Loss of inhibitory control from descending serotonergic (5-HT) fibers deregulates the excitability of spinal motoneurons and interneurons following spinal cord injury (SCI). Exaggerated synaptic activity triggers long-lasting excitatory postsynaptic potentials (long EPSPs) in hyperexcitable motoneurons to ultimately drive muscle spasms at the chronic stage of SCI. Deep dorsal horn (DDH)...