Contraction of a Muscle

Module Four Lesson One Practice By: Brielle Winner


A sarcomere is one unit of skeletal muscle. It is made up of millions of different fibers, the two main fibers are actin and myosin.


Actin is a protein that makes up the thin myofilaments in skeletal muscle.


Myosin is a thick filament of protein found in skeletal muscle cells that will bind to, and pull on actin filaments during contraction. It needs two chemicals in order to attach to actin. They are ATP and tropomyosin.

ATP (adenosine triphosphate) is the main energy source for cell functions. When ATP is produced, calcium can be actively pumped in the body making muscle contractions.

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What happens during a muscle contraction:

A sarcomere contains actin and myosin filaments. Myosin needs two chemicals to attach to actin: ATP and tropomyosin. When myosin finally gets access to myosin the calcium is released by action potentials and will grab onto the tropomyosin. It moves the block out of the way. Myosin can loses its phosphate head. It will let go of the phosphate and creep along the actin. Myosin slides along actin to contract the muscle.During contraction (in the presence of calcium ions) myosin heads bind to actin filaments and pull them closer to each other and shorten the sarcomere. When you relax a muscle it slides back to its original spot. So relaxing the muscle will lengthen the sarcomere. The ends of each sarcomere are defined by the Z-disc. I bands contain actin filaments and A bands contains only myosin. During muscle contraction, each sarcomere shortens and bring the z-discs closer together. Actin filaments slide past myosin filaments towards the middle of the sarcomere. Skeletal muscle is triggered by nerve impulses which will release calcium ions.
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