What is the micturition reflex?

Micturition reflex is the neurological pathways involved in urination.

When your bladder fills, the detrusor muscles expand and proprioceptors within the walls of the muscle activate. It sends signals (afferent nerves) that go to your medulla and also to the parasympathetic system in the sacral spinal cord. The signal to the parasympathetic system elicits the response to urinate by relaxing the inner external sphincter and constricting the detrusor muscles. However, first your upper brain is consulted and decides whether it is appropriate to go or not.

If your brain allows, it will elicit the response by inhibiting the sympathetic nervous system and inhibiting the somatic nervous system. The somatic nervous system controls the external sphincter and by inhibiting it, you are relaxing it. The external sphincter is much more powerful than the internal sphincter.

If your brain says no, it will activate the sympathetic nervous system (which relaxes the muscle walls and constricts inner sphincter) and your somatic control continues to constrict the external sphincter.