Upper motor neurons are nerves that live within the spinal cord or the brain. When an upper motor neuron is damaged (e.g. a spinal cord injury above the level of T12), messages that would normally pass through that neuron aren’t able to connect to the nerves below the damage.
The reflexes that normally happen below the damage still work because the nerves that connect from the muscles to the spinal cord are functional, however the brain is not able to control them properly.
An upper motor neuron lesion usually results in a reflex bladder.
A reflex bladder means that the bladder tries to empty when it is full but the brain can’t send messages to make the emptying process happen properly. The bladder tries to empty itself but it can’t control the process properly without being connected to the brain.