The actual contractile units of muscles extend from z disc to z disc. they are

There are four units of muscles extend from z disc to z disc. They are thick filaments,myofibrils, sarcoplasmic reticulum and sarcomeres. Cross sections of the sarcomere in areas where thick and thin filaments overlap show that thick filament is surrounded by 6 thin filaments - each thin is surrounded by 3 thin

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