yes, neurological. Many of us CFS patients get defensive when called depressive, including myself because we think it's a purely psychological issue and whe have a physical problem.
But now I know for sure that most if not all psychological issues have underlying physical abnormalities so a depressed patient can have CFS and the other way around too. Depression is infact a serotonin deficiency, which then leads to behavioural abnormalities, but at the core its a physical thing.
I too have serotonin deficiency and since I was at a CFS clinic, the professor and leader of the clinic told me, that most CFS patients have a neurotransmitter abnormality. You can also read my signature link on that subject. I think that's a major aspect for us to look into