148 Muscles /8 : 4 



the myofibril, than any other type of molecule. Present theories do not 

 assign any specific role to these water molecules, aside from forming a 

 medium through which the contractile molecules act and also through 

 which the energy-carrying molecules diffuse. The various organelles 

 within the muscle, for example, nuclei and mitochondria, have the same 

 composition as those of other cells. The ionic concentration within 

 the muscle fibers is similar to that within nerve fibers described in 

 Chapter 4. The one unique component, outside of the myofibrils, is 

 the protein myoglobin. This is a red pigment similar to the hemoglobin 

 of red blood cells except that myoglobin has about one-fourth the mole- 

 cular weight and only one iron atom per molecule (hemoglobin has four 

 iron atoms per molecule). Myoglobin is generally believed to act as a 

 storage for oxygen within the muscle fiber. 



The myofibrils contain unique molecules not found in other tissues. 

 Three proteins, myosin, actin, and tropomyosin, are all found in high 

 concentrations. All three are members of a general class of proteins 

 called globulins, when classified in terms of their solubilities. (Proteins 

 are condensation polymers formed from small monomers known as 

 amino acids. The structure of proteins, including those in muscle, is 

 discussed more fully in Chapter 15.) The actin is similar to many other 

 globulins in that it can exist in either a globular (sphere-like) form or a 

 fibrillar form. Small changes in the ionic strength, pH, or temperature 

 can convert some globulins reversibly from the fibrillar to the globular 

 form. (In the fibrillar form, they are believed to be arranged in a 

 helical structure described in Chapter 15.) 



The striking physical changes which take place as myosin and actin 

 shift from one form to the other suggest that they might be the molecules 

 actually responsible for contraction. Present evidence, discussed more 

 fully in Section 6, supports the conclusion that these three proteins form 

 the contractile elements. However, the premise that they change from 

 globular to fibrillar form appears to be completely fallacious. Rather, 

 it appears that myosin, actin, and tropomyosin are always in the fibrillar 

 form in intact muscles. They are formed into thin filaments, visible 

 only with the electron microscope. These filaments are believed to 

 develop the actual contractile forces. 



The proteins, myosin, actin, and tropomyosin are large molecules 

 organized into filaments that are long on an atomic scale. When the 

 myofibril contracts and relaxes, it uses chemical energy which is derived 

 from a much smaller molecule called adenosine triphosphate, or A TP. This 

 small molecule is the source of immediately available chemical energy 

 for chemical syntheses, for muscular contraction, and for the active 

 transport of ions and metabolites across cell membranes. A wide 

 variety of systems within all vertebrate cells can use ATP as a source of 



