Muscles Unit 5

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  • Created by: Gemma
  • Created on: 27-02-14 20:32
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  • Muscles
    • Ultra-structure of Muscle fibres
      • electron micrographs show muscles as striped
      • i band-light band, thin, actin hollywood
      • A band- dark band- myosin + actin- thick
      • Zline = actin x2
      • M line = myosin x2
      • H zone = myosin
      • Dark band= A band
      • light band= I band
      • dark line down I band = Z line
      • I band = actin
      • A band = myosin
      • lighter section of A band= H zone
      • Zline-zline = sarcomere
      • sarcoplasm has a network tubules called sarcoplasmic reticulum
    • Skeletal muscles
      • features
        • Striped as look striped under the electron microscope
        • multinucleate as has many nuclei
        • the cytoplasm in the fibres is sarcoplasm
        • Sarcolemma is like the cell surface membrane
          • each muscle fibre is surrounded by the membrane
        • sarcolemma contains mitochondria and myofibrils
          • myofibrils are composed of protein filaments called myofilaments
            • each unit of myofibril= sarcomere
        • one unit of myofibril is called a sarcomere
        • made up of specialised cells- muscle fibres
      • attached to skeleton
      • voluntary muscle- you decide whether it
      • striped muscle as it appears as striped under the microscope
      • multinucleate
    • Muscle contraction
      • z lines move closer
        • sarcomere shortens
      • H zone decreases/ shorter
      • I band decreases/ shortens
      • A band stays the same
      • Sliding filament hypothesis of muscle contraction
        • basis- actin slides between myosin filaments using energy from ATP
      • How the filaments move
        • 1-myosin head binds to actin-the actomyosin crossbridge forms
          • action potential arrives-nerve impulse causes actin to enlarge
          • ADP and Pi must be bonded to each head
        • 2-Myosin head tilts- forces actin to move in relation THE POWERSTROKE ADP and Pi are released
          • 3-ATP binds with the myosin heads
        • 3-ATP binds with the myosin heads
        • 4-head hydrolysis ATP to ADP and P- provides energy to release myosin head from actin
          • the head flips away
        • Key point
          • This is the RATCHET MECHANISM as actin moves one step at a time by the myosin head
          • ATP is used to break the cross bridge between A + M
          • without ATP myosin locks onto actin. eg in death causing rigor mortis
      • Supplying energy for muscle contraction
        • muscle contraction requires a lot of energy (ATP-ADP+Pi)
        • the energy is needed for :
          • 1- movement/ disatchment of myosin heads
          • 2- reabsorption of Ca2+ (SR via A.trans
        • ATP lasts only 3/.4 seconds muscles fibres have PHOSPHOCREATINE- used to create ATP rapidly by Pi+ADP = ATP (repleaces broken down ATP)
        • supplies are limited but generate ATP for the muscle until more ATP is produced by mitochondria
    • Structure of myosin and actin
      • Actin
        • Thin filaments are made up of actin and two other proteins
          • TROPOMYOSIN
          • TROPONIN
          • these proteins are involved in muscle contraction
          • calcium ions are also involved
        • a plait like structure
      • Myosin
        • consists of tail and 2 heads of globular protein
        • heads point in 6 different directions
        • heads arranged in a spiral
        • head has ATPase enzyme activity which breaks down ATP realising energy
        • heads link with actin to form actomyosin crossbridge
    • role of tropomyosin, troponin and calcium ions in muscle contraction
      • muscle contraction is turned on/off by calcium switch
      • at rest actin covered by tropomyosin
      • when nerve impulse arrives at a muscle fibre, calcium channels in the muscle fibre open up and CA2+ diffuse into sarcoplasm
      • tropomyosin moves exposing the binding sites on the actin
      • myosin now binds to actin and forms the actomyosin cross bridge
    • The Neuromuscular Junction
      • where the neurone communicates with a muscle fibre
      • motor end plate- end of the axon-contains vesicles filled with neurotransmitter acetylcholine
      • The Tubule System
        • the transverse or t-tubules are in-foldings of the plasma membrane or sarcolemma
        • In muscles calcium ions are stored in the sarcoplasmic reticulum of the cytoplasm
          • they accumulate as a result of active transport
        • action potential arrives the calcium ions are released which sets off a sequence of events leading to the contraction of the myofibril
      • The sequence of events following a nerve impulse arriving at a neuromuscular junction
        • 1-a nerve impulse arrives at the neuromuscular junction
        • 2-acetycholine is realised into the synaptic cleft
        • 3-an action potential Is released from the sarcolemma
        • 4-the action potential is transmitted down the T-tubule
        • 5-Ca2+ released from the sarcolemma reticulum
        • 6-Ca2+ bind to the calcium switch protein--troponin (attached to actin)
        • 7-Tropomyosin moves and the myosin binding sites are exposed- on actin
        • 8-Myosin heads to the exposed sites on the actin filaments and form actomyosin cross bridges
        • 9-energy from the hydrolysis of ATP is used to move actin filament
    • sequence of action potential stops
      • 1- acetylcholine is destroyed at the neuromuscular junction by the enzyme acetyl cholinesterase
      • 2-stops stream of action potentials along sarcolemma
      • 3-S.R stops releasing Ca2+
      • 4-Ca2+ pumped back into S.R uses ATP
      • 5-decreased level of Ca2+ causes tropomyosin to move back into its original place/posistion
      • 6-actin binding sites blocked- no binding
      • 7-muscle contraction stops

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