Introduction:

Almost everything you do requires actions of muscles. There are muscles big and small, but they are all over your body. Each muscle consists of individual fibers that when stimulated by a somatic motor neuron contract. For every contraction of a muscle, an imulse must happen. The cells that control the contraction and relaxation of muscles are at a microscopic level. Without the proper function of muscles, you would not be able to maintain homeostasis in your body. Muscles are used for everything it seems like, from every breath you take, to blinking your eyes. The actions that the muscle must go through in order to contract, and relax is more complex then most people know. Smooth, skeletal, and cardiac are the three main categories of muscles in the body.



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Concept #1: Structure of a muscle

All skeletal muscles are usually attatched to a bone. Tendons are a tough connective tissue that attaches the muscle to the bone. The origin is the less movable part of the muscle, while the insertion is the movable part. Agonist muscle is the prime mover of any skeletal muscle. When the opposite action is produced by a flexor, which decreases the angle of the joint, and an extensor, which increase the angle of the bone at the joint, are antagonistic muscles. Episysium, which is within a fibrous connective tissue within the tendons, wraps around the muscle. A fasicle are columns within the muscle, that are surrounded by the perimysium. Each muscle is composed of muscle fibers, or myofibers. The sarcoplasmic reticulum has interconnected sacs and tubes that surrounds each myofibril. Myofilaments are very small structures inside each myofibril. There are different kinds of filaments inside the myofibril, thick and thin. Myosin is the thick filament, while actin is the thin filament. Every muscle fiber is surrounded by a plasma membrane, or a sarcolemma. Unlike most other cells, muscle fibers have multiple nucleus'. Muscles are straited, because of the alternating dark and light bands on the fiber. A bands are the dark bands, and I bands are the light bands. In the middle of the I band, a Z line can be seen. The neuromuscular junction is where the neurotransmitter is released in order to stimulate a contraction. A motor end plate is the place of the muscle fiber at the neuromuscular junction. The motor unit includes the somatic motor neuron and all of the muscle fibers it innervates. When a contraction is stimulated, all of the muscle fibers work together to acheive that goal. All of these structures of a muscle must be working properly in order for the muscle to either contract or relax.



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Concept #2: Muscle contraction

All of the parts of the muscle must work together in order for a contraction. There are thick and thin filaments which are in the myofibrils. Thin filaments belong in the I band, and thick filaments are in the A band. The H band is in the center of the A bands, and are lighter. In the center of the I band, are Z lines which are thin and dark. The Z line is located towards the outer edge, and there are two Z lines in each sarcomere. From the Z line on one part to the Z line on the other part is called a sarcomere. The M lines help to anchor the filaments during a muscle contraction. After a muscle contracts it returns to it's original state, which is assisted by titin that is an elastic fiber. On the bottom of a sarcomere, which is Z line to Z line, are the thick filaments of myosin. Coming off of the thick myosin are a myosin protein that has a head and an arm. The myosin heads are located on both sides of the sarcomere, so when a cross bridge is formed that the myosin pulls the actin from both sides. Each myosin head contains and ATP binding site, that can attach to the actin. Myosin ATPase is an enzyme that breaks down ATP into ADP and finally into Pi. ATP is the energy that is needed for the muscle to contract, also known as adenosine triphosphate. When the ATP breaks down on the myosin head it becomes cocked, and now has the potential energy for contraction. Once the myosin attaches to the actin, Pi is released, which then produces a power stroke in the cross bridge. The power stroke pulls the thin filaments closer together in the middle of the A band. After the power stroke, ADP is released and a new ATP molecule binds to the head of the myosin. The release of the ADP and binding of ATP is needed for the myosin to let go of the actin. ATP is needed for both contraction and relaxation. Muscles can't be contracted all the time, so within the actin lies a protein called tropomyosin. Troponin is attached to the tropopmyosin rather than the actin. Troponin and tropomyosin act as the on and off switch for muscles. Both proteins work together to regulate the attachment of cross bridges to actin. When the muscle is relaxed tropomyosin blocks the attachment of a cross bridge to the actin. In order for a muscle to contract then the tropomyosin must be moved. Calcium ions, Ca2+, interacts with tropomyosin to move it, so a cross bridge can be formed. Ca2+ levels increase during contractions, and decreases during relaxation. A troponin complex are three subunits that are attached to the tropomyosin rather than the actin. When Ca2+ latches onto the troponin complex, moving the tropomyosin, the cross bridges can attach and a power stroke can occur. All of these complicated steps must take place in order for a muscle to contract. It's amazing the rate of speed that these tiny cells can perform their job, and move such large things.



