Muscles Unit 5
- 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
- myofibrils are composed of protein filaments called myofilaments
- 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
- features
- 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
- 1-myosin head binds to actin-the actomyosin crossbridge forms
- 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
- z lines move closer
- 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
- Thin filaments are made up of actin and two other proteins
- 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
- Actin
- 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
- Ultra-structure of Muscle fibres
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