72 PHYSIOLOGY OF MUSCLE AND NERVE 



other words, the destruction of the myoplasmic material must first be 

 made good by anabolic changes before the muscle can again respond. 

 This period during which the muscle remains inexcitable to a second 

 stimulus, is known as the refractory period. Its duration is only about 

 0.0015 sec. Thus, a muscle is in a position to react to stimuli only if 



Fig. 38. — Summation of Contractions. 

 M and B, make and break shocks indicated by an electro-magnetic signal. Time 

 in Moo sec. As the break contraction occurs during the period of relaxation of the 

 make contraction, it is added to the first. 



they recur with a lesser frequency than one in every 0.0015 sec. If their 

 rate is increased beyond this limit, not every stimulus will be capable 

 of producing a reaction. As will be shown later, the refractory period 

 is of especial functional significance in the case of cardiac muscle. 



Fig. 39. — Fusion and Tetanus. 

 S, summation; F, fusion; T, tetanus. Time in seconds. The individual make and 

 break shocks are repeated so quickly that a continuous contraction is obtained. 



Tetanus. — A tetanic contraction of muscle exhibits a greater 

 height and length than the simple twitch. It must be evident from the 

 preceding discussion that a tetanus is really composed of a multi- 

 tude of single contractions which have been fused into a continuous- 

 curve by permitting the stimuli to enter the muscle at very brief in- 



