8 : 5/ Muscles 151 



other forms of chemical energy are more clearly understood. This 

 process is a result of the oxidation of many substrates, most of the free 

 energy liberated being used to form ATP. Figure 7 shows several of 

 the major groups of steps in the use of glucose to form ATP. In the 

 absence of oxygen, or in the presence of limited amounts of oxygen, the 

 process stops at lactic acid, as is the case in an active muscle. After 

 activity, the muscle slowly oxidizes the lactic acid the rest of the way to 

 C0 2 and water. These processes are not unique to muscle but occur in 

 all vertebrate cells. 



Another important compound in muscles is creatine. Just as ADP can 

 be phosphorylated to store energy, creatine can be made to store energy 

 in the form of a phosphate compound, creatine phosphate. In the muscle, 

 there is a dynamic balance between the creatine-creatine phosphate 

 system and the ATP-ADP system. Thus, creatine-phosphate acts as a 

 storage depot whose energy can be utilized about as readily as that of 

 ATP. 



Chemical studies have revealed many of the basic energy trans- 

 formations that accompany the changes from relaxed-muscle + glucose 

 + oxygen to contracted-muscle + C0 2 + water. Inherently, how- 

 ever, these methods cannot describe the molecular details of the actual 

 mechanical changes which occur in the active muscle. 



5. Electron-Microscope Studies of Muscles 



In Section 2, the structure of striated muscles was discussed. In the 

 present section, this discussion will be further amplified to include 

 observations made by electron microscopy and by X-ray diffraction. 

 As was noted earlier, each striated muscle can be broken down into 

 large bundles of small groups of single muscle fibers. Each muscle fiber 

 is some 10-100 /x in diameter and is very long, perhaps as long as the 

 entire muscle. The muscle fiber contains nuclei, mitochondria, and 

 other formed elements as well as myofibrils. 



The myofibrils are about 1 /x in diameter and may have lengths 

 comparable to that of the entire muscle fiber. Each myofibril is 

 striated with the same bands as the entire muscle fiber. The myofibrils 

 consist of units similar to that shown in Figure 8 which start with the 

 Z disc or membrane and contain one-half of an / band, an A band with 

 a H zone in the middle, one-half of the next / band, and then another 

 Z disc. 



Electron-microscope techniques have shown that the myofibrils, in 

 turn, are made up of smaller filaments of two types, thick and thin. The 

 thick ones are about 100 A (that is, 0.01 fx) in diameter and about 2 ft 



