Muscle and Electric Organs 621 



All evidence agrees, therefore, that, like the end-plate potential, the dis- 

 charge of an electric organ is associated with the liberation of acetylcholine. 



Electric tissue contains much phosphocreatine, -■^' ^^ which diminishes 

 after prolonged activity. An old study" of heat utilization by the electric 

 organ in activity should be repeated with modern thermopiles. The voltage 

 output of Malo'pterus is reported to have a temperature optimum at 19° C. 



The electric organ of fishes has long been an object of wonder and curi- 

 osity; it is now most useful in leading to an understanding of the physico- 

 chemical processes in the nervous activation of muscle. 



CONCLUSIONS 



There are many kinds of muscles, functionally and structurally. Muscles 

 can be arranged in orderly fashion according to physiological and histologi- 

 cal properties. 



Speed— fast contraction and relaxation, high fusion frequency, short ex- 

 citation constant— is associated with sharp transverse striations and long 

 fibers, with sparse sarcoplasm and abundant fibrils. Conduction is slow and 

 electrical activity complex in sheets of short fibers, particularly when the fibers 

 are branched, or connected by protoplasmic bridges. Local responses, both 

 electrical and mechanical, may be one link in the excitation of fast muscle, 

 but become the dominant response when innervation is multiple. Ability 

 to maintain tension is associated with an abundance of connective tissue or 

 with a "set" in the pattern of contractile elements. Increase in tension with 

 increase in frequency of stimulation reflects facilitation of neuromuscular 

 excitation as well as facilitation of mechanical properties. There are dif- 

 ferences among muscles in the agents of neuromuscular transmission and 

 in the phosphagens. 



The mechanical properties of muscles make possible their gross function. 

 Correlations of physiological and histological properties of muscle with 

 phylogeny are few, and within an individual animal a number of muscle 

 types exist. Within one muscle there may be fast and slow portions. 



Comparison of the forms of phosphagen, one of the principal sources of 

 energy in muscle, supports the echinoderm theory of chordate origin, and 

 suggests that annelids may be farther phylogenetically from arthropods, and 

 tunicates farther from true chordatei than is sometimes supposed. 



Electric organs are modified neuromuscular junctions, greatly enlarged 

 and altered so that the production of action potentials is their principal 

 function. 



Study of the comparative physiology of muscle can be most useful in a 

 correlation of the structure— histological and molecular— with the functional 

 properties of diflFerent kinds of muscle; this approach should help to explain 

 the fundamental nature of contractile tissues. 



REFERENCES 



1. AcHEsoN, G. H., Fed. Proc. 7:447-457 (1948). Chemical aspects of neuro- 

 muscular transmission. 



2. Alvarez, W. C, and Mahoney, L. J., Am.J. Physiol. 59:421-430 (1922). Myo- 

 genic movements of intestine. 



3. Ambache,N., J. Physiol. 104:266-287(1946). Effect of ions on intestine. 



