454 MUSCLE-NERVE PHYSIOLOGY 



The amount of heat energy developed during a single contraction will vary 

 sharply according to the tension under which the muscle contracts. The heat 

 production follows closely the energy of work produced, and apparently 

 obeys the same laws. 



The Work Energy Liberated by a Simple Muscle Contraction. When 

 a muscle contracts against a resistance and a load is moved, work 

 energy is liberated. In fact the liberation of work energy and heat energy 

 are the specific functions of the muscles among the warm-blooded animals. 

 A frog's gastrocnemius weighing i gram and loaded with 50 grams will 

 contract from 0.5 to 0.6 cm, i.e., will do 25 to 30 gramcentimeters of work 

 for each simple contraction. The amount of work done is intimately associ- 



TABLE SHOWING THE RELATION BETWEEN LOAD AND WORK. 



Load or Tension. Height Lifted. Work Done. 



Grams. Centimeters. Gramcentimeters. 



o 1.2 O 



40 0.8 32 



80 0.5 40 



120 O.4 48 



160 0.2 32 



200 O.I 2O 



240 o.o o 



ated with the tension under which the muscle contracts. As the tension in- 

 creases from no load up to 100 or 150 grams (for a i-gram muscle), the work 

 increases. But as the tension continues to increase, the work "falls off until 

 a point is reached at which the load is not lifted at all. 



CONDITIONS WHICH AFFECT THE IRRITABILITY OF 



THE MUSCLE AND THE CHARACTER OF THE 



CONTRACTION. 



There are a number of conditions which influence both the irritability 

 of a muscle and the power and character of its contractions. Irritability 

 and contractility may vary independently, but as a rule any condition which 

 decreases the one also decreases the other. The most important of these 

 conditions are: relation of the muscle to the central nervous system, con- 

 dition of nutrition, stimulus, temperature, fatigue, drugs, disease, etc. 



Effect of the Strength of Stimulus. A strength of current that is 

 just sufficient to give the contraction of a muscle is called a minimal stimulus. 

 This is a comparatively weak induction current, one which can scarcely be 

 detected by the tip of the tongue. As the strength of the current is very 

 gradually increased,' the height of the contraction curve increases until the 

 maximal stimulus is reached, which produces a contraction of an amplitude 

 beyond which no increase occurs even though the strength of the stimulus be 



