Abstract
In the present study the factors which influence the critical level of membrane potential for the spike were investigated.
1. The relation between critical level and resting membrane potential was approximately linear, their average values were 50.5±6.9 mV (S. D.) and 82.9 ± 7.0 mV (S. D.) respectively and the correlation coefficient between them was 0.75. The critical level was independent of the time necessary to reach it. 2. When the membrane potential was changed artificially, the critical level was linearly related to the membrane potential up to 95 mV, above which the critical level showed a tendency to flatten off.
3. The critical level was observed to tend gradually towards the lower value as the external potassium was raised in either case the membrane potential was changed with the change of potassium concentration or maintained at the initial level.
4. The resting potential was little influenced by the external sodium concentration and the critical level was also almost constant until 50 mM, below which it showed a tendency to decrease.
5. It is concluded that the critical level depends on the membrane potential and the external potassium concentration but not on the sodium concentration of the external fluid.
