Bladder is an internal organ, which is mainly made up of smooth muscles. The contractions of the smooth muscles are not under voluntary control. The contraction of these muscles are directed by brain, without any external control.
The autonomic nervous system is a part of nervous system, which is responsible for regulation of the all the conditions, which occur involuntarily. It directs the movements of all smooth muscles.
Hence, the movements of the bladder are controlled by the autonomic nervous system.