Structure of skeletal muscle

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11.1 structure of skeletal muscle

3 types of muscle - cardiac (heart), smooth (walls of blood vessels and gut), skeletal muscle (bulk of body muscle in vertebrates) it is attached to a bone and acts under voluntary, conscious control.

Individual muscles are made up of millions of tiny muscle fibres called myofibrils. myofibrils are arranged to give maximum force. 

The seperate cells have become fused together into muscle fibres. these muscle fibres share nucelio, and cytoplasm, called sarcoplasm usually found in the circumference. in the sarcoplasm there is a large concentration of mitochondria and endoplasmic reticulum. 

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Microscopic structure of skeletal muscle

Myofibrils are made up of two types of protein filament:

  • Actin - thin, consists of two strands twisted around one another. 
  • Myosin - which is thicker and consist of a long rod-shaped fibres with bulbous heads that project to the side. (http://www-rohan.sdsu.edu/course/ens304/public_html/section1/Image6.gif)
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Myofibrils appear striped due to their alternating light-coloured and dark-coloured bands. The light bands are called isotropic bands (I-BANDS). They appear lighter because the actin and myosin filaments do not overlap in this region. The dark bands are called anisoropic bands(A-bands). They appear darker becausethe actin and myosin filaments overlap in this region. 

At the centre of each anisotropic band is a light-coloured region called the H-zone. At the centre of each isotropic band is a line called the z-line. the distance between adjacent z-lines iscalled a sarcomere. When a muscle contracts, these sarcomeres shorten and the pattern of light and dark bands change. 

Two other important proteins are found in muscle:

  • Tropomyosin - forms fibrous strandaround the actin filament.
  • a globular protein (troponin) involved in muscle contraction.  
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Types of muscle fibre

Two types of muscle fibre:

  • Slow twitch fibres - these contract slow and provide less prowerful contractions over a longer period. They are therefore adapted to endurance work, such as running.  In humans they are more common in muslces like the calf music, which need to contract constantly to maintain the body's upright position. they are suited tothis rolde by being adapted for aerobic respiration in orderto avoid a build up of lactic acid, which would cause them to function less effectively. these adaptions include having:
    - a large store of myoglobin (molecule which stores oxygen, which accounts for the red colour of slow-twitch fibres)
    - a supply of glycogen to provide a source of metabolic energy.
    - a rich supply of blood vessels to deliver oxygenand glucose
    - numerous mitochondria to produce ATP 
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- fast twitch fibres - these contract more rapidly and produce powerful contractions but only for a short period. They are therefore adapted to intense exercise, such as weight lifting. As a result they are more common in muscles which need to do short bursts of intense activity, like the biceps muscle of the upper arm. Fast-twitch fibres are adapted to their role by having:

  • thicker and more numerous myosin filaments.
  • a high concentration of enzymes involved in anaerobic respiration
  • a storeof phophocreatine, a molecule that can rapidly generate ATP from ADP in anaerobic conditions and so provide energy for muscle contraction.
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Neuromuscular junctions

A neuromuscular junction is the point where a motor neurone meets a skeletal muscle fibre. There are many such junctions along the muscle. If there was only one juntion of this type, it would take time for a wave of contraction to travel across the muscle, in which case not all the fibres wouldnt contract simultaneously, and the movement would be slow.  as rapid muscle contraction is frequently essential for survival there are many neuromuscular junctions spread through the muscle. This ensures that contraction of a muscle is reapid and powerful when it is simultaniously stimulated by action potentials.

All muscle fibres supplied are by a single motor neurone act together as asingle functional unitand are known as a motor unit.  if only slight force is needed, only a few untis are stimulated. If a greater force is required a larger number of  units are stimulated.   

 When a nerve impulse is recieved at the neuromuscular junction,the synaptic vesicles fuse with the presynaptic membrane and release their acetylchloine. The  acetylcholine diffuses with to the possynaptic membrane, altering its permeability to sodium ions, which enter rapidly depolorising the membrane. 

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The acetylcholine is broken downby acetylcholinesterase to ensure that the muscle is not over-stimulated. The resulting choiceand ethanoic acid diffuse back into the neurone, where they are recombinedtoform acetylcholineusing energy provided by the mitchondira found there. 

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