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Skeletal Muscle Organization and Structure

Skeletal muscle is composed of single muscle cells called fibers.  Each fiber is composed of myofibrils. 

MACROSTRUCTURE
Skeletal muscle shape and organization is maintained by three pieces of connective tissue: 
 

The Epimysium:  The outermost layer of connective tissue, the epimysium wraps around the entire muscle and is continuous with the tendon. 

The Perimysium:  The middle layer of connective tissue, the perimysium attaches one bundle of muscle fibers to another bundle of muscle fibers.  A bundle of muscle fibers is termed a fasciculus. 

The Endomysium:  The innermost layer of connective tissue, the enomysium wraps each individual muscle fiber and allows them to be bound into a fasciculus. 

  

MICROSTRUCTURE

A myofibril can be divided into two myofilaments:  Actin and Myosin.  Actin is a thin protein, while Myosin is a thick protein.  Actin and myosin make up two of the four contractile proteins.  

The myofilaments are suspended in a gel-like substance called the sarcoplasm.  The sarcoplasm is the cytoplasm of a muscle cell.  Mitochondria are also embedded in the sarcoplasm, along with a structure called the sarcoplasmic reticulum.  The sarcoplasmic reticulum is a specialized form of the endoplasmic reticulum, and stores calcium ions.  Hence, the highest concentration of calcium in the cell is located within the sarcoplasmic reticulum.   

The sarcoplasm is wrapped by the sarcolemma, the membrane of the muscle cell. 

Deep invaginations in the sarcolemma and sarcoplasm exist.  These invaginations are called the Transverse tubules.  The transverse tubules terminate between two adjacent sarcoplasmic reticulum.  The Transverse tubules, sometimes called the T tubules, allow the depolarization (resulting from an action potential) to propagate down the sarcolemma and to pass within close proximity of the sarcolasmic reticulum, thereby eliminating the need for the depolarization to travel through the sarcoplasm.  This is advantageous for muscular contraction because the process of propagation through the sarcoplasm is much slower than propagation down the sarcolemma. 

 
STRIATIONS / SARCOMERE STRUCTURE

Actin and myosin are arranged in a fashion that causes visible striations, or alternating regions of light and dark.  The smallest functional unit of muscular contraction, called a sarcomere, is used to demonstrate the different bands and lines , which have been assigned names:  the I band, A band, Z line, and H band.

The I band stands for isotropic band, or "light passing" band because a beam of light can be shined through it.  The light can pass through this area because it contains only the thin filament, actin, and does not contain the thick filament myosin.  The I band is divided in half by the Z line, the point at which actin is anchored.  A sarcomere is defined as one Z line to another Z line, meaning that half of the I band lies in one sarcomere while the other half lies in another sarcomere.  The I band can be found at each end of a sarcomere.

The A band stands for anisotropic band, or "no light passing" because the area is darker and light does not pass as freely through it.  The light does not pass as freely because the A band contains both the  thick filament, myosin, and the thin filament, actin.  The H zone is the zone that contains only myosin filaments, and is located midway through the A band.

During skeletal muscle contraction the Z lines become closer together and the I band shortens.  The H zone also shortens until it eventually disappears completely.  The A band stays the same length. 

 

 

 
 
 

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