The measurement of “accommodation” in nerve

Abstract

The time-constant of the accommodation process in nerve was measured through excitation by exponentially increasing currents. Hill has deduced theoretically that if the exciting current approaches the threshold exponentially, the threshold current (I) divided by the rheobasic constant current (7o) should have a linear relationship to the time-constant of current rise ([alpha]), the slope of the line being 1/[lambda]. Experimentally, exponentially increasing currents were obtained by the use of a variable condenser and resistances. The threshold for instantaneous rise of current was first detd., then thresholds for currents with increasing time-constants of rise, and finally a return series with decreasing time-constants. The average value for each duration was divided by the rheobase observed with instantaneous current rise, and plotted against the time-constant of current rise. Muscle twitch was taken as an index of nerve excitation. Wooden non-polarizable electrodes were used for the nerve contact. The effect of salts was tested in the case of frog muscle both by alterations in the Ringer solution surrounding the muscle, and by injection of salts into the lymph spaces of the intact frog; in marine animal muscles by addition of isotonic solutions to sea-water; and in the human, by the administration of salts by mouth. For the latter, the ulnar nerve was stimulated, and tendon twitch at the wrist used as index of nerve stimulation. The effect of skin capacity on the form of current rise was checked by an oscillograph. Within the range where the time-constant of current rise is large compared with k, the time-constant of the decay of "local potential" to resting value, a linear relationship, confirming Hill, is obtained for frog, elasmobranch and teleost fish, crustacean and human nerve. The average [lambda] in msec. varied from 11 in the teleost to 4160 in Carcinus. The deviation from a linear relation predicted by Hill''s theory for currents with short time-constants of rise is confirmed. Increasing Ca concn., and to a lesser extent increasing K, Mg, Sr, or Ba concn., lowers [lambda]. Decreasing temp. or decreasing Ca concn. increased [lambda]. The effect of Ca on [lambda] was greater than on k, confirming the independence of the 2 time-factors. The deviations in values of [lambda] for nerves of different normal frogs suggest that the value of ionised Ca varies with the individual and with the season. In man, no such deviations occur.