Muscles

AP 
--> ACTIN MYOSIN X-LINK
  • MoA:
    • @ rest -->  tropomyosin and troponin complex are bound to actin
    • AP → sarcoplasmic reticulum (via transverse tubules) releases Ca++ into cytosol
    • -->myosin binding site inhibition released (on actin)
    • -->myosin heads (w/ ATPase) binds to actin x-link, pulls itself along actin
    • -->ATP binds myosin head-->releases x-link
    • -->ATPase hydrolyzes ATP --> ADP released
    • Myofibrils are connected via desmin intermediate filaments, & anchored to sarclomenna by dystrophin
  • Sarcomere: 
    Draw Z lines
    Draw myosin free floating
    Connect actin lines
    H = myosin only
    AI: A = actin & myosin
    I = actin only

  • Recruitment of motor units: smaller > larger, weaker > stronger
|----Muscular dystrophy
TYPES OF MUSCLES
  • Type I: slow-twitch red fibers 
    ↑ mitochondria, ↑ oxidative mets, ↑ myoglobin

  • Type II: fast-twitch white fibers

    Fast fatigue-resistant (IIA): aerobic capactity, able to generate rapid contraction, but can maintain over several minutes

    Fast fatigable (IIB)    : large glycogen stores, anaerobic catabolism, lactic acid accumulates rapidly

  • Cardiac mm: striated, mononuclear, gap junctions
  • Smooth mm: sheets of elongated spindle-shaped cells, no myofibrils. Contractile elements are loosely arranged complexes of actin/myosin II. Contraction is initiated by ↑ cytosolic Ca++ which binds calmodulin; the Ca/Calmodulin complex activates MLCK (myosin light chain kinase) → phosphorlyates myosin-II light chains→ contraction. Smooth muscles does not contain troponin. Slower than striated mm, but generates larger movements over longer distances
  • Nuclei: periphery
|-----(atrophy)-------Myotonic Dystrophy, Congenital Myopathy
<---(hypertrophy)--Wernig-Hoffman


|---(atrophy)---------Myasthenia gravis, denervation, disuse, paraneoplastic, steroids
<--(hypertrophy)--Congenital fiber-type disproportion
ATROPHY & DENERVATION
Picture
Denervation atrophy, small angular fibers
Picture
Fiber type grouping
Picture
Group atrophy
Picture
group atrophy & target fibers (MoA: no oxidative enzyme in center, surrounded by rim of intense activity)