Complex interaction
This is a complex interaction between your will-powered (somatic) and non will-powered (autonomic) nervous systems.
In the will-powered nervous system, we differentiate between the parasympathetic and the sympathetic systems. During the bladder filling phase, the sympathetic nervous system dominates, while your parasympathetic system dominates during the emptying phase.
Urine urging
When you feel the urge to urinate, nerve impulses travel from the receptors in the bladder walls to the primary micturition centre in the pons, which is located in the midbrain, and on to the cerebral cortex. From here, you can activate or pause the emptying of your bladder by willpower. When you decide to empty your bladder, the pressure in your urethra lowers, the urethra opens up and your bladder contracts until it is completely empty. Afterwards, your bladder returns to the filling phase and the pressure in your urethra returns to being higher than the pressure in your bladder.