REFRACTORY PERIOD 



259 



is known as the absolute refractory period. Then follows 

 the relative refractory period, during which the excitability 

 is steadily rising to the normal. This is succeeded by a 

 period of heightened excitabihty, the supernormal phase. 

 It will be noted that the refractory period of nerve differs 

 from that of muscle in being much shorter. 



It is now known that these three phases in the change 



< 



ex! 

 o 



z 





o 

 x: 



uJ 



100- 



50- 



-^ — i—, — ■ > I 

 •01 -oz 



TIME IN SECONDS AFTER. 

 FIR5T STIMULUS 



■03 



Fig. 38. — Diagram (after Adrian and Lucas) to show recovery of excit- 

 ability after the passage of an impulse : A, absolute refractory 

 period ; B, relative refractory period ; C, supernormal phase. 



in excitabihty are accompanied by corresponding changes 

 in conductivity. 



It follows as a corollary from this that if a nerve is 

 stimulated while it is in the supernormal phase due to 

 a previous stimulus, the initiation and propagation of the 

 second impulse will be facihtated — the disturbance will be 

 greater and will travel more rapidly than it would had 

 there been no previous stimulus. If two stimuh, each of 

 which acting alone would be ineffective, are sent into a 

 nerve such that the second enters while the nerve is in 



