140 Muscles /8 : 2 



is surrounded by a special membrane called the sarcolemma. (The 

 prefixes myo- and sarco- both are used widely to identify muscle and 

 muscle-like structures.) The organelles include small bodies associated 

 with oxidative mechanisms known as mitochondria, as well as many 

 nuclei. Thus, one may regard the striated muscle fiber as a single, 

 polynuclear cell, but the entire concept of cell becomes rather meaning- 

 less in this connection. 



Three types of striated muscles are known : ( 1 ) the skeletal muscles 

 which form long, unbranched fibers with the nuclei distributed just 

 inside the outer edge of the fiber, (2) special muscles of the face and head 

 region, which are made up of branched fibers with cell nuclei located 

 just inside the outer edge of the fiber, and (3) cardiac muscle in which 

 the nuclei are at the center of the fiber cross section and in which all of 

 the fibers branch to such an extent that very few ends can be found. In 

 addition, cardiac muscle has intercalated discs which occur between the 

 cell nuclei and divide the fibers into units resembling cells. This 

 chapter emphasizes vertebrate skeletal muscles. Chapter 9 describes 

 various aspects of the action of cardiac muscle. 



As can be seen in Figure 1 , there are a number of bands present along 

 the striated muscle fiber. They are common to all striated muscles. 

 The bands which stain dark are also birefringent ; that is, they split 

 unpolarized light into two beams. Any such substance also transmits 

 light at a velocity which depends on the angle between the plane of 

 polarization and the fiber axis. This birefringence is believed to be due 

 to the lining up of large protein macromolecules, but the exact molecular 

 basis is not well understood in the muscle striations. The birefringent 

 bands are labeled A, for anisotropic, that is, index of refraction depends 

 on direction of the incident light. 



By contrast, the less heavily stained bands have no polarizing pro- 

 perties. They are labeled /, for isotropic. In many ways, the / bands 

 are harder to understand than the A bands, for it is believed that the 

 protein molecules are oriented in both. 



In the center of the / band is a darker staining disc called the Z disc. 

 In the center of the A band is a lighter staining region called the H zone. 

 Because the cell concept is not too helpful in discussing muscle fibers, 

 the repeating unit is called a sarcomere. It is chosen to run from one Z 

 disc to the next. A sarcomere may include no nuclei, or one, or even 

 more than one ; it is in no sense of the word a cell. 



Vertebrate muscles which are not striated are called smooth because 

 they are not made up of bundles of small groups of fibers. Smooth 

 muscles, by contrast to striated ones, consist of short spindle-shaped 

 cells of isotropic material. The cells usually are 15-20 /z long, though 

 some reach a length of 500 \i. A diagram of a typical smooth muscle 



