Exam 3 Review:  Chapter 09:  Sarcomere Appearance

sarcomere - One of the segments into which a myofibril of striated muscle is divided; "the unit of contraction;" the sarcomere has dark a central A band split in the middle by the lighter H zone which contains the M line and, at each end, has one half of a lighter I band which is split by the Z line; the borders of the sarcomere are the Z lines.

Z disc = Z line - The lateral boundaries of a sarcomere; a dark line splitting the I band which appears dark because it contains a meshwork of fibrous proteins which serve as the anchoring points for actin = thin filaments.

A band - The repeating dark area in the banding pattern observed in striated muscle when examined by a microscope; the A band is darker because within the A band both myosin and actin fibers are present and block the passage of much light; the center of each A band is marked by a lighter H zone.

I band - The repeating lighter area in the banding pattern observed in striated muscle when examined by a microscope; the I band is lighter because within the I band only actin fibers are present and they block the passage of only a small amount of light; the center of each I band is marked by a darker Z line.

H zone - The thin pale stripe or light area in the center of each A band observed in striated muscle when examined by a microscope; the H zone is lighter because within the H zone only myosin and no actin fibers are present block the passage of light; the center of each H zone is marked by a darker M line.

M line - The center of a sarcomere; a dark line splitting the H zone within th A band which appears dark because it contains a meshwork of fibrous proteins which serve as the anchoring points for myosin = thick filaments.

Sketch and Label:

4. A sarcomere (including the various protein components).

Below is the appearance of the sarcomere in the electron microscope.  The second image is at a somewhat higher magnification and includes a diagram which identifies the components of the sarcomere.  The M symbols in the first picture identify mitochondria.