Complex sentence: ADHD, which in severe cases can be debilitating, has symptoms thought to be caused by structural as well as biochemical imbalances in the brain; in particular, low levels of the neurotransmitters dopamine and norepinephrine, which are responsible for controlling and maintaining attention and movement.
Split into simpler sentences: ADHD, which in severe cases can be debilitating, has symptoms thought to be caused by structural as well as biochemical imbalances in the brain. In particular, low levels of the neurotransmitters dopamine and norephinprine, which are responsible for controlling and maintaining attention and movement.