82 



The Conduction of Impulses by Nerves /4 : 4 



a comparative point of view, it is a huge axon. It is possible to shove all 

 sorts of electrodes and shafts inside this axon. Experiments with squid 

 axons confirm that the resting potential and the spike potential depend 

 only on the membrane, not on the bulk of the axoplasm. This is in 

 accord with the charge distribution shown in Figures 2 and 4. Similar 

 experiments have % shown that this pattern is valid also for all other 

 axons, for muscle fibers, and for many long algal cells. 



The local response has the same form as the spike potential shown in 

 Figures 4 and 7a. A small depolarization applied externally results in a 

 flow of ions, so that a greater depolarization of the membrane occurs. 



Subthreshold Stimulus 



Produces Local 



Response 



Two Subthreshold 

 Stimuli Add to 



Trigger Conducted 

 Spike Potential 



Second Response 

 Inhibited by First 

 for Greater Time 

 Between 



Figure 7. Temporal summation of subthreshold responses. 



Figure 7b shows a second subthreshold stimulus following closely after a 

 first one. These add and give rise to a conducted spike potential. 

 Figure 7c illustrates two stimuli slightly further apart in time. In this 

 case, the first local response inhibits the second one. 



Both the local responses and the conducted spike potential involve a 

 flow of ions. In the initial or resting condition, the K + concentration 

 inside the fiber is greater than that outside and the Na + less than that 

 outside. The ions are maintained with this distribution at the expense 

 of metabolic energy. The spike potential does not merely result in a 

 depolarization of the axon membrane, since the potential actually 

 reverses in sign. Rather, measurements of the Cv ">lex impedance Z, 

 per unit surface area, have shown that ionic condui tion increases both 

 during the regenerative phase of the spike and agai.x during the recovery. 



Tracer experiments and others described in Chapter 24 have shown 

 that Na + flows into the axon during the regenerative phase and K + flows 

 out during the recovery phase. This may be summarized by the 



