Low-conductance NMDA channels in SpS neurons are less susceptible to Mg²⁺ blockade at negative membrane potentials. A, Left, Channel records from an outside-out patch that contains both high- and low-conductance type channels. Right, Amplitude distribution and fit with the sum of four Gaussians for voltage of -60 mV. B, Channel activities during voltage ramps were recorded, and traces containing only one kind of opening (left) were separated, and the currents were averaged to produce an instantaneous current-voltage relationship (right). Note that the inward current through 30/19 pS channel is significantly larger (closed squares) than that through 51/41 pS channel (open squares) over a wide voltage range (-10 to -70 mV).