Histology Midterm

• Mechanism of contraction

  1. Nerve impulse triggers release of Acetyl Choline [neurotransmitter] from the synaptic knob into the synaptic cleft

  2. ACh binds to ACh receptors in the motor end plate of the neuromuscular junction, initiating a muscle impulse in the sarcolemma of the muscle fiber

  3. As the muscle impulse spreads quickly from the sarcolemma along T tubules, calcium ions are released from terminal cisternae into the sarcoplasm

  4. Calcium ions bind to troponin

  5. Troponin changes shape, moving tropomyosin on the actin to expose active sites on actin molecules of thin filaments

  6. Myosin heads of thick filaments attach to exposed active sites to form crossbridges

  7. Myosin heads pivot, moving thin filaments toward the sarcomere center. ATP binds myosin heads and is broken down into ADP and P

  8. Myosin heads detach from thin filaments and return to their prepivot position

  9. The sarcomere shortens and the muscle contracts

  10. When the impulse stops, calcium ions are actively transported into the sarcoplasmic reticulum

  11. Tropomyosin re-covers active sites, and filaments passively slide back to their relaxed state
• Type II A [Fast Oxidative Glycolytic Fibers]

  • Intermediate fibers seen in fresh tissue
  • Medium size w/ many mitochondria and a high myoglobin content
  • Fast-twitch fatigue-resistant motor units that generate high peak muscle tension
  • They possess characteristics that are intermediate between fast fibers and slow fibers
  • Use both aerobic and anaerobic metabolism
  • Primarily for movements, such as walking, that require more energy than postural control but less energy than an explosive movement, such as sprinting
• Type II B [Fast Glycolytic Fibers]

  • Large fibers, w/c appear light pink in fresh specimens, contain less myoglobin & fewer mitochondria than type I & IIa fibers
  • Low levels of oxidative enzymes but exhibit high anaerobic enzyme activity and store a considerable amount of glycogen
  • Fast-twitch fatigue-prone motor units & generate high peak muscle tension
  • Used to produce rapid, forceful contractions to make quick, powerful movements
  • These fibers fatigue quickly, permitting them to only be used for short periods
  • This type of fibre can be turned into type IIa fibres by resistance training
  • Found in large quantities in the muscles of the arms