Abstract
(1) The potential change across the membrane of the squid giant axon during passage of electric current is investigated using internal longitudinal stimulating and recording electrodes.
(2) The threshold condition for constant current pulse is determined by the electrical constants of the resting membrane when the duration of pulse is short (less than 2 msec. at about 16°C.). For longer currents, up to 5 msec.(at about 16°C.), some amount of subthreshold potential change contributes to the condition. In both cases the critical depolarization for the spike shows the same value which is called the resting critical depolarization.
(3) Shifting the membrane potential to various levels with short current pulses it is concluded that the critical depolarization is a certain potential level at which the dV/dt immediately following changes its sign and is often followed by a plateau potential which represents an unstable equilibrium potential level.
(4) The minimal gradient for stimulation with a linearly increasing current is mainly determined by delayed rectification of the membrane while the late phase of accommodation is due to the increase of the critical depolarization.
(5) There are two certain current intensities for producing two spikes with a constant current. The one limits the longest interval between them and the other limits the shortest one. Delayed rectification is mainly concerned with the existence of the former and the increase of critical depolarization contributes to the latter.
