Muscles and Muscle Contractions

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Skeletal Muscle

  • Muscles act in antagonistic pairs against an incompressible skeleton
  • The skeleton provides the basic structure of levers and joints to which the muscles are attached
  • Muscle fibres/cells have  -
    • a cell surface membrane called the sarcolemma
    • an endoplasmic reticulum called the sarcoplasmic reticulum
    • a cytoplasm called the sarcoplasm
    • consist of many protein filaments
      • thick myosin filaments
      • thin actin filaments

Sliding Filament Mechanism of Muscle Contraction:

  • Actin filaments are pulled closer together over myosin filaments
  • Actin filaments have binding sites for the myosin heads
  • Myosin filaments are bound together so they can provide tension against each other to pull actin over themselves
  • Actin filaments slide over myosin filaments

Neuromuscular Junction

Defined as the place where a motor neurone synapses with a muscle fibre.

When an action potential reaches the junction, synaptic vesicles containing the neurotransmitter acetylcholine, fuse with the presynaptic membrane and release the neurotransmitter into the synaptic gap. Acetylcholine diffuses across the gap and attaches to the plasma membrane on the sarcolemma (the muscle cell surface membrane).

The sarcolemma is depolarised and an action potential is transmitted along the muscle fibre.

The action potential causes the release of calcium ions into the sarcoplasm (the cytoplasm of the muscle cell). The presence of the calcium ions cause a protein called tropomyosin to uncover the myosin head binding sites on the actin filaments. Myosin heads begin to form cross bridges with the actin filaments ~ called actino-myosin cross bridges.

This allows the actin filaments to be pulled closer together by the myosin filaments, thus making the sarcomeres of the muscle fibres shorten or contract. This is known as

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