 The amygdala is responsible for threat detection and the conditioned and unconditioned fear responses that are carried out as a response to a threat.The HPA axis is responsible for coordinating the hormonal response to stress.Given the strong cortisol suppression to dexamethasone in PTSD, HPA axis abnormalities are likely predicated on strong negative feedback inhibition of cortisol, itself likely due to an increased sensitivity of glucocorticoid receptors.