Final answer:
The defecation reflex is mediated by the parasympathetic nervous system, the enteric nervous system, and the central nervous system, which includes the spinal cord.
Step-by-step explanation:
The three nervous system components that mediate the defecation reflex are the parasympathetic nervous system, the enteric nervous system, and the central nervous system. The parasympathetic nervous system is responsible for stimulating the muscles in the colon and rectum to promote bowel movement. The enteric nervous system, which is a network of neurons governing the function of the gastrointestinal system, controls the local reflexes involved in defecation. Finally, the central nervous system, comprising the brain and spinal cord, coordinates the overall reflex by processing sensory information and facilitating the voluntary and involuntary aspects of defecation.