The release of cortisol during the fight or flight response occurs at the end of the HPA axis (hypothalamic-pituitary-adrenal) pathway. Fear is modulated via the amygdala, the part of the brain that is involved in the fear response, also called the fight or flight response, adrenal response, and the stress response. When the amygdala senses fear, the amygdala (in the brain) activates two systems downstream (in the body):
- Sympathetic nervous system
- HPA axis
The autonomic nervous system is involved with regulation of body organs beyond your conscious control…it is involuntary. The autonomic nervous system controls your heart rate, blood flow, blood pressure, respiratory rate, digestive system, reproductive system, urination, and pupillary response. The autonomic nervous system has two parts: the sympathetic nervous system, involved in the fight or flight response, and the parasympathetic nervous system, involved with digestion, reproduction, and rest.
In the fear response, fear activates the amygdala. The amygdala then activates the locus coeruleus, which in turn activates the sympathetic nervous system. The sympathetic nervous system is activated in the fight or flight response, and it is responsible for increasing your heart rate, increasing blood flow, constricting blood vessels, increasing blood pressure, widening your pupils, stimulating your sweat glands to perspire, causing piloerection (goosebumps), slowing the movement of your large intestine, widening your bronchial passages, and increasing your rate of breathing. The sympathetic response also shuts down your gastrointestinal (GI) system and your reproductive system, as these are non-essential systems for preparing for battle or running from it.
The sympathetic nervous system also activates the adrenal medulla, located in the adrenal glands which sit on top of the kidneys, to release adrenaline (epinephrine) and noradrenaline (norepinephrine) into the bloodstream. Adrenaline and noradrenaline are stress hormones, which increases heart rate, increases blood pressure, increases respiratory rate, increases blood glucose, increases blood flow to the skeletal muscles, and tenses the skeletal muscles. Noradrenaline is responsible for increased alertness and focus to danger- also called hypervigilance.
The HPA axis is involved with changing the neural signals from the brain and amygdala into hormonal signals to the body. In the fight or flight response, the amygdala also signals the hypothalamus to release the hormone CRF (corticotropin releasing factor), which signals the pituitary to release ACTH (adrenocorticotropic hormone). The ACTH then travels in the bloodstream and activates the adrenal cortex (in the adrenal glands) to release cortisol, one of the stress hormones involved in the fight or flight response. Cortisol increases blood glucose, suppresses the immune system, and aids in the metabolism of carbohydrates, proteins, and fat.
Together, the sympathetic neural response, and the hormonal release of adrenaline, noradrenaline, and cortisol into the bloodstream constitute the fight or flight response.