MP Chapter 12

• Skeletal muscles


  *Striated muscle attached to bones of the skeleton

  *control body movement

  *voluntary control

  *respond to somatic motor neurons
• Cardiac muscle


  *Striated muscle found only in the heart

  *moves blood through the circulatory system

  *involuntary control; responds to autonomic innervation, spontaneous contraction

  *modulated by the endocrine system
• Smooth muscle


  *Primary muscle of the internal organs and tubes

  *Influence the movement of material into, out of, and within the body

  *Involuntary control; responds to autonomic innervation, spontaneous contraction

  *Modulated by the endocrine system
• Skeletal muscles


  • Usually attached to bones by tendons
  • Origin: closest to the trunk or to more stationary bone
  • Insertion: more distal or more mobile attachment
  • Flexor: brings bones together
  • Extensor: moves bones away
  • Flexor-extensor pairs form antagonistic muscle groups
• Muscle fibers


  Muscle cells, long and cylindrical, fused cells with many nuclei, satellite cells (stem cells) differentiate into muscle for growth or repair
• Sarcolemma


  Cell membrane
• Sarcoplasmic reticulum


  *Endoplasmic reticulum

  *longitudinal tubes with enlarged ends called terminal cisternae
• Myofibrils


  Contractile structures of muscle fibers
• Transverse tubules (t-tubules)


  Continuous with the sarcolemma, allow action potentials to penetrate nearer to the internal structures of the fiber
• Triad


  1. tubule + 2 flanking terminal cisternae
• Regulatory proteins


  Tropomyosin and troponin
• Crossbridges


  Myosin heads that interact with actin filaments
• Sarcomere


  The contractile unit of the myofibril
• Z disks


  Have filaments between them
• I band


  Made of thin filaments only (actin)
• A band


  Darker regions where both light and heavy filaments overlap
• H zone


  Clear band in the middle of the A band, heavy filaments only
• M line


  Represents the proteins to which heavy filaments attach
• Titin


  Elastic protein molecule, stabilizes the position of the contractile elements, elasticity returns stretched muscles to their resting length
• Nebulin


  Inelastic protein, aligns actin filaments
• Muscle tension


  Force created by muscle
• Load


  Weight or force opposing contraction
• Contraction


  Creation of tension in muscle
• Relaxation


  Release of tension
• Major steps leading up to skeletal muscle contraction

  1. Events at the neuromuscular junction

  2. Excitation-contraction (E-C) coupling

  3. Contraction-relaxation cycle
• Sliding filament theory of contraction


  Overlapping actin and myosin myofibrils, fibrils are fixed length, slide past each other in energy-dependent process
• Myosin crossbridges move actin filaments

  1. Power stroke: myosin crossbridge swivels and pulls actin toward M line

  2. End of power stroke: myosin releases actin and resets and binds another actin

  3. Heads are not released in unison, power stroke is repeated multiple times

  4. Myosin ATPase: energy places the myosin head into the "cocked position", when myosin comes into contact with actin, the pull is initiated
• Troponin


  Complex of 3 proteins, troponin C binds reversibly to calcium, controls position of tropomyosin
• Tropomyosin


  Partially covers myosin binding site on actin, prevents myosin and actin from interacting
• Contraction steps


  1. Calcium release from terminal cisternae

  2. Calcium binds troponin

  3. Troponin pulls tropomyosin from myosin-binding sites on actin

  4. Myosin binds tightly to and moves actin

  5. Repeated as long as binding sites are uncovered and ATP is available
• Myosin heads step along actin filaments

  1. Start: rigor state where myosin tightly bound to G-actin after a power stroke

  2. ATP binds and myosin detaches

  3. ATP hydrolysis provides energy for the myosin head to rotate and reattach to actin

  4. Power stroke begins in response to Pi release, allows head to swivel, pulling actin toward M line

  5. Myosin releases ADP
• Rigor state


  *Occurs when no ATP or ADP are bound to myosin, very brief

  *rigor mortis: muscles "freeze" if no ATP is available to release myosin
• Acetylcholine initiates excitation-contraction coupling


  1. Acetylcholine (ACh) is released from the somatic motor neuron

  2. ACh initiates an action potential in the muscle fiber

  3. ACh binds to receptors on the sarcolemma, ions enter and exit the cell, depolarization leads to end-plate potential and muscle action potential

  4. Muscle action potential triggers calcium release from the sarcoplasmic reticulum via L-type calcium channel dihydropyridine (DHP) receptor on t-tubule and ryanodine receptors (RyR) on sarcoplasmic reticulum

  5. Calcium combines with troponin to initiate contraction
• Relaxation


  Calcium is pumped back to sarcoplasmic reticulum via Ca-ATPase
• Latent period


  Short delay between the muscle action potential and beginning of muscle tension development, time required for calcium release and binding to troponin
• Phosphocreatine


  Breakdown produces a short burst of energy, creatine kinase (CK) catalyzes the reaction
• Anaerobic glycolysis


  Produces lactate and acid, quick, no oxygen required, small amount of energy released
• Aerobic respiration


  Citric acid cycle and electron transport chain, slow, oxygen required, large amount of energy released
• Central fatigue


  Due to central nervous system
• Peripheral fatigue

  Due to neuron or muscle