BRINK AND OTHERS: CHEMICAL EXCITATION OF NERVE 481 



When part of a squid giant fiber is deprived of calcium ions, trains 

 of impulses are initiated in this region and conducted to the recording 

 electrodes. The nerve behaves, in' this respect, exactly like the 

 myelinated nerve of a frog, but here conditions are more favorable for 

 recording the difference of potential between the chemically excited 

 region of the fiber and a remote portion. When this is done, it is found 

 that the conducted impulses are preceded by a series of local periodic 

 potential changes of variable amplitude and relatively constant fre- 

 quency^^ (figure 21). The spacing between adjacent peaks of the 



Figure 21. Local electrical response, recorded from a chemically excited region of giant axon 

 of squid, is oscillatory and precedes the conducted impulses. The last ten oscillations on the right 

 of the record initiated propagated impulses, which are much larger in amplitude than shown. 

 Stimulation by topical application of isotonic sodium chloride. 



local response, just before the conducted impulses appear, is the same 

 as the spacing between the conducted impulses. It is obvious, as 

 Arvanitaki concluded,^^ that the frequency of conducted impulses along 

 the giant axon is determined by the frequency of the local excitatory 

 process. 



If relatively little calcium is removed from the nerve, local periodic 

 electric changes may be observed which do not initiate propagated im- 

 pulses. The local process is an essential part of the excitatory mecha- 

 nism, but the cyclic changes initiate impulses only when a given cycle 

 is of sufficient magnitude. Furthermore, the frequency of the local 

 process is essentially independent of whether or not a conducted impulse 

 is initiated by each cycle. 



Figure 22 gives the frequencies of conducted impulses observed in 

 nerves treated with solutions containing different concentrations of Ca"^, 

 or with sodium citrate. The frequencies vary from 250 to 400 cycles 

 per sec. This range is comparable to that which is characteristic of the 

 undamped natural frequency of the nerve membrane, as calculated by 

 Cole and Baker^^ from impedance measurements on squid nerve. This 



