When does sadness become depression?

Chris Smith put this question to mental health expert Anne-Laura van Harmelen from Cambridge University...

Anne-Laura - We know certain things about what's going on in the brain when people are more depressed. One of the things is for instance the serotonin system is a neurotransmitter system that makes you happy and that system is shown to be there's a bit less serotonin in the brain in certain regions.

Another thing that is kind of well replicated or well known now is that there are certain networks of brain regions that are working differently or they're more or less active in people with depression. For instance the ventral limbic system is elevated in its connectivity and that means that it is excessively actively giving you negative mood. We also know that the frontal striatal network is lower and that is related to loss of interest and motivation and less pleasure in activities we also know that the default mode network is more active and that is related to more negative thinking about yourselves. But the problem is that you can have different kind of sorts of depression and a researcher has shown that there are over a thousand different types of depression.

So we know that there are lots and lots of different types of depression that also means there are lots and lots of different types of depression in the brain. And in addition to that you can have the same type of depression but with different reasons for that depression. So you can have depression because you had really severe stress in early life or you can have depression because you have financial difficulties. So even if you have the same type of depression or the same symptom clustering, you can still have different causes and therefore different mechanisms through which you get to these systems. So we don't really know yet and the reason for that is is that it's so multifaceted.