Can someone describe the differences between "normal" anxiety and anxiety as an organic symptom of M.E?
Anxiety is a normal and healthy human response that appears in appropriate situations. For example, if you were suddenly dragged into an alley at gunpoint, you would naturally experience a strong anxiety response.
The areas of the brain that are known to mediate such anxiety responses are the amygdala, hypothalamus, prefrontal cortex and locus coeruleus. These areas you can call the "anxiety circuits" of the brain.
So if dragged into an alley at gunpoint, your anxiety circuits would be strongly activated as part of the brain's normal reaction to such circumstances. When the brain is functioning properly, its anxiety circuits create the appropriate anxiety responses as a survival mechanism in these type of dangerous or precarious situations.
Anxiety will also arise to a lesser extent when facing difficult challenges, like say having to give a speech in front of dozens of people; or when your exams are coming up next week, but you have not yet done any revising.
However, in the condition of generalized anxiety disorder (GAD), in which an individual continually experiences anxiety with no good reason, it would appear that these anxiety circuits have become permanently switched on as a result of a dysfunction of the brain, leading to constant anxiety symptoms, even though no anxiety-provoking circumstances are present.
In other words, in GAD you can have constant anxiety, sometimes incredibly severe, but for no external reason whatsoever.
The other thing to consider is that if you suffer from GAD, and then you experience an anxiety-provoking situation that would induce anxiety even in healthy people, your anxiety level may go through the roof due to the combined effect.