Basic muscle contraction

Muscles shorten or contract to move the skeleton. The origin of the muscle is where it begins and the insertion is where it attaches. The insertion moves toward the origin during contraction. A muscle has a connective tissue covering called the epimysium. A little deeper are bundles of muscle fiber called fasciculi which are covered by perimysium. Each individual muscle fiber has a covering called the endomysium.


Muscle fibers ability to contract is due to the sliding filament theory. The smallest functional contractile unit of a muscle fiber is called a sarcomere. Sarcomeres contain myofibrils which are composed of the filaments actin and myosin. Actin is the thin filament and myosin is the thick filament. A nerve fires which depolarizes the muscle fiber. The sarcomere has a significant influx of calcium which move the troponin-tropomyosin complex away from the actin binding site. The myosin filament binds the actin binding site giving the myosin head ATPase ability. The breaking of this high-energy phosphate bond provides energy for the sarcomeres to shorten and muscle to contract. As the sarcomere shortens, portions of it are only the actin filament which is the lightest. There is myosin alone which is darker. The portion of sarcomere with overlapping of actin and myosin are the darkest. This different color banding pattern is called a muscle being striated.

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