REFRACTORY PERIOD 41 



relaxed muscle and the muscle contracted isometrically 

 has been aptly compared with the difference between a 

 stretched spiral of lead and a stretched spiral of steel. Since 

 practically no work is done in an isometric contraction all 

 the energy hberated appears in the form of heat. 



As an example of an isometric contraction which occurs 

 physiologically we may take the contraction of the ventricle 

 before this chamber has begun to empty its contents into 

 the aorta. 



3. The Change in Excitability 



When a muscle has been stimulated to contraction there 

 occurs an interval of time during which it is incapable of 

 responding to a second stimulus. This is known as the 

 refractory period. In skeletal muscle the refractory period 

 is of shorter duration than the time occupied by a single 

 contraction. It is therefore too short to prevent the 

 fusion of repeated contractions into tetanus. In the case 

 of cardiac and unstriated muscle, on the other hand, the 

 refractory period outlasts contraction and relaxation com- 

 bined. It is upon the great length of the refractory period 

 in these types of muscle that the capacity for rhythmic 

 contraction depends, fusion into tetanus being impossible. 



4. The Chemical Changes Accompanying Contraction 



Dextrose fed to a beating heart disappears, but whether 

 it is oxidised or converted into glycogen we have no direct 

 evidence. Indirect evidence pointing to oxidation is 

 given by the rise which takes place in the respiratory 

 quotient (see p. 150). That the respiratory quotient is 

 subject to change according to the diet indicates that both 

 fat and carbohydrate can be oxidised. 



During contraction there is no increase in protein meta- 

 bolism. There is a change in the non-protein N. meta- 

 bohsm, but depending upon the form of contraction|and 

 the manner in which it is produced. The amount of 

 creatinine and purine bodies has been shown to be increased 



