Skeletal Muscle

Created by: YoungLady
Created on: 04-05-16 21:28

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SKELETAL MUSCLE
- Structure
  - Stimulated to contract by neurones
  - Act as effectors
  - Actively move
    - E.g. biceps and triceps in lower arm
  - Muscle Fibres
    - Large bundles of long cells
    - Make up skeletal muscle
    - Sarcoplasm
      - Cytoplasm of the muscle cell
    - Sarcolemma
      - Cell membrane of muscle cell
      - Parts fold inwards along muscle fibre
        Stick into sarcoplasm
        
        Transverse (T) tubules
        
        Help to spread electrical impulses throughout sarcoplasm
        Can reach all parts of the muscle fibre
    - Sarcoplasmic reticulum
      - Network of internal membranes
      - Runs throughout sarcoplasm
      - Stores and releases calcium ions
        Needed for muscle contraction
    - Contain many mitochondria
      - Provide ATP for muscle contraction
    - Multinucleate
      - Contain many nuclei
    - Contain myofibrils...
      - Long, cylindrical organelles
      - Made up of proteins
      - Highly specialised for contraction
      - Contain bundles of myosin and actin filaments
        Move past one another to allow muscle contraction
        
        Myosin is thick
        
        Actin is thin
      - Z line
        Marks the end of each sarcomere
      - M line
        Centre of each sarcomere
        
        Middle of myosin filaments
      - H zone
        Only contains myosin filaments
        
        Around M line
      - Alternating light and dark bands viewed under an electron microscope
        A bands
        Thick myosin filaments
        
        Some overlapping actin filaments
        
        Dark bands
        
        I bands
        Thin actin filaments only
        
        Light bands
- Sliding Filament Theory
  - Actin and myosin filaments slide over one another
    - Makes sarcomeres contract
      - Simultaneous contraction of many sarcomeres means myofibrils and muscle fibres contract
    - Myofilaments themselves DON'T contract
  - Sarcomeres return to their original length as the muscle relaxes
  - During contraction...
    - A band remains the same length
    - I band gets shorter
    - H zone gets shorter
    - Sarcomeres get shorter
      - Sarcomeres return to their original length as the muscle relaxes
  - Myosin filaments
    - Globular heads
      - Hinged so they can move backwards and forwards
    - Binding sites for...
      - Actin
      - ATP
  - Actin filaments
  - Process
    - Resting
      - Unstimulated
      - Actin-myosin binding site blocked by tropomyosin
        Held in place by troponin
        
        Myofilaments can't slide past each other because myosin heads can't bind to actin-myosin binding sites on actin filaments
    - Contraction
      - Action potential stimulated muscle cell
        From motor neurone
        
        Depolarisation of sarcolemma
        Spreads down T tubules to sarcoplasmic reticulum
        Releases calcium ions into sarcoplasm
        Calcium ions bind to troponin
        Troponin changes shape
        Tropomyosin pulled out of actin-myosin binding site
        Exposed so myosin head can bind
        Actin-myosin cross bridge bond formed
        Destroys actin-myosin cross bridge
        Myosin head detaches from filament
        Reattaches to a different binding site further along the actin filament
        Cycle repeats
        Action potential stimulated muscle cell
        From motor neurone
        
        Depolarisation of sarcolemma
        Spreads down T tubules to sarcoplasmic reticulum
        Releases calcium ions into sarcoplasm
        Calcium ions bind to troponin
        Troponin changes shape
        Tropomyosin pulled out of actin-myosin binding site
        Exposed so myosin head can bind
        Actin-myosin cross bridge bond formed
        Destroys actin-myosin cross bridge
        Myosin head detaches from filament
        Reattaches to a different binding site further along the actin filament
        Cycle repeats
        Until...
        When excitation stops...
        Calcium ions leave binding sites on troponin molecules
        Moved by active transport
        
        Requires ATP
        
        Troponin returns to it's original shape
        Pulls attached tropomyosin molecules into actin-myosin binding site
        Becomes blocked again
        
        No myosin heads attached to actin filaments
        
        No actin-myosin cross bridges
        
        Actin filaments slide back into relaxed position
        
        Sarcomere lengthened
        
        Activate ATPase enzyme
        Breaks down ATP
        Forms ADP and Pi
        
        Provides energy for muscle contraction
        
        Energy moves myosin heads
        Pulls actin filament along in a rowing action
        
        Until...
        When excitation stops...
        Calcium ions leave binding sites on troponin molecules
        Moved by active transport
        
        Requires ATP
        
        Troponin returns to it's original shape
        Pulls attached tropomyosin molecules into actin-myosin binding site
        Becomes blocked again
        
        No myosin heads attached to actin filaments
        
        No actin-myosin cross bridges
        
        Actin filaments slide back into relaxed position
        
        Sarcomere lengthened
        
        Activate ATPase enzyme
        Breaks down ATP
        Forms ADP and Pi
        
        Provides energy for muscle contraction
        
        Energy moves myosin heads
        Pulls actin filament along in a rowing action
- Energy for Muscle Contraction
  - ATP used up very quickly during contraction
    - A lot of energy is needed
  - ATP continuously generated
    - Allows exercise to continue
  - Aerobic respiration
    - Most ATP generated by oxidative phosphorylation
    - Occurs in cells mitochondria
    - Only works when oxygen is present
    - Good for long periods of low-intensity exercise
  - Anaerobic respiration
    - ATP made rapidly from glucose
    - Pyruvate converted to lactate
      - End product of glycolysis
      - Process of lactate fermentation
      - Lactate can build up and cause fatigue
    - Good for short periods of high-intensity exercise
  - ATP-Phosphocreatine (PCR) system
    - ATP made by phosphorylating ADP
      - Add a phosphate group
        Removed from a PCr molecule
        PCr stored inside cells
    - Generates ATP very quickly
    - Per runs out in a few seconds
    - Used for short bursts of extreme high-intensity exercise
    - Anaerobic
    - Alactic
      - No lactate formed
- Types of Muscle Fibres
  - Different muscles have different proportions of each kind
  - Each type has it's own special properties
  - Slow twitch
    - Contract slowly
    - Used for posture
    - Endurance activities
    - Can work for a long time without tiring
    - Slow energy release
    - Aerobic respiration
    - Contain many...
      - Mitochondria
        For aerobic respiration
      - Blood vessels
        To supply enough oxygen
    - Reddish in colour
      - Rich in myoglobin
        Red coloured
        
        Protein
        
        Stores oxygen
  - Fast twitch
    - Contract very quickly
    - Used for fast movement
    - Short bursts of speed and power
    - Tire very quickly
    - Anaerobic respiration using glycogen stores
    - Few mitochondria and blood vessels
    - Whitish in colour
      - Lack of myoglobin
        Can't store much oxygen
- Also known as...
  - Striated Muscle
  - Striped Muscle
  - Voluntary Muscle

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Skeletal Muscle

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