Fig. 9. Effects of N -methyl-d-aspartate (NMDA) on the spontaneous discharge pattern of an inspiratory bulbospinal neuron. Fn= neuronal discharge frequency in hertz; PNG = phrenic neurogram. (Left ) Cycle-triggered histograms (CTHs; 8–10 cycles) of the neuronal discharge unit activity (Fn) and ensemble averages of phrenic activity (PNG). NMDA picoejection at a constant dose rate produces a parallel upward shift of the neuronal discharge pattern compared to control. The dashed lines indicate the duration of the inspiratory phase. (Right ) The plot of the discharge frequency, Fn, during constant-dose NMDA versus the control values is linear, with a slope of 1.01 and an intercept of 56.5 Hz. Values were obtained from the CTHs at corresponding times for each 50 ms throughout the inspiratory phase. The solid triangles superimposed on the NMDA discharge pattern in the left panel represent the sum of the control CTH values plus 56.5 Hz. Linearity is demonstrated by the constant NMDA-induced net increase in Fnin the presence of increasing endogenous excitatory drive to the neuron throughout the inspiratory phase. See text for details.