138 Muscles /8 :2 



the fields of active research except for experiments on specialized topics 

 such as body resonances and tissue elasticity. These topics are part of 

 biophysics (although they are not described in this text) . 



Starting some time in the 1920's, muscles were studied as biochemical 

 complexes. At the same time, biophysicists related the heat changes 

 which occurred in muscles to a mixture of chemical and mechanical 

 effects. These studies markedly influenced the direction of biochemical 

 research as a whole and still form part of the basis for current models of 

 oxidative mechanisms in protoplasm. 



A slight refinement in the above-mentioned biochemical and thermal 

 studies involves the use of extraction techniques. The muscles are 

 ground up; certain compounds, for example, myosin, are extracted and 

 purified; and then their properties are studied. It is believed that the 

 nature of the contractile process should be related to the properties of 

 the chemical constituents of muscles. 



Recent advances in research on the contractile process in muscles have 

 come about through the use of highly specialized physical instrumenta- 

 tion and by the introduction of the ideas and concepts of molecular 

 structure and form. Thus, muscle studies are increasingly falling within 

 the scope of biophysics and biophysical chemistry. For example, the 

 enzyme reactions and the optical density changes in living muscle have 

 been followed by using specially constructed spectrophotometers. Like- 

 wise, microelectrode techniques have made it possible to observe the 

 magnitude and form of the electrical surface potentials, as well as the 

 action potential spikes which precede contraction. Perhaps most 

 important of all, a special physical tool, the electron microscope, has 

 been used to extend the range of observation to smaller size pieces of 

 muscle than can be seen with the light microscope. The interpretation 

 of electron micrographs of muscles has dramatically altered the accept- 

 able models of muscular contraction, at the same time emphasizing the 

 need for further studies of protein structure before muscular contraction 

 can be understood on a molecular level. 



2. Anatomy 



Muscles are found in all of the more advanced animals, both invertebrate 

 and vertebrate. All are transducers converting chemical energy into 

 electrical energy, heat energy, and useful mechanical energy. Muscles 

 appear in a variety of sizes and shapes ; they differ in the forces they can 

 exert and in their speed of action. In this chapter, only vertebrate 

 muscles will be discussed. 



Anatomically, muscles can be classified in many ways, in terms of 



