Exercise XVIII 



THE NERVE IMPULSE 93 



Analysis of a diphasic 

 action potential 



+ 



-»- Time 



^ 



Voltage-measuring 



m: 



Active 

 region 



Nerve 

 fiber 



b 



U 



o- 



Wm>M/M'^y/^y,.ymmm. 



weak response. At each point, however, the gun- 

 powder will give everything it has. That is, the 

 response will vary in strength from point to 

 point with the amount of gunpowder, but at 

 each point it will be all or nothing. In the same 

 way, and for much the same reasons, the re- 

 sponse of a nerve fiber is independent of the 

 strength of the stimulus, provided it is strong 

 enough to excite at all; and is also all or nothing. 

 We measure nerve response by placing two 

 electrodes on the nerve and connecting them 

 through a sufficiently sensitive recording device. 

 The quantity measured is the potential. (The 

 nerve membrane has a very high resistance, so 

 that the current flow is very small.) The two elec- 

 trodes, touching the outside of the fiber, nor- 

 mally have zero potential between them. If we 

 now excite the fiber, as the nerve impulse comes 

 under the first electrode that electrode becomes 



zm. 



negative to the other electrode. The instrument 

 records that negativity. As the impulse sweeps 

 on, it may come to involve equally both elec- 

 trodes. In that case there is momentarily again 

 no difference of potential between the two elec- 

 trodes. Then the nerve impulse has passed the 

 first electrode and involves only the second, 

 which now momentarily becomes negative to the 

 first; the recorded potential is now just opposite 

 to what it was before. Then the impulse is past, 

 and again, as at rest, there is zero potential be- 

 tween the electrodes. 



The result is a so-called diphasic change of po- 

 tential : the potential starts at zero, sweeps up to 

 a maximum in one polarity, comes down again 

 through zero to a maximum in the opposite 

 polarity, and returns to zero. These changes 

 should be obvious from the accompanying dia- 

 gram. 



