282 BIOPHYSICAL STUDIES ON NERVE AND MUSCLE 



the rate at which the physical process occurs. Thus the faster the process the 

 less efficient it is: i.e., the greater the fraction q' cx /(A^' + q' ex ) which is 

 lost as heat, and the less is the fraction A$7'/(A;F' + q' ex ) which is realized 

 as external work. However, whether the work is done fast or slowly, nu- 

 merically the same amount of work is done; and therefore, because Ao' is 

 independent of speed, so must q' tx be, provided the same amount of fuel is 

 consumed. It is a well-established experimental finding that the total heat, 

 £•) ', given out during a shortening (the "contraction heat ,, ) 



is constant, independent of speed of shortening. However, ~/ ', is propor- 

 tional to the distance (a) shortened; i.e., °c ax; and the constant a has the 

 same dimensions (energy/distance, or force) and numerical value (~400 g 

 wt/cm 2 area of cross section) as the a in the force-velocity relationship. The 

 significance of this coincidence is not yet clear. 



Careful measurements, with small thermocouples imbedded in the muscle 

 and fast galvanometers to record small electrical currents, have shown that 

 the contraction heat is composed of two parts: a rapid initial surge follow- 

 ing stimulation, and completed before contraction starts; and then the con- 

 traction heat proper. The first has been called the heat of activation, .4, by 

 analogy with the terminology of the threshold in chemical kinetics. There- 

 fore q' ex can be written as 



q' ex = A + ax 



in Hill's terminology, the first term being the activation heat and the second 

 the contraction heat proper. + 



Discussion of the production of enthalpy, JC , by biochemical reactions in 

 muscle is beyond our scope in this book. A few notes suffice. Muscle glyco- 

 gen is the primary fuel, being oxidized to lactic acid with — A// = 16.2 

 Cal/mole of lactic acid produced. This energy is used in the synthesis of 

 creatine phosphate (CP) which acts as a secondary fuel. Both glycogen and 

 creatine phosphate supply free energy for the synthesis of adenosine tri- 

 phosphate (ATP), the hydrolysis of which is the immediate source of free 

 energy for the physical work of contraction. Regeneration of the hydrolysis 

 product, the diphosphate (ADP) is effected by reaction of ADP with CP — 

 the famous Lohmann reaction. The enzyme myosin, which has the con- 

 tractile property, adsorbs ATP and catalyses its hydrolysis. 



+ In Hill's terms (Ref. 23, for example) the extra metabolic energy involved in contraction is 

 composed of three parts: the work done (AJP), the activation heat (.-J), and the heat of short- 

 ening (ax). The total energy will include q' bm and T A S ■ 



