CONDUCTION OF THE NERVE IMPULSE 



89 



TABLE I . Resistances and Capacities nf the Myelin Sheath, the Squid Axon and the Nodal Membrane 



farad/cm 



farad/cm* 



ohn 



ohm-cm^ 



Myelin sheath (fiber diameter 12 fi) 

 Squid giant axon (diameter 500 ii) 

 Nodal membrane 



.6 X lo-i 

 .6 X 10-" 

 I -5 y-t^^ 



5 X 10-9 



ID"* 



(3-7) X lo-s 



2.9 X 10' 

 (6-15) X 10' 



41 Mn* 



10* 



(1-2.5) X io» 

 8-20 



Data from references (20, 61, 125). 



* Values for one whole node of Ranvier of the toad motor ner\e fiber. 



figure iii5, alter treatint; this node with a sodium- 

 free Ringer's solution or with a dilute cocaine- 

 Ringer's solution. The details of the principle of the 

 method can be found elsewhere (125). Since it is 

 difficult to estimate the area of the nodal membrane, 

 the figures for a unit area of the nodal memijrane are 

 somewhat inaccurate. For comparison, the membrane 

 constants of the squid giant axon are also listed in the 

 same table. 



It is interesting to note that the capacity of ijoth 

 the myeHn sheath and of the nodal membrane is ex- 

 tremely insensitive to changes in the temperature and 

 the chemical composition of the surrounding fluid 

 medium, while their resistance can be strongly modi- 

 fied by slight changes in the environinent C'-4> 125). 

 There is, however, one siinple way of increasing the 

 capacity of the inyelin sheath, that is, by dissolving 

 the fatty substance of the myelin sheath by an appli- 

 cation of a saponin-Ringer's solution or some other 

 detergent solution. During the early stage of a 

 saponin treatment of the myelin sheath, the capacity 

 increases as the resistance decreases, the product 

 c,„r,„ remaining almost unchanged. This fact strongly 

 suggests that the capacity of the inyelin sheath is 

 dielectric in nature, determined by the thickness of 

 the sheath and the dielectric constant of the myelin 

 substance. The dielectric constant of the myelin sheath 

 is known to be similar to that of many other fatty 

 compounds (66, 125). 



CONDUCT.\NCE OF THE MEMBR.ANE DURING .ACTIVITY 



VVe have seen in the preceding section that the 

 development of the action potential represents a tran- 

 sient variation in the potential difference across the 

 surface membrane of the nerve fiber. In 1939, Cole & 

 Curtis C19) demonstrated in the .squid giant axon 

 that this variation in the meinbrane potential is asso- 

 ciated with a pronounced change in the resistance of 

 the membrane. Tasaki & Mizuguchi C'SS) showed a 

 similar change in the membrane at the node of Ran- 



\ier. We shall discuss the principle of measuring the 

 membrane impedance during activity under relatively 

 simple experimental conditions. The method to he 

 described is slightly different from that employed by 

 Cole & Courtis but the principle is the same. 



In the arrangement shown in the upper part of 

 figure 12, a long silver wire electrode about 100 ^i in 

 diameter is thrust into a squid a.xon immersed in sea 

 water This internal electrode and a large electrode 

 immersed in sea water surrounding the a.xon are 

 connected to one arm of an alternatino current 



FIG. \i. Measurement of the membrane impedance of a 

 squid giant axon during activity with an a.c. impedance 

 bridge. The bridge was balanced for the impedance of the 

 resting membrane. The two records on the left were taken at 

 nearly the same stimulus intensity, but the bridge output was 

 amplified 10 times the normal (ix) in the lower record. The 

 upper trace in the records displays the unfiltered bridge out- 

 put; the potentials recorded are slightly reduced and distorted 

 by the bridge. The bridge a.c, 20 kc per sec; temperature, 

 22 °C. (Further discussion in text.} 



