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Muscular system

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Muscle contraction.PNG

The muscular system is a biological system that produces body movement and it is also primarily responsible for the movement of blood throughout the body. In vertebrates the muscular system is controlled primarily by the nervous system. There are some muscles that work autonomously from the nervous system which allow the body to run more smoothly. The muscles of the body allow for every movement in your body, and are also used in things not normally noticed. The movement of blood through your veins, the moving of the eyes is all controlled by your muscular system.[1]

Muscle tissue is composed of cells that have the special ability to shorten or contract in order to produce movement of the body parts. The tissue is highly cellular and is well supplied with blood vessels. The cells are long and slender so they are sometimes called muscle fibers, and these are usually arranged in bundles or layers that are surrounded by connective tissue. Actin and myosin are contractile proteins in muscle tissue.[2]

Muscle tissue can be categorized into skeletal muscle tissue, smooth muscle tissue, and cardiac muscle tissue. Skeletal muscle fibers are cylindrical, multinucleated, striated, and under voluntary control. Smooth muscle cells are spindle shaped, have a single, centrally located nucleus, and lack striations. They are called involuntary muscles. Cardiac muscle has branching fibers, one nucleus per cell, striations, and intercalated disks. Its contraction is not under voluntary control.[2]

Structure

Muscles of the human arm.

The muscular system is a highly organized and complex system in the body. The amazing complexity of the muscular system both on the microscopic level and the macroscopic level shows more subtle evidence that humans did not come from evolution but from a creator. The muscles fibrous make up allows for a more taught and strong muscle. It is much like a rope. With many small fibers you can make a extremely strong rope.

Cell

A muscle is primarily composed of muscle cells (also known as muscle fibers). More specifically, the muscle is a group of muscle cells that bound in a group of connective tissues which are then bundled together.[3] The skeletal muscle cell is a multinucleate cell meaning it has more than one nucleus per cell. Muscle cells are highly complex and have many proteins and other substances that comprise them. On the interior of a muscle cell are myofibrils.[4]

Structure of Skeletal Muscle.

Groups of myofibrils (Muscle fibers) are encased in the sarcoplasmic reticulum. The sarcoplasmic reticulum is a modified smooth endoplasmic reticulum. The main difference between the two is that the S.R. is concerned more with pumping calcium ions than the production of proteins. The S.R. is then surrounded by a plasma membrane which aids in the protection of the muscle cell. Myofibrils are composed of a specific sequence of protein units (called sarcomeres) that are highly organized and repetitive in skeletal muscle giving them their striated appearance.

Sarcomere

Each muscle cell contains many small bundles of contractile proteins, called myofibrils. These contractile proteins do the work of muscle contraction.

Sarcomeres are the repeating units of protein fibers that perform the contraction of the muscle cell. They are visible in striated muscle as sets of dark and light stripes. The units within the sarcomere, which runs horizontally along the cell is the "Z" line, after this lies the "M" band which is enveloped by the "H" zone which in turn is surrounded by the "A" band, and finally the "I" band. Sarcomeres lay directly between the two Z-lines in the muscular cell sequence. Sarcomeres are also the portions of the cell that are responsibly for muscle contraction.[5]


  • Z-line
The Z-line is the portion of the muscle cell that anchors in the thin actin filaments.
  • H zone
The H zone is the portion of the muscle cell where the actin and myosin filaments do not overlap. This section appears light because the filaments do not overlap when the muscle is relaxed.
  • M band
The M band which lies inside of the H zone is seen as a dark strip. It is dark because it is the location that contains proteins that hold the myosin filaments in place. They are held in place by the protein titin which is most likely the largest polypeptide chain in the human body.
  • A band
The A band is the central band in the muscle cell and it contains all of the myosin filaments.
  • I band
The I band is another location where the actin and myosin filaments do not overlap creating another section that appears light because the filaments do not overlap when the muscle is at a relaxed state.

Types of Muscle

In the body there are three different types of muscles. There is the striated (or skeletal) muscle which is a "voluntary" muscle, meaning you have to purposely think about it to cause that muscle to move. Next is the smooth muscle which is an "involuntary" muscle because it acts without the body consciously having to think about it. The last type of muscle is the cardiovascular muscle. This muscle is also a "involuntary" muscle but it has its own group because it is a highly complicated and unique set of muscles.

Voluntary

Striated

Main Article: Skeletal muscle

Skeletal muscle, is striated, voluntary muscle that is connected to the bones of the body by tendons. Examples of these muscles are your quadriceps, hamstrings, biceps, triceps and many other muscles in the body. Skeletal muscle is primarily used for locomotion. There are two types of skeletal muscles. The amount of each type is based on heredity but can be changed to a small degree by training. These two types of skeletal muscles are Slow-twitch muscles, which are also called red muscle, and Fast-twitch muscles which also go by the name of white muscle.

