AQA Biol 4 Muscle Contraction

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The Components of a Muscle

  • A muscle is an organ composed of many different tissues (muscle, connective)
  • Muscle tissue is composed of muscle cells calles muscle fibres
  • Each muscle fibre (cell) is packed with organelles called myofibrils
  • Myofibrils are composed mainly of two mucle filaments called actin and myosin
  • Myofibrils can be divided into functional units, each called a sarcomere


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Muscle Stimulation

  • An action potential arrives at the neuromuscular junction.  This causes calcium ion channels to open and Ca2+ ions to diffuse in.  Vescicles bind to the muscle end plate and release acetylcholine.
  • Acetylcholine binds to receptors on the muscle fibre membrane, causing Na+ to diffuse in, causing depolarisation.
  • Depolarisation wave travels down T Tubules (T system) 
  • This depolarisation leads to Ca2+ release from the sarcoplasmic reticulum in the muscle
  • Ca2+ binds to the proteins in the muscle - troponin - which causes tropomyosin to move and reveal myosin-binding sites, this leads to muscle contraction
  • Acetylcholinesterase in the cleft rapidly breaks down acetylcholine so that contraction only occurs when impulses arrive continuously
  • Acetyl and choline diffuse back across the cleft.
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Neuromuscular Junction

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How Do Muscles Work?

  • Skeletal muscles cause the skeleton to move at joints
  • They are attached to the skeleton by tendons, which transmit the muscle force to the bone and can also change the direction of the force
  • Tendons are made of inelastic fibres and are very strong and stiff
  • The skeleton provides leverage, magnifying either the movement or the force
  • Muscles are either relaxed or contracted
  • In the relaxed state muscle is compliant (can be stretched)
  • In the contracted state muscles exert a pulling force, causing it to shorten, or generate force
  • Since muscles can only pull (not push), they work in pairs called antagonistic muscles.  The muscle that bends (flexes) the joint is called the flexor muscle, and the muscle that straightens (extends) the muscle is called the extensor muscle.
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Muscle Contraction

  • The action potential moves through the fibres by travelling through T-tubules that branch through the sarcolemma.
  • The action potential moves through the tubules until it reaches the sarcoplasmic reticulum.
  • This opens the calcium ion channels in the sarcoplasmic reticulum.
  • Calcium ions diffuse out into the muscle.
  • Calcium ions cause troponin to change shape so that the binding sites on the actin filament are exposed.
  • The myosin head forms a cross bridge with actin by binding with the receptor site.
  • An ATP molecule attaches to the myosin head and thus causes it to detach.
  •  Calcium ions activate the enzyme ATPase which hydrolyses the ATP and releases energy to allow the myosin to resume its natural shape.
  • The myosin head now has a new ADP molecule that will allow it to bind with a new receptor site somewhere along the actin filament.
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Fast Twitch Fibres

  • Fast contraction speed
  • Powerful contraction power
  • Short length of contraction
  • Adapted for short bursts of energy, ie sprinting, weight lifting
  • Found mainly in upper thigh and upper arms to move faster
  • Has anaerobic respiration because aerobic takes too long
  • Adaptions
    • Thicker and more numerous myosin filaments
    • High concentration of enzymes involved in anaerobic respiration
    • A store of phosphocreatin, a molecule that can rapidly generate ATP from ADP in anaerobic respiration
    • Produces lactate so fatigues quickly
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Slow Twitch Fibres

  • Slow contraction speed
  • Short contraction power
  • Long length of contraction
  • Adapted for endurance activities, for long periods of time
  • Found mainly in calf and lower arms 
  • Has aerobic respiration 
  • Adaptions
    • Large store of myoglobin
    • A supply of glucose to produce a source of metabolic energy
    • Rich supply of blood vessels to deliver oxygen and glucose
    • Numerous mitochondria to produce ATP
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