The fight or flight response is basically your body's way of protecting itself from any dangers in the environment. It occurs as a reflex to danger...no conscious awareness is needed for the fight or flight response to occur- it is involuntary. So when a danger is perceived, a signal is sent to the amygdala, a part of the brain that processes danger and fear signals. The amygdala then sends 2 signals- one to the locus coeruleus, and the second one to the pituitary gland. The locus coeruleus then triggers the sympathetic nervous system, which is well-known to be involved in the fight or flight response. The sympathetic nervous system is composed of nerves which innervate major organ systems in the body, like the heart, lungs, and blood vessels. So when the sympathetic nervous system is activated, it affects your organs, like increasing your heart rate, increasing your rate of breathing, constricting blood vessels in non-essential areas (like the skin, digestive, and reproductive organs), widening your pupils, increasing blood flow to your skeletal muscles and brain, and increases your sweating. So these are the physical manifestations of the fight or flight response.
In addition to activating the sympathetic nervous system (a neural system), the amygdala also activates the hypothalamus, and sets-off the hypothalamic-pituitary-adrenal axis, also known as the HPA axis (a hormonal system. So after the amygdala sets off the hypothalamus, the hypothalamus releases a hormone, CRF or cortisol releasing factor, and this signals the pituitary to release ACTH or adrenocorticotropic hormone into the bloodstream. The ACTH then signals the adrenal glands to release cortisol into the bloodstream, and this contributes to the stress response.
The adrenal glands also release adrenaline in response to the sympathetic nervous system being triggered by amygdala (via the locus coeruleus). Adrenaline is released from the adrenal glands and this also contributes to the physical symptoms you see in the fight or flight response.
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