Fig. 3. Plots showing the effect of different adjunctive drugs on the unstable area (area C) of the relative inhibitory postsynaptic potential area versus inhibitory postsynaptic potential (IPSP) decay–time diagram. a shows the decrease in unstable area caused by N -methyl-d-aspartate blockade. b shows the increase in unstable area caused by augmented excitatory postsynaptic potential amplitude, such as might be induced by 5-(2-cyclohexylideneethyl)-5-ethyl barbituric acid. c shows the decrease in unstable area caused by augmentation of tonic hyperpolarizing (potassium or chloride) currents, amounting to a −2.0-mV shift in resting potential. d shows the increase in unstable area caused by inhibition of tonic currents, amounting to a +2.0-mV shift in resting potential. The trajectories are for increasing concentrations of isoflurane ( solid line ) and enflurane ( dashed line ), as in figure 2.