In order to move, all animals must use some form of muscle to move. For example, mammals are comprised mainly of muscle (aside from internal organs), along with other organic materials, such as fat. Amphibians and fish also have muscle similar to that of mammals. Other animals have striated muscle, such as insects. Insects have muscle similar to that of mammals, although it is comprised of different, yet similar, materials. An interesting example of muscle is in the starfish. Starfish do not use their muscle to interact with the outside world, but rather to move water throughout their bodies. In order to move, starfish employ a system that utilizes suction. This suction is created through pressure differentials inside the starfish, which are controlled by muscle.

The muscle easiest to study is the muscle found in mammals, fish and amphibians, as all of these types of animals have striated muscles that are large enough to study. For example, the human body is about 40% muscle. This muscle is divided into three basic categories: cardiac, skeletal, and smooth. Cardiac muscle tissue is only found in the heart and involves the pumping of blood throughout the body. Skeletal muscles are those muscles involved in the actual movement of the body, and are the only type that can be consciously controlled by the body. Smooth muscle tissue is a broad category that encompasses the muscle tissue that cannot be consciously controlled by the body. The muscles' main function generally involves the movement of blood, and digestion.


Cardiac Muscle



The cardiac muscle tissue is specialized to respond to electrical impulses from the SA node (a natural heart pacemaker), which is in contrast to the skeletal muscle system, which only responds to nervous system electrical impulses. This unique structure of the SA node allows the heart to perform its function without constantly receiving impulses from the brain. Furthermore, cardiac muscle tissue is thicker in the the left ventricle then the right ventricle, in mammals, since the left ventricle is responsible for pumping blood throughout the body, whereas the right ventricle only has to pump deoxygenated blood to the lungs, a much shorter distance, requiring much less effort. This structure and function relationship allows the simultaneous excretion from the heart from both ventricles.
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The heart is made up of Cardiac Muscle

Skeletal Muscle



Skeletal muscle is the type of muscle most typically used for voluntary movement. This is the most familiar type of muscle, as in humans it includes muscles such as the hamstring and the bicep. This type of muscle is constructed of bands of muscle fiber. Depending on their size and function, each muscle fiber can range in size from 1 mm to 30 cm. Each muscle fiber is one cell and this cell can have multiple nuclei. In the multiple dissections completed, there was a large amount of unity throughout many of the organisms dissected. First of all, the skeletal muscles were similar in appearance throughout all of the organisms, sort of a skin/peach color. However, had the organisms been alive, as they are during surgery, the muscles would have quickly turned red as the result of both blood and direct exposure to oxygen. The initial surgical incisions through the skin, subcutaneous tissue layer exposes skin colored muscle, that by the middle of the surgery has turned red, in accordance with the classical anatomical charts.
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Smooth Muscle



Smooth muscles are not under the conscious control of the brain. One of the best examples of smooth muscle comes in the digestive system, where the peristaltic muscle movements that progress the food through the digestive tract are not consciously stimulated. I cannot "tell" my digestive tract to start contracting, it does that of its own accord. Unlike skeletal muscles, smooth muscles are not striated or banded. Smooth muscle is found primarily in the arteries and the internal organs. In air respiring organisms such as humans or pigs a diaphragm is necessary for manipulating the pressure inside the body to allow oxygen in and out of the body. The diaphragm is essentially a large sheet of smooth muscle and can be seen in the picture below. Smooth muscle contractions require less calcium and energy than skeletal muscles, allowing for the maintenance of essential homeostatic functions at low energy costs.
diaphragm
The diaphragm is smooth muscle which aids in respiration



Muscle Structure



In order to construct a full muscle, there needs to be a collection of pieces. The first level of organization is the entire muscle itself. There are many skeletal muscles on the body. Often, the only way to tell the difference between muscles is by looking at the striations or the lines in the muscle tissue. Muscles have an antagonistic relationship to one another as skeletal muscles can only provide a pulling force. This means that as one muscle pulls or contracts, there must be another muscle to pull and contract in the opposite direction. The muscle level of organization consist of many fascicles grouped together that are surrounded by a covering called the epimysium. Like muscles, fascicles are simply bundles of muscle cells, or muscle fibers, that are surrounded by anther covering called the perimysium. Muscle fibers are the basic muscle cells. They have many nuclei and mitochondria and can be very long. Muscle fibers are filled with proteins organized into what is known as myofibril. Each myofibril consists of many chains of proteins that are known as myofilaments. Actin and myosin are the most well known of the myofilament. Myosin slides actin over itself in order to provide a contracting force which in turn contracts the muscle cell, the fascicle, and the muscle. However as stated before this is he result of what is known as a neuromuscular junction, the connection between the nervous system and each muscle fiber. It is in the neuromuscular junction that groups of muscle cells, or motor units, become excited as the result of impulses from the brain and begin the process of contraction.
I found some detailed information on the relation between the muscular and nervous systems. This "neuromuscular junction" occurs when the neuron undergoes depolarization and eventually an action potential arrives at the axon of the neuron. This causes calcium channels to open and sends Ca2+ ions into the extracellular fluid, eventually leading to the release of acetylcholine into the synaptic cleft. The acetylcholine binds to the receptors which lie on the muscle cells. These receptors are ion channels which open (due to acetylcholine) and let potassium and sodium to flow into the cell, causing the original message from the nerve cell to be transported to the muscle cell [edited by Casey].

The Sliding Filament Theory of Human Skeletal Muscle Contraction:

In order for a muscle to contract, a nerve impulse has to be sent out from the brain. The impulse moves to a motor neuron, where acetylcholine is released from the presynaptic vesicles. The acetylcholine binds to an acetylcholine receptor along the surface of a muscle cell, and causes the release of calcium from calsequestrin, which is located in the sarcoplasmic reticulum. The calcium then binds to troponin, which causes troponin-tropomyosin complex to move, which thus exposes the binding sites of the myosin and actin. So far, all of these processes have taken place on the thick filaments, which are comprised of myosin. The myosin head, the binding site of the thick filament, then forms a crossbridge with the actin filament and perform a "power stroke", which pulls the actin filaments. When these filaments "slide" past one another, a muscle contracts. This sliding causes the thin filaments and thick filaments in a sarcomere to slide past one another. In the entire process, none of the filaments change size, they rather just most past each other in order to contract the entire muscle.



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http://www.ivy-rose.co.uk/Topics/Muscles/Sliding-Filament_Theory.jpg