A twitch is caused by the spread of the action potential of a muscle through the T tubule system of the muscle which in turn causes a minimum unit of contraction. A twitch is measured by the force of tension produced by the muscle. The more fibers that "twitch" the higher the action potential and force generated by the muscle is.

Slow-twitch
A slow twitch muscle is a muscle that is capable working for a long period of time. This is the case because the muscle contains an abundance of myoglobin which is a molecule that is efficient and capable of binding large amounts of oxygen. Because they muscle is oxygenated continuously it can be "put to the test" for long periods of time. This quality increases the overall metabolism of the muscle making it work for long periods of time with out stopping. Slow-twitch muscles are abundant in Olympic level long distances runners.

Fast-twitch
Fast-twitch muscle has a low concentration of myoglobin decreasing its oxygenative ability and decreasing the ability to metabolize quickly. They also have fewer mitochondria. Because of these circumstances fast-twitch muscles are not capable or lasting for long periods of time. They are better suited for sprinting and fast moving activities that do not take long.[6]

Involuntary

Smooth

Smooth muscle is the muscles in the body that work as "involuntary" actions. The mind does not have to specifically think about performing an action for smooth muscles to work. Smooth muscles are muscles that move food through the digestive tract, control the bladder, and also sends the blood back and forth through the circulatory system in the body. Smooth muscles are not straited like skeletal muscles but have a rather smooth appearance because the filaments of the cells are very un-regular arrangement which causes it to have its smooth look.[7]

Some sections of smooth muscles have unique proprietress. The smooth muscles in the intestines react instantaneously through the process of gap junctions. A electrical charge goes through the gap junctions of the muscle which allows the muscle to react in unison of itself. All of the smooth muscle cells are controlled by the autonomic nervous system.[7]

Cardiac

Cardiovascular Muscle
Main Article: Heart

The third type of muscles is the cardiovascular muscle. This is also a Involuntary muscle type. The cardiovascular muscle, primarily the heart is responsible for sending blood throughout the body. The cardiac muscle is different than both the skeletal muscle and the smooth muscle in appearance because it forms a mesh work rather than being just straited or scaled. The mesh work of the cardiac muscle is important because it allows the muscle to be stronger enabling the heart to keep from rupturing with the extreme blood pressure pumping through it continuously. Like the electrical contact in smooth muscle cells, the cardiac muscles also are in electrical contact which allows the heart to beat evenly all at once. The rhythm of the heart beat is controlled by specialized cells called pacemaker cells. This makes the heart beat myogenic because the heart is regulating itself rather than being regulated by the nervous system.[7]

Contraction

Main Article: Muscle contraction

Muscle contraction occurs by a process called the sliding filament theory. This theory is based on the fact that when the muscle contracts the sarcomeres bringing the two Z-line's together. This being done the A and I band are pulled together causing the myosin and actin filaments to slid in against each other. When the muscle begins to contract the myosin binds to the actin. This occurs because myosin has "heads" which are points where ATP can hydrolyze it with ATPase causing it to change form. When this form has change it moves backward connecting itself to the actin. This process is reversible and will re contract when the muscle loosens. The myosin molecule has more than one head per filament allowing for multiple reactions to occur. Actin is a helical shaped filament that has two other proteins twisted around it that aid in the binding of myosin and the gleaning of calcium ions to allow the electrical charges that move them muscles to react.[8]

The response for the cell to contract originates from the actin-myosin interactions with calcium ions. When the action potential is sent and received by the neurons to the neuromuscular junction. A single synapse for a muscle has a large enough gap junction that it can affect up to hundreds of muscle fibers at once. The charges sent by the synapse cause the S.R. to collect calcium ions. When dormant nothing occurs but cells that lie between the T tubules and the S.R. are charge sensitive. When they are affected by a charge changing their shape. When the shape changes a calcium ion is let through the S.R. into the myofibril causing the filaments to get excited causing them to contract.[8]

References

  1. Muscle by Wikipedia
  2. 2.0 2.1 Muscle Tissue SEER Training Modules, National Cancer Institute.
  3. Purves, William. Life the Science of Biology. 2005. Couriers Company Inc. p907-908.
  4. Myofibril by Wikipedia
  5. Purves, p906-907
  6. Purves, p911-912
  7. 7.0 7.1 7.2 Purves, p905
  8. 8.0 8.1 Purves, p908-909