Bladder. Artwork of a human bladder and the nerves of its reflex arc. Two ureters (orange, upper left/right) carry urine from the kidneys to the bladder where it is stored before expulsion in the urethra (bottom centre). The internal urethral sphincter muscle (pink ovals at base of bladder) is controlled by a reflex arc, a nervous response that occurs without conscious control. Sensory nerves (green) in the bladder tell the spinal cord (sectioned, upper centre) when it is full, causing motor cells (purple) in the spinal cord to open the internal urethral sphincter. Voluntary bladder control using the external urethral sphincter (on either side of urethra) develops after infancy.