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Concept #3: Cardiac and smooth muscle

Although all muscles seem to have the same job, they are all different in many ways. Cardiac muscle controls the heart, and only the heart. Like skeletal muscle, cardiac muscle is also straited. Myocardial cells contain the actin and myosin, are short, branched, and interconnected. Gap junctions are electrical synapses. Each myocardial cell is joined, and the gap junctions are more concentrated towards the end of each cell. Unlike skeletal muscle which needs nervous stimulation, cardiac muscle's impulses are spontaneous. Since the gap junctions are more concentrated towards the end of the myocardial cells, this allows the impulses to be sent from cell to cell easily. Gap junctions appear like dark lines underneath the microscope, which are called intercalated discs. The myocardium is a mass of myocardial cells where an action potential originates. Action potentials are sent very fast because the cells are so close together. Because the action potentials happen so fast, the myocardium behaves as one single unit. All of the myocardial cells contract to their fullest each time, unlike skeletal muscle. Smooth muscle are in the walls of the blood vessels and bronchioles and are arranged in a circular pattern. There are other places throughout the body that contain smooth muscle as well. Smooth muscles do not contain sarcomeres, but do have alot of actin and some myosin. Unlike skeletal muscle which contains troponin, smooth muscle has a protein called calmodulin. Calmodulin has similarities to troponin. The entire surface of the smooth muscle has receptor proteins for neurotransmitters. Neurotransmitters are released along a part of the autonomic nerve fiber which is a distance away from the smooth muscle. Varicosities look like bulges, and this is where the transmitters are released along the autonomic fiber. Neurotransmitters are released from the varicosities and stimulate the smooth muscle. Without the proper function of both cardiac and smooth muscle our bodies would not work. The cardiac muscle contracts all day everyday, until we die. Smooth muscles help us breath, carry our blood, digest our food, and much more. The action of both of these muscles is awesome.

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Application:

Not everyone in the world has proper function of their muscles. Being in the health care field, understand how muscles work is essential. If I see a patient who needs to, or is, exercising more frequently, being able to explain how their muscles are growing is key. There are many diseases, disorders, and conditions that affect the muscles in our body. Keeping the muscles healthy is needed to live a healthy life. In the world, there are many people who have Parkinson's disease. Dopamine is the neurotransmitter that is too low compared to acetylcholine which causes the uncontrollable tremor. My boyfriend has restless leg syndrome, which is described as a lack of dopamine as well. Being able to treat and take care of a patient is important, but it's also very important to understand why this is happening to them. There are quite alot of muscles in our body, and three main kinds of muscle. The number of conditions that involve skeletal, smooth, and cardiac muscle is huge. Education on the reason why, will help me be a better nurse.

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Essential question:

The sliding filament theory of contraction is basically what happens when the muscle is contracting. When the muscle contracts the sarcomere, which is from Z disc to Z disc, shortens. The A bands do not shorten, but they do move closer to each other. In the I band, which is the distance between the A bands, the thin filaments called actin do shorten. In order of the muscle contraction to be completed the sarcomere must get smaller, or come together. The thick myosin, and thin actin don't actually change their size, but they slide over each other to shorten the sarcomere. The sliding filament theory comes from this observation of the filaments sliding to complete a muscle contraction. Actin is on either side of the A band get closer towards the middle of the sarcomere and overlap the myosin. The H and I bands also get closer during contraction. Sliding of the filaments could not happen without the action of the cross bridges. Cross bridges are part of the myosin that extend from both sides of the thick filaments. A myosin head and a myosin arm come out from the thick filaments. The myosin head must bind with ATP before the start of a contraction happens. After the binding of the ATP, myosin ATPase breaks down the ATP into ADP, and then into Pi, or phosphate. When the phosphate binds to the myosin head, it causes it to become cocked, or bent. Once the myosin head is bent, it forms with actin. First before the myosin can bind with actin, the actin must move the tropomyosin. The actin contains a protein that blocks the binding site for myosin called tropomyosin. Troponin is attached to the tropomyosin rather then the actin itself. The troponin has three subunits which is then called the troponin complex. Tropomyosin has to be moved in order for the myosin binding. Calcium ions, Ca2+, help to move the tropomyosin out of the way. During contraction the levels of Ca2+ are increased, and during relaxation the levels are decreased. Inside the sarcoplasmic reticulum is where the excess of Ca2+ is stored until it's needed for contraction. The Ca2+ concentrations, which are found within the sarcoplasm, must be lowered in order for relaxation. When the tropomyosin is moved, to expose the binding site on the actin, the myosin head binds with the actin. Once the myosin is bonded with the actin, the myosin releases it's phosphate, which then creates a power stroke. The power stroke is the force that pulls that actin towards the center of the A band. Once the power stroke is complete the ADP is released and a new ATP molecule binds to the myosin head. In order for the muscle to relax, ADP must be replaced with ATP. ATP, or adenosine triphosphate, is needed before a contraction and after a contraction for relaxation.

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References from top to bottom:

http://www.youtube.com/watch?v=PJDrR3sZPZU

http://www.nlm.nih.gov/medlineplus/ency/images/ency/fullsize/19917.jpg

http://www.youtube.com/watch?v=XoP1diaXVCI

http://people.eku.edu/ritchisong/301images/muscle_structure.jpg

http://www.tmd.ac.jp/artsci/biol/textbook/sarcomere.jpg

http://faculty.irsc.edu/FACULTY/TFischer/AP1/cross%20bridge%20cycle.jpg

http://www.youtube.com/watch?v=WRxsOMenNQM

http://www.colorado.edu/intphys/Class/IPHY3430-200/image/14-7h.jpg

http://faculty.etsu.edu/forsman/Histology%20of%20musclefor%20web_files/image006.jpg

http://www.istockphoto.com/file_thumbview_approve/15227315/2/istockphoto_15227315-nurse-muscle-power-strength.jpg

http://edoc.hu-berlin.de/dissertationen/kabaeva-zhyldyz-2002-11-11/HTML/objct2.png