Professor Trevor Robbins describes the noradrenaline system, which is highly involved in arousal.

The noradrenergic system of the brain originates in brain stem structures such as the locus ceruleus, and also behind the locus ceruleus, in an area called the medulla. There are two main branches of the noradrenaline [also known as norepinephrine] system. One is called the dorsal ascending system, which projects all over the forebrain, including the neocortex and the hippocampus. One of the places it does NOT go to is the striatum. We don't know why that is. The ventral bundle projects mainly to the hypothalamus and to other visceral parts of the brain including the limbic system, which is involved in emotion. The noradrenaline system, globally speaking, is involved in arousal. It is least active when you're asleep, especially when you are dreaming and it is most active when you are aroused, either moving a lot or thinking a lot, when it will affect your cortical electroencephalogram [EEG]. It is really turned on by novelty. So, novelty will make this system work very fast. Too much stimulation, including too much novelty, may cause stress and that may be linked to anxiety. So, this is the way in which the noradrenaline system may go wrong as well.

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