EXCITATION AND CONDUCTION 59 



Excitability. The excitability of the nerve fiber has 

 three criteria: (i) the degree of irritability, i.e., the 

 ease with which it can be stimulated; (2) velocity of 

 the nerve impulse, i.e., the speed with which the state 

 of excitation travels from one point to another; (3) 

 the direction of the nerve impulse. All nerves are 

 classified into two general functional types: efferent 

 and afferent, the former conducting away from the 

 nerve center (brain, etc.), the latter toward the center. 

 We shall consider somewhat in detail in this chapter 

 what relation the metabolic condition bears to these 

 three phenomena in the nerve. 



Degree of excitability. Not all nerves can be stimu- 

 lated equally well by the same strength of stimulus. 

 The threshold value the minimum strength of stimulus 

 which can call forth functional activity is different 

 in different nerves. Not only have the different nerves 

 different degrees of excitability, but the same nerve can 

 be made excitable in different degrees under a variety of 

 conditions. If we study metabolic activity in nerves 

 under different conditions which we know affect the 

 state of excitability, we find that there is a very close 

 relation between metabolism and excitability. 



a) If the sciatic nerve is removed from a frog, it 

 exhibits electrical phenomena for many* hours. Since 

 electrical changes are characteristic of living nerves 

 only, we consider that the isolated nerve does not die for 

 many hours. Such a nerve, although it shows large 

 electrical responses, is nevertheless less excitable than a 

 fresh one. If measurements are made on an isolated 

 nerve at successive time intervals for many hours, we 

 find that the carbon dioxide production steadily dimin- 



