How Muscles in the Human Body Work
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Muscle Nerves
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Muscle tissue is innervated by a series of motor neurons. When the brain sends signals a motor neuron all the muscle fibers connected to that neuron contract at once. The number of fibers per neuron depends on the demands placed on that particular muscle. For example, a runner will have more fibers per neuron in the leg muscles than a non-runner. However, motor neuron connections can change as demand changes. If a non-runner starts running regularly, the motor neurons will recruit more fibers to accommodate the increased demand. Conversely, if that runner stops running, over time, the motor neurons will recruit fewer fibers. Because every fiber on the neuron must fire, muscle contraction has the potential to use a lot of energy. A variable rate of recruitment helps the body conserve energy by transferring energy use from high-demand to low-demand areas.
Sliding Filament Theory
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Muscle contraction doesn't end with the nerve impulses from the brain, it actually relies on a series of complex chemical and mechanical processes. Each muscle fiber is made up of thick and thin filaments that lie parallel to each other and give muscle its striated (or lined) appearance. The muscle fibers are arranged in a long tube called a sarcomere. In the sarcomere, the thick and thin fibers are staggered lengthwise so that the thin fibers take up the outer and middle areas of each end, and the thick fibers make up the middle and run through the center of the sarcomere.
The thick fibers have a series of branches with bulbous heads that extend from the fiber like oars on a boat. Each fiber also has a small calcium store and a receptor for the neurotransmitter acetylcholine. At rest, the thick and thin fibers don't touch.When a motor neuron fires it releases acetylcholine, which attaches to the receptors on the muscle fibers and triggers an electric charge along the fibers. This electrical charge stimulates the release of calcium ions which attaches to the thin fibers. The branches from the thick fibers then connect to the thin fibers, pivot and pull the thin fibers toward the center of the sarcomere. This happens simultaneously on all fibers connected to that motor neuron, causing the muscle to contract.
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Muscle Fuel
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In addition to electrical impulses, muscles also need a fuel source in order to effectively contract. Muscles get their fuel in the form of adenosene triphosphate (ATP), which is synthesized from glucose in the mitochondria of the muscle cell. Most glucose comes from carbohydrates in our diets. However, the liver has a small store of glycogen and, if needed, that store is synthesized into glucose for use in ATP production.
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