Final answer:
The sympathetic nervous system and the hypothalamic-pituitary-adrenal (HPA) axis are activated during a stressful event, leading to a physical fight-or-flight response and hormonal changes like the release of cortisol.
Step-by-step explanation:
When an individual goes through a highly stressful event, the sympathetic nervous system becomes activated. This system triggers the fight-or-flight response, preparing the body to either confront or flee from the stressor. Examples of physiological changes include dilated pupils, increased heart rate and blood pressure, relaxed bladder, and glucose release by the liver. Additionally, the hypothalamus signals the adrenal glands through the hypothalamic-pituitary-adrenal (HPA) axis, unleashing hormones like adrenaline and cortisol into the bloodstream. Cortisol, known as a stress hormone, gives a burst of energy but can impair the immune system if levels remain high over time.