The muscular system is the reason that we are able to walk, play, eat, breathe, and to just about any other activity you can think of. It is what gives us motility. There are three types of muscle tissue - skeletal muscle, cardiac muscle, and smooth muscle.

Skeletal Muscle: Skeletal muscle is attached to the bones by tendons, and is responsible for all movements we make voluntarily. It is made up of muscle fibers, bundles of long cells. Skeletal muscle is also called striated muscle due to the striped pattern seen under a microscope. This pattern is caused by the regular arrangement of sarcomeres, the basic contractile unit of the muscle.

Cardiac Muscle: Like skeletal muscle, cardiac muscle is striated and has many of the same properties. It makes up the contractile wall of the heart and is responsible for heart contractions. The fibers of cardiac muscle branch and interconnect through intercalated disks. These disks help to synchronize the heartbeat by relaying signals between cells.

Smooth Muscle: Smooth muscle lacks the striations of cardiac and skeletal muscle and has spindle-shaped cells. It can be found in the walls of internal organs and the digestive tract. These muscles take care of involuntary body activities, such as the constriction of arteries. They are controlled by a different type of nerves than skeletal muscles.

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The Sliding-Filament Model

When a muscle contracts, the filaments that it consists of do not change in length; they instead slid past each other, increasing overlap. This is known as the sliding-filament model. The phenomenon is caused by interactions between myosin and actin molecules. The "head" of the myosin molecule binds to ATP, and then hydrolizes it into ADP and phosphate. This hydrolysis allows the myosin to bind to actin, forming a cross-bridge between the filaments. The myosin then proceeds to pull the thin filament towards the sarcromere's center. A new molecule of ATP binds to the myosin head, and the myosin releases the filament. Then the entire cycle begins all over again.

Of course, none of this would be possible without the aid of calcium ions. When a muscle is at rest, a set of regulatory proteins called the troponin complex binds to the actin strands, effectively covering the myosin binding sites. However, accumulating calcium ions in the cytosol bind to the troponin complex and cause the proteins to shift position. The mysoin binding sites are uncovered, and the muscle is able to contract. The calcium ions are released into the cytosol when the neurotransmitter acetylcholine triggers an action potential.

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Muscular System Diseases/Disorders

Compartment Syndrome
Compartment syndrome is excessive muscle pressure that causes a decrease in blood flow and sometimes nerve damage. Symptoms of this include pale skine, severe pain, decreased sensation, and weakness. The only way to treat the syndrome is surgury. Long cuts through the fascia must be made in order to relieve the pressure, and then they must be sewn back up in a second surgury two to four days later. A study in the United States showed that prevalence of compartment syndrome is only 14%.

Muscular Dystrophy
Muscular dystrophy is a genetic disease in which the muscles are susceptible to damage and become progressively weaker. The muscles may even begin to draw inward and then become fixed in that position. This is called contracture. The most common type of muscular dystrophy occurs in boys, and symptoms begin to surface as soon as the child begins to walk. Signs and symptoms can include falling often, a waddling gait, trouble jumping or running, and difficulty getting up from a sitting or lying down position. There is no cure for this disease, but corticosteroids can improve muscle strength and slow progression. Braces may be used to support the weak muscles and fight contractures. Contractures can also be loosened by removing tendons that pull the joints inward. Thankfully, this terrible disease is listed as a "rare disease" and only 500-600 infants are born with it each year.

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Sources:
~ Campbell AP Biology Textbook, Eighth Edition
~ http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002204/
~ http://emedicine.medscape.com/article/88014-overview#a0156
~ http://www.livestrong.com/article/201935-common-muscular-system-disorders/
~ http://www.mayoclinic.com/health/muscular-dystrophy/DS00200