96 



HANDBOOK OF PHVSIOLOGV -^ NEUROPHYSKJLOGV I 



RECTANGULAR CURRENT PULSES 



SLOWLY INCREASING CURRENT PULSES 



5 msec 



FIG. 17. Upper portion: Stimulation of a squid giant axon by rectangular current pulses applied 

 through a long intracellular metal electrode. The membrane potential was recorded with another 

 intracellular electrode. Stimulus durations used are indicated by the bars in the records. Lower por- 

 tion: Stimulation of a squid giant axon by slowly rising current pulses. The time courses of the current 

 pulses used are indicated by the broken lines. [From S. Hagiwara et al., unpublished.] 



were applied through one of the internal wire elec- 

 trodes and the change in the membrane potential 

 was recorded with the other electrode. Under these 

 experimental conditions, the axon memiarane is 

 traversed by the applied current uniformly over the 

 whole area under investigation. The intensity of the 

 stimulating pulses was adjusted to the threshold at 

 every stimulus duration. 



It is seen in the figure that the threshold membrane 

 potential defined as the highest subthreshold level of 

 the membrane potential is approximately constant 

 (within about 5 per cent), irrespective of the stimulus 

 duration. As in the nodal membrane of the toad 

 myelinated nerve fiber, the decay of the membrane 

 potential in barely subthreshold stimulation is ex- 

 tremely variable. In response to long current pulses 

 (see record £)), however, a phenomenon we have not 

 discussed before is seen. A barely subthreshold, long 

 current pul.se sets up an approximately exponential 

 change at the beginning; later, in spite of maintained 

 flow of the constant current, the memijrane potential 

 is found to fall gradually. This is the behavior of the 

 membrane associated with the phenomenon classi- 

 cally known as ' accomodation' [see Erlanger & Blair 

 (27)]. In the nodal membrane, the process of accom- 

 modation progresses more slowly than in the squid 

 axon and is not apparent in figure 16. 



It has been known for many decades (79) that a 

 slowly increasing current fails to excite a nerve fiber 

 even when its intensity rises well abo\e the rheobase.^ 

 Evidently, this phenomenon is related to the ' accom- 

 modative fall in the membrane potential' just men- 

 tioned. This point is illustrated by the records in 

 the lower part of figure i 7. When the rate of current 

 increase is greater than a certain critical \alue, a 

 full-sized action potential starts when the membrane 

 potential reaches the threshold level. When the 

 membrane current rises .slower than the critical 

 rate, the potential begins to fall while the current 

 intensity is increasing. Once such an accommodative 

 fall in the membrane potential has taken place, the 

 potential can rise well above the ordinary threshold 

 le\el without initiating an action potential. 



Now, let us turn to the corresponding obsersation 

 on the toad myelinated nerve fiber. Figure 18 shows 

 the beha\ior of the nodal membrane in threshold 

 stimulation bv linearlv rising \oltage pulses. The 

 experimental arrangement used is the same as that 

 used in the experiment of figure 16. Since there is a 

 high ohmic resistance in the axis-cylinder between 



" This is the threshold for a long rectangular pulse. For 

 pulses longer than 5 msec, the threshold is practically in- 

 dependent of duration. 



