Skeletal Muscle

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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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