Structure of a skeletal muscle

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  • Structure of skeletal muscle
    • Muscles are effector organs that respond to nervous stimulation by contracting that bring about movement.
    • Three main types of muscle.
      • Cardiac muscle
        • found exclusively in the heart - not under conscious control.
      • Smooth muscle
        • found in the walls of blood vessels and the gut - not under conscious control.
      • Skeletal muscle
        • makes up the bulk of the muscle and is attached to bone - acts under voluntary and conscious control.
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    • Individual muscles are made up of millions of tiny muscle fibres called myofibrils.
      • collectively very powerful.
      • myofibrils are lined up paraelle to one another in order to maximise strength and force.
      • muscles are composed of smaller units built up into larger ones
    • Muscles are not made up of individual cells joined end to end, because they would not be able to perform the function of contraction very efficiently, due to the junction between adjacent cells creating a point of weakness, decreasing the overall strength.
      • Muscles have developed the structure, where seperate cells have become fused together as muscle fibres, which share a nucleus and cytoplasm, called a sarcoplasm.
      • each sarcoplasm contains large amounts of mitochondria and endoplasmic reticulum.
    • protein filaments that make up the myofibrils.
      • Actin
        • thin and consists of two strands twisted around one another
      • Myosin
        • thick and consists of long rod-shaped fibres with bulbous heads the project to the side.
      • Dark coloured bands
        • anisotropic bands
          • appear darker because the actin and myosin filaments overlap.
          • has a lighter coloured region in the centre of each band known as the H-zone
      • Light coloured bands
        • appear lighter because the actin and myosin filaments do not overlap.
        • has a line in the centre known as the Z line. The distance between adjacent Z lines is called a sarcomere.
      • Tropomyosin
        • forms a fibrous strand around the actin filament.
      • Troponin (gobular protein)
        • involved in muscle contraction.
    • Types of muscle fibres
      • slow twitch fibres
        • contract slowly and provide less powerful contractions over a longer period.
        • adapted for endurance work
        • more common in muscles such as calf muscle, which needs to contract constantly to keep us in an upright position.
        • suited to their role by being adapted for aerobic respiration in order to avoid the build up of lactic acid, which causes them to function less effectively.
          • adaptations include...
            • a large store of myoglobin, which is a bright red molecule that stores oxygen.
            • supply of glycogen to provide a store of metabolic energy.
            • rich supple of blood vessels to deliver oxygen and glucose
            • numerous mitochondria to produce ATP.
      • fast twitch fibres
        • contract rapidly and produce powerful contractions,but only for a short period.
        • adapted for intense exercise, such as weightlifting
        • more common in muscles which need to do short bursts of intense activity, e.g. biceps
        • adapted to their role...
          • thicker and more numerous myosin filaments
          • high concentrationof enzymes which are involved in anaerobic respiration.
          • a store of photocreatine, a molecule tha can rapidly generate ATP from ADP in anaerobic conditions and so provide energy for muscle contraction.

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