By increasing the amount of serotonin in the spinal cord, an experimental drug helps nerve connections work better. Bee Smith/Ocean/Corbis hide caption

itoggle caption Bee Smith/Ocean/Corbis

Sofia Jarvis, seen here with her family at a press conference in February, is one of several dozen children in California who have been diagnosed with a rare paralytic syndrome. It has left her left arm paralyzed. Martha Mendoza/AP hide caption

itoggle caption Martha Mendoza/AP

Kent Stephenson, a research participant at the University of Louisville's Kentucky Spinal Cord Injury Research Center, has his level of muscle activity and force measured by Katelyn Gurley. Courtesy of the University of Louisville hide caption

itoggle caption Courtesy of the University of Louisville

Sophia Jarvis, 4, of Berkeley, Calif., is one of the few children diagnosed with the polio-like disease, which left her arm paralyzed. She attended a press conference Monday at Stanford University with her dad, Jeff. Martha Mendoza/AP hide caption

itoggle caption Martha Mendoza/AP