Islam and mental illnesses

Dear Muftisaab
Assalaamualaikum
I am a lecturer in the psychology department at Wits and one of my areas of interest is around Islam and mental illness and mental well being. Alhumdullilah in the last few years I have through my research managed to disseminate a number of Islamic ideas via my research in a field that is primarily secular and in a fair number of cases atheistic. I have now been asked to contribute a chapter on the Islamic conceptualisation of mental illness for a book on cultural views on mental illness and I am struggling with some issues. My Islamic education is limited in that I only completed Class 10 at the Benoni Madressah and following this I have read books in the area but I am not conversant with Quraanic injunctions on mental illness and I have no in-depth knowledge of the Ahaadith books particularly the Book of Medicine from Bukhaari Sharif. Hence I am requesting your assistance.
I am aware that Islam recognises a mental illness as distinct from a physical illness in that there are laws that exempt insane persons from certain fardh acts. However I have not come across any definitions of what constitutes insanity in Islam. What is recognised as a mental illness? How does Islam distinguish between illnesses that are physical, mental and spiritual?
It is clear from the Quraan and ahaadith what physical illnesses are and how they may be treated and what spiritual illnesses are and how they may be treated but how does mental illness fit into this? Is a mental illness equated with a spiritual illness? What are the treatments for mental illness?
I would be most grateful if you could provide some input on these issues or pass this e-mail onto someone who can.
Jazakallah for taking the time to read this e-mail. May the Almighty reward you.