Figure 2.

Examples of neurons with altered discharge identity or discharge rate after codeine administration from several animals. Several of the neurons shown became breathing modulated (examples 1–3). In others, the time of maximum discharge rate during the respiratory cycle changed (4 and 5). Discharge rates of most of the neurons shown increased (1, 2, and 7) or decreased (4 and 5) after codeine. Cycle-triggered histograms of neuronal activity (colors) overlying phrenic discharge (gray). E-P, expiratory phasic; E-T, expiratory tonic; I-T, inspiratory tonic; n, neuron; NBM, nonbreathing modulated. Anesthetized, paralyzed, and artificially ventilated cats. Codeine (1–3,000 µg/kg) was administered via the vertebral artery.