NERVE-CONDUCTION IN DILUTED AND IN CENTRATED SEA-WATER.
Ring-shaped strips of subumbrella tissue of the scyphomedusa Cassiopea xamachana were deprived of marginal sense-organs and placed in concentrated sea-water in order to determine the effect of concentration of electrolytes upon their rate of nerve-conduction.
These results, combined with those previously obtained' with diluted sea-water, are as follows: The sea-water was concentrated by evaporation in sunlight, and its alkalinity reduced to that of normal sea-water by adding a slight amount of hydrochloric acid, the hydrogen-ion concentration being determined colorimetrically by comparison with a graded set of thymol sulphonephthalein tubes prepared and standardized by Professor J. F. McClendon. These readings were also checked by comparison with those of a Leeds and Northrup potentiometer.
The hydrogen-ion concentration of the sea-water at Tortugas is usually between 8.1 and 8.23 and many experiments made in 1917 showed that the rate of nerve-conduction remains practically constant and normal in sea-waters ranging from 7 to 8.65 P.; the relatively acid water is made from normal sea-water treated with HC1 and the alkaline water from normal sea-water treated with and If, however, the water is made as acid as 6.7 P., the rate falls slightly so as to be about 98, that in normal sea-water being 100; and at 6.3 P. the rate declines to 80 with greatly reduced muscular activity; but it recovers in all respects almost immediately upon being replaced in normal sea-water.
At 30° C. the neutral point is about 6.8 P. and it then appears that in sea-water any excess of over OH' is decidedly depressant, while correspondingly alkaline sea-water produces no ill effects.
Assuming the electrical conductivity of normal sea-water of 36.24 0/00 salinity and 8.22 P. to be 100, it appears from table I and figure 1 that the rate of nerve-conduction augments in practically a straight line as the relative electrical conductivity of the sea-water increases from 51 to about 108, above which the rate falls off rapidly, becoming about 77 in concentrated sea-water having an electrical conductivity of 152.9.
The formula is y = 0.945x +4.4 ; y is the rate of nerve-conduction, that in natural sea-water being 100; x is the electrical conductivity of the sea-water, that of normal sea-water being 100.
The electrical conductivity of the sea-water was determined by Kohlrausch's method, using a tunable telephone and a Leeds and Northrup revolving drum bridge. The figures given in table I are for sea-water at 30° C.
It was found that the electrical conductivity of sea-water of 36.24 0/00 salinity and 8.22 P., at 30° C. was 4.21 times that of N/10 KCl
at the same temperature. If, however, the sea-water be compared with this potassium-chloride solution, both at 25° C., the ratio of their electrical conductivities became as 1 to 4.179.
The salinity of the sea-water was determined by titration with silver nitrate, using potassium chromate as an indicator and titrating against a sample of standard sea-water from Professor Knildsen's laboratory.
The fact that the rate of nerve-conduction increases in a straight line ratio in comparison with the increase of electrical conductivity suggests a causal relation between the two, as was pointed out by Ralph S. Lillie, 1916,' but the same ratio exists between the concentration of the dissociated cations Na•, Ca", Mg", and K•, surrounding the nerves and the rate of nerve-conduction; and thus the rate may be dependent upon the concentration of these electrolytes rather than upon their electrical conductivity as such. Thus, in experiments made in 1917 upon Cassi4pea, we find that if the electrical conductivity of 0.601 m. NaCI be taken as 1.00, that of an isotonic solution composed of 81.1 c.c. of 0.601 m. NaC1 + 14.36 of 0.39 m. is 0.932; yet if the rate in the pure sodium chloride be 100, that in the NaC1+ is 115. Thus the rate has increased while the electrical conductivity has dimin ished. This experiment is a striking illustration of Loeb's law that a bivalent cation tends to offset the injurious effects of a univalent cation and even magnesium, a well-known depressant, acts as a stimulant for rate of nerve-conduction in association with sodium.
Comparing the effects of these solutions with that of normal sea water, all being practically isotonic one with the other, we find: Rate of nerve-conduction is 100 in natural sea-water of 36.24 0/00 salinity, 8.22 P., and 1.00 relative electrical conductivity. Rate of nerve-conduction is 79.6 in 0.601 m. NaCI of 8.2 P., and 0.996 relative electrical conductivity.
Rate of nerve-conduction is 92.2 in 81.1 c.c. of 0.601 m. NaCI+ 14.36 of 0.39 m. of 8.2 P., and 0.94 relative electrical conductivity.
It may be of interest to see that diluted sea-water is less depressant for nerve-conduction than abnormally concentrated sea-water.
The injurious effects of concentrated sea-water upon regeneration and growth have been studied by Loeb' and by Goldfarb,' and there is a general resemblance between their curves and ours for the rate of nerve-conduction, excepting that for regeneration somewhat dilute sea-water seems to be more favorable than normal sea-water, whereas in nerve-conduction the highest rate is obtained in slightly concen trated sea-water.