zone - Both filaments

  During contraction I bands and H zone gets shorter

  
• Neuromuscular junction:

  1. An action potential reaches the junction. Synaptic vesicles fuse with the presynaptic membrane, releasing the neurotransmitter acetylcholine which diffuses across the synaptic gap
  2. Acetylcholine binds to receptor proteins on the sarcolemma, depolarising it and causing an action potential to be generated
  3. The AP causes the diffusion of calcium ions into the sarcoplasm
  4. This causes a protein, tropomyosin, to uncover the myosin head binding sites on the actin filaments. Myosin heads bind creating actin-myosin cross bridges
• sliding filament theory - the theory that the contraction of the muscle is due to the sliding of the myofilaments
• Muscle contraction (part 1):

  1. Un-stimulated muscle tropomyosin blocks myosin head binding sites on actin filaments, ADP is attached to myosin heads putting them under tension
  2. Depolarisation of sarcolemma causes release of calcium ions from the reticulum, this makes tropomyosin move and uncover the binding sites
  3. Myosin heads form actin-myosin cross bridges
  4. Myosin heads change shape and perform the power stroke, pulling the actin filaments closer together. ADP is released.
• Muscle contraction (part 2):

  5. ATP attaches and releases myosin heads, breaking the cross bridges

  6. Hydrolysis of ATP releases energy to pull the myosin heads back. ATP hydrolase enzyme needed - activated by calcium ions

  7. Myosin heads form new actin-myosin cross bridges further back to continue to pull on the actin filaments

  8. The cycle repeats as long as nervous stimulation continues and enough ATP and calcium ions are present
• Muscle relaxation:

  • Nervous stimulation stops
  • Calcium ions actively transported back into sarcoplasmic reticulum
  • Tropomyosin re-covers and blocks binding sites again on the actin filaments
  • Antagonistic muscle pulls actin filaments back - muscle elongates
• Slow twitch fibres:

  • Contracts more slowly and with less force
  • Contains lots of myoglobin which stores oxygen
  • More mitochondria for aerobic respiration
  • Stores of glycogen to release glucose for respiration
  • responsible for prolonged contractions such as posture
• Fast twitch fibres:

  • Contract faster and powerfully (response speed is important)
  • Less myoglobin and fewer mitochondria, but more anaerobic enzymes
  • Myosin filaments are thicker
  • have a store of phosphocreatine to quickly supply phosphate groups which are condensed with ADP to ATP