510 ANIMAL BIOCHEMISTRY 



and in the muscles of the heart, intestinal, and respiratory systems. 

 Since little of this energy produces physical displacements of either 

 the individual or parts of the environment, most of it is converted to 

 heat and lost as such. The increments beyond this minimum level 

 of activity are used to overcome gravity, move from place to place, or 

 transport objects. Again much of the energy appears as body heat but 

 some is converted into potential energy of position. 



Intense emotion may increase the expenditure of energy by 5 to 10 

 per cent above the basal level, a relatively small contribution when 

 compared with exercise. Profound mental activity has an even lower 

 energy requirement, making it unnecessary to allot anything in the 

 diet as food for thought. 



Turning to the operation of muscle, agreement has not been 

 reached on the mechanism of muscle action, although some general 

 requirements are well established. In the first place, high-energy 

 phosphates participate as intermediates in the transfer of chemical 

 energy from metabolites to muscle. A limited supply of creatine phos- 

 phate (phosphocreatine) acts as an energy reserve, making available 

 the adenosine triphosphate (ATP) which functions as the actual 

 energy carrier. The creatine phosphate is formed by energy transfers, 

 starting with glycogen. Thus muscle glycogen becomes the major 

 starting point for muscular energy. 



Nerve impulses induce muscular contraction by splitting ATP into 

 ADP and phosphate and converting the chemical energy thus made 

 available into mechanical energy. The exact nature and sequence of 

 events are still under debate. The higher animals are about 40 per 

 cent muscle, which in turn is mainly water and protein. Some of the 

 proteins are dissolved in the cellular fluid, while two others called 

 actin and myosin make up the contractile fibers. Myosin and actin 

 comprise respectively about 38 per cent and 14 per cent of the proteins 

 of rabbit muscle. 



One theory postulates that contraction involves the shortening of 

 actomyosin, which is a complex of about 3 parts of myosin with 1 of 

 actin. Since muscular contraction is very rapid, fast reactions are re- 

 quired and these are visualized as ionic changes in electrical potential 

 near the surface of the actomyosin complex. As a result, attractions 

 between unlike charges on the actomyosin cause a shortening of the 

 complex which is muscle contraction. This process is represented 

 schematically in Figures 13-1 and 13-2 with the fixed charges of the 

 protein surrounded by ion atmospheres shown as ions of ATP and 

 potassium. 



It is proposed that stimulation by a nerve alters the ion atmosphere 



