What is the roleof calcium ions, troponin and F - actin during contraction in striated muscles in humans ? Represent diagrammatically the stages in the cross bridge formation.

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Calcium ions play a significant role in muscle contraction. The calcium ions are released from sarcoplasm on receiving action potential (signal for contraction). Calcium ions bind with troponin subunits of actin which mask the binding sites for myosin on actin. Binding of calcium to troponin allows the binding of myosin head to actin to form a cross bridge. Cross bridge formation pulls the attached actin filament towards the  centre  of A band. The Z line attached to the actin is also pulled in, leading to shortening the length of  sarcomere .This ultimately causes muscle contraction. 

Troponin is a complex of three types of regulatory proteins that regulate the contraction of cardiac and skeletal muscles.

Actin  is a type of protein that makes microfilament. F-actin is a polymer of actin protein which is essential for the functions like mobility and contraction of  cell  at the time of division. 
 

Flow chart of muscle contraction

 
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