The contraction of skeletal muscle cells is under voluntary control by the nervous system. ... These steps initiate muscle contraction. Calcium released from the SR binds to a protein called tropomyosin. This protein normally blocks sites on actin that bind myosin heads during contraction. How does it happen? Well When an impulse reaches the muscle fibres of a motor unit, it stimulates a reaction in each sarcomere between the actin and myosin filaments. This reaction results in the start of a contraction. And the three different types of of muscle contractions are : Concentric contractions (shorten) Eccentric contractions (lengthen) Isometric contractions (remain the same)
