98 INTRODUCTION TO GENERAL PHYSIOLOGY 



Refractory Period 



A word must be said next about another important property 

 of muscle and other excitable tissues. If a second stimulus arrives 

 at a brief interval, a fraction of a second, after a previous one, the 

 second stimulus produces no effect. This interval of time during 

 which the muscle is inaccessible to stimulation is known as the 

 " refractory period," and is exhibited while the muscle is in the 

 initial stages of giving effect to the first stimulus (E., p. 208). If 

 we remember the evidence that an essential part of the process 

 of excitation consists in an increase of permeability of the cell 

 membrane, we see that it cannot be repeated until the membrane 

 has recovered its normal state of semi-permeability. 



" Staircase " 



Another interesting phenomenon is that of the " staircase." If 

 a muscle has been at rest for some time, it will be found that the 

 strength of the contraction increases for each successive stimulus 

 during a few contractions (E., p. 207). It appears that a certain 

 very small degree of acidity is that best adapted for maximum 

 contraction. As we saw, lactic acid is produced in contraction, and 

 a trace is left after each contraction, gradually increasing until it is 

 oxidised as fast as it is formed. 



Tetanic and Voluntary Contraction 



The simple form of contraction which follows a single electrical 

 shock lasts an appreciable time, varying with the particular muscle 

 in question. In the frog it lasts about a tenth of a second. Now 

 the refractory period referred to above lasts only something over 

 a thousandth of a second. If, therefore, a second stimulus arrives 

 later than this, but during the time in which the muscle is shorter 

 than at rest, a further shortening takes place, and another stimulus 

 and shortening may be superposed on this. Each succeeding 

 stimulus after the first, however, has somewhat less effect on the 

 length of the muscle than the one before it, so that, after a certain 

 number, the height becomes practically steady, but much higher 

 than that produced by a single stimulus (E., p. 206). This state is 

 known as a "tetanic" contraction, and is similar to that resulting 

 from a normal discharge from the nervous system, which consists 

 of a series of stimuli, varying in number according to the length of 

 time that the muscle is required to remain in contraction. 



