VOL. 4 (1950) 



PERMEABILITY AND NERVE FUNCTION, II 



lOI 



complete within about 30 min. Attainment of the steady state is accompHshed when 

 all of the Na inside the nerve has been exchanged for Na^*. Under such conditions, 

 substituting in the permeability equation, the values of 0.162 M for Cg (Webb and 

 Young), 0.52 M for Q, 934.3 cts/min//:d for a^ (Table II) and 293.6 cts/min/^/.g for 

 a^ (Fig. 3) with t = 0.5 h and d = 0.05 cm, gives a value for the permeability constant 

 of 5.76-10-2 cm/h. 



TABLE II 



Na^* PENETRATION 



Nerves exposed to sea water containing 0.39 M Na^^Cl +0.13 Na^^Cl for var^'ing periods of time. 

 Standards (S^ and Sj) : sea water diluted 1:100 and then 0.4 ml evaporated in duplicate (2.1 micro- 

 moles/0.4 ml). Counts per min indicate the actual count, uncorrected for time decay of radioactivity. 



c: 6.0 

 6 5.0 



The degree of scattering appears to be slightly larger in the case of Na than of K. 

 This could, to some extent, be due to a slight contamination of the samples with radio- 

 active sea water since the sea water contained such a high concentration of radioactive 

 Na. Another factor may be the individual variations in Na content of these nerves. The 

 data of Steinbach and Spiegelman indicate that the values vary considerably from 

 one nerve to the next : 3 to 4 hour exposure of axons to sea water gave Na values varying 

 from 7.8 to 17.4 meq. per cent. No apparent effort was made in their work to determine 

 whether or not all of these nerves main- 

 tained conduction. It is, therefore, not 

 certain that such large deviations are 

 actually within the normal range of s 4.0 

 variation. Nevertheless, it is quite con- 

 ceivable that marked individual devia- 

 tions occur. 



The rates of penetration of Na into 

 Squid nerves are plotted against time 

 in Fig. 6. It will be noted that the initial 

 rate of penetration of Na into fibres is 

 extremely high but falls to a very low 

 level within 15 to 20 min. The rate of 

 penetration after 40 min of exposure 

 has fallen to a value about one twen- 



3.0 



2.0 



1.0 



15 



30 



60 



90 

 Min. of exposure 



Fig. 6. Rate of Na penetration across the mem- 

 brane of the giant of Squid when exposed to arti- 

 ficial sea water containing either 0.13 M or 0.065 ^I 

 Na^^Cl. Total NaCl concentration is 0.52 M. The 

 rate of penetration of Na^* in millimoles (mM)/ioo 

 g/min is plotted against time of exposure in min. 



References p. 114. 



