Once the bladder has been filled (250-500 ml) the nerves supplying the bladder send a signal to the brain that produces the desire to urinate.Urination begins with contraction of the muscle of the bladder and the relaxation of the sphincters (closing mechanisms) situated at the neck of the bladder and the urethra, permitting the exit of urine through the urinary meatus (the external orifice of the urethra).