FIG. 1. Schematic representation of the major components of the motor hypothesis of 5-HT function. When voluntary tonic or rhythmic motor outflow is initiated, forebrain motor commands are simultaneously sent to alpha motoneurons (MN) and/or brainstem or spinal cord central pattern generators (CPG) and to brainstem 5-HT neurons. The output of 5-HT neurons serves several functions simultaneously: It facilitates CPG and a motoneuron activity, orchestrates autonomic and neuroendocrine (not shown) output with motor outflow, and inhibits sensory information processing. 5-HT neurons are shown to be driven by an endogenous oscillator (sine wave with arrow) and to be phasically activated by various afferents. Not shown is the possibility that different groups of 5-HT neurons may facilitate different motor functions (e.g., NRPa and respiration, DRN and oral–buccal movements, etc.). (Adapted from ref. 24.)