In addition to the loss of descending cortical drive, monoaminergic input to the spinal cord is also often disrupted in SCI. As hypothesized by Jacobs and Fornal (Jacobs and Fornal 1993; Jacobs et al. 2002), serotonin acts to facilitate motor output and inhibit sensory input. Removal of monoaminergic input in humans may therefore lead to motor weakness and hyperexcitable reflexes resulting in impaired motor function.