In a muscle cells join up to make muscle fibres, resulting in long strands of cells sharing nuclei and cytoplasm (aka sarcoplasm). Also inside these fibres are myofibrils: these are the structures that allow contraction. Myofibrils are made up of proteins.
Fibers are grouped together into bundles which are grouped further to create muscles.
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For this to happen, Ca2+ binds to troponin, which changes its positioning moving tropomyosin out of the binding site of actin.
This allows myosin heads to bind there, as they do they change angle and release a ADP molecule. ATP then attatches to the head causing it to detach from the actin. The ATP turns to ADP providing energy for the head to return to where it was before so that it can repeat the process.