How does the nervous system help control urination?

Before toilet training, a simple reflex controls the timing of urination. Stretch-detecting sensory nerves in the detrusor muscle signal the spinal cord that the bladder is filling with urine. In response, motor nerves from the spine signal the bladder muscles to contract and the urethra and pelvic floor muscles to relax, allowing urination. As the brain matures, it is able to override the spinal nerves' automatic signal for the bladder to empty. The bladder gradually becomes capable of storing greater amounts of urine, and a child learns to urinate when it is convenient and socially acceptable.