Figure 6:

Transformation of the motoneuron frequency-current (F-I) function by neuromodulatory input. As monoaminergic input from the brainstem increases from low (red trace) to medium (blue) to high (green), the threshold of the cell is lowered markedly. In addition the PIC becomes larger and its threshold is lowered. As a result, the primary range disappears and firing is dominated by the PIC. There is an initial acceleration in firing (the secondary range) followed by a more shallow but usually still positive tertiary range (this phenomenon has variously been referred to as rate limiting, saturation and the “preferred” firing range).