Muscle contraction

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Effectors – Muscle Contraction


Myosin Filaments have Globular Heads and Binding Sites


Myosin filaments have globular heads that are hinged, so they can move back and forth.

  1. Each myosin head has a binding site for actin and a binding site for ATP.
  2. Actin filaments have binding sites for myosin heads, called actin-myosin binding sites.
  3. Two other proteins called tropomyosin and troponin are found between actin filaments. These proteins are attached to each other and they help myofilaments move past each other.


Binding Sites in Resting Muscles are blocked by Tropomyosin

In a resting (unstimulated) muscle the actin-myosin binding site is blocked by tropomyosin, which is held in place by troponin.

So myofilaments can’t slide past each other because the myosin heads can’t bind to the actin-myosin binding site on the actin filaments.


Muscle Contraction is triggered by an Influx of Calcium Ions


  1. When an action potential from a motor neurone stimulates a muscle cell, it depolarises the sarcolemma. Depolarisation spreads down the T-tubules to the sarcoplasmic reticulum.
  2. This causes the sarcoplasmic reticulum to release stored calcium ions into the sarcoplasm.
  3. Calcium ions bind to troponin,


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