Sarcomeres - The Basic Contractile Unit

 
Each muscle fiber has hundreds of sarcomeres.  Sarcomeres are the basic contractile unit of the muscle.  The length of one sarcomere extends from Z line to Z line.  Z lines are often referred to as Z discs.  In addition, Z lines provide stability by attaching to the sarcolemma, the membrane surrounding the muscle fiber.

Banding patterns are created by the individual components of the sarcomere.  This primarily includes myosin (thick filaments), actin (thin filaments), and Z lines.  Usually, banding patterns are separated into dark and light areas.  This can be seen in the picture on the left. 

The A band represents the length of the myosin.  This band includes the overlapping actin.  The I band is the region that contains only actin filaments.  Along the same idea, the H zone is in the center of the A band and is made up of purely myosin

During muscle contractions, the distance between Z lines is decreased.  Also, the A band remains the same length, I bands shorten, and the H zone disappears.
Image courtesy of John Kimball - http://www.ultranet.com/~jkimball/BiologyPages/M/Muscles.html

 
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