Your muscles are made up of thousands upon thousands of muscle fibers. Within each muscle fiber lies a sarcolemma, the protective sheath around a muscle fiber and a sarcoplasm, the protection around myofibrils. One single muscle fiber contains many microscopic myofibrils. The myofibrils are the portion of the muscle fiber that contains the mechanism for contraction.
Myofibrils
Inside a myofibril are two myofilaments known as actin and myosin. Actin and myosin are the molecules responsible for contracting a muscle fiber. A full myosin filament is made of about 200 myosin molecules that are found in a strand-like formation with small heads that come off the filaments. These small protrusions are known as cross-bridges. Actin lies just above and below each myosin filament. It is shaped like a double helix. Two additional proteins, tropomyosin and troponin, are located on the actin filament and assist muscle contractions as well.
The Sliding Filament Theory
The most basic way of explaining the sliding filament theory is that an interaction between actin and myosin causes actin to slide inward with assistance of the myosin cross-bridges. This movement shortens the muscle fiber, which results in a contraction. There are two phases that occur during a contraction, the excitation phase and the contraction phase.
Excitation-Contraction Coupling
In order for a muscle to contract, it must be signaled to do so by the brain. The excitation phase of a contraction starts when the nerve ending to the muscle excites a muscle fiber. This nerve impulse releases calcium ions into myofibrils. Calcium interacts with the troponin and tropomyosin on the actin filament, moving it away and exposing a site for the myosin cross-bridges to grab onto the actin.
Contraction Phase
ATP, the source of energy for the human body, is present on myosin and when the energy from the ATP is released, it allows the myosin cross-bridge to bind to the actin filament. ADP is produced when the energy from ATP is released, which allows the myosin head to flex into a bent position and move the actin filament. This is called the power stroke. The myosin will stay attached to the actin until another ATP is present, then it will release it and do the whole process again until the contraction is completed. Although it seems like a time consuming process, all of these steps take just seconds for your body to perform.
References
- "Essentials of Strength and Conditioning: National Strength and Conditioning Assciation" Thomas R. Baechle, Roger W. Earle, Editors; 2000
- "Anatomy and Physiology"; Kenneth S. Saladin; 2004
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