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Once the muscle cell has been excited it will contract.
• A muscle action potential will trigger the release Of Ca2+ ions into the sarcoplasm.
• The Ca2+ ions bind to the regulatory proteins and trigger contraction.
• Within skeletal muscle cells are structures that provide the ability to contract forcefully.
• Myofibrils: rod-like structures that extend the length of the cell and contain repeating units called sarcomeres.
• Sarcomeres: contractile units of a skeletal muscle cell that contain actin (thin) and myosin (thick) filaments.
• When these filaments move over each other, the muscle cell contracts.

☆ Muscle cell structures - actin filament
• The actin filament consists of actin proteins and regulatory proteins.
• The actin protein has binding sites for myosin molecules.
• The regulatory protein, tropomyosin, is wrapped around the actin molecules, covering the myosin binding sites.
• A second regulatory protein, troponin, contains binding sites for Ca2+ ions and is attached to actin and tropomyosin.
• In the presence of Ca2+ ions, troponin binds with calcium and pulls the tropomyosin away from the myosin binding sites and uncovers them.

☆ Muscle cell structures - myosin filament
• The myosin filament consists of myosin molecules, each with a head (or crossbridge) and tail.
• ATP binds to the crossbridge.
• When ATP binds with myosin it is hydrolyzed, releasing energ to move the myosin protein.
• Myosin crossbridges attach to specific myosin binding areas.
• When the actin and myosin binds, any movement of the myosin filament will also move the actin filament.

☆Muscle cell structures - titin filament
•The elastic titin filament allows the sarcomere to return to its resting length after contracting or stretching.

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