The neurobiology of hyperarousal involves the activation of the fight or flight response, where the amygdala is activated by the memories of the trauma (nightmares or flashbacks) or is triggered by reminders of the trauma, as seen in PTSD (post traumatic stress disorder). So when you are in hyperarousal, you have increased sympathetic nervous system activity, where the activated amygdala triggers the locus coeruleus to secrete norepinephrine, with resultant increase in heart rate and blood pressure. In addition, the amygdala activates the HPA axis (hypothalamic-pituitary-adrenal axis), with the subsequent release of stress hormones such as cortisol, with resultant increase in metabolic rate and temperature. The activation of both the sympathetic nervous system and the HPA axis leads to the hyperarousal symptoms seen in people who have PTSD.
The following flowchart illustrates the neurobiology of hyperarousal: