Abstract
Hyperpolarizing responses of the somata of median giant fibers of crayfish were investigated under current clamp and voltage clamp conditions in normal saline. The responses consisted of a peak [mean ΔVm (peak)=-217mV] followed by a plateau (-107 mV), and could be classified into N-type and G-type. In the N-type cells, the current clamp I-V curve showed a transient phase to the peak. The phase corresponded to the negative slope region of the voltage clamp I-V curve. There was no negative slope region in the G-type cells. In the transient and the peak phases, the instantaneous I-V curves showed nonlinearity, and the shifts of the electromotive forces (ΔEMF) from the resting level were slightly negative or zero. In the plateau phase, however, the instantaneous I-V curve was linear and ΔEMF was large and positive (33.2 mV). The membrane resistance, which was larger (×2.3) at the peak than at the resting state, decreased (×0.23) gradually during the plateau phase.ΔEMF during the hyperpolarizing response consisted of two components: a fast component (1.7 msec) and a slow one (113 msec). The amplitudes of ΔEMF (27.4 mV) of these components in the depolarizing direction increased gradually, once the response had reached the peak. The fast and slow ΔEMF's attained 17.0 and 10.4 mV respectively at the end of 100 msec current pulse. It is shown that constant charge triggers the plateau process and that this threshold voltage-current curve is hyperbolic. It is a possiblity that the plateau is caused by membrane breakdown.
