The two components of the fight or flight response are the neural response and the hormonal response. The neural response is from the activation of the sympathetic nervous system, which is part of the autonomic nervous system, involved with regulation of body organs. The hormonal response is from the activation of the HPA (hypothalamic pituitary adrenal system.
The neural response is from the activation of the sympathetic nervous system. Fear stimulates the amygdala (part of the brain), which then stimulates the locus coeruleus. The locus coeruleus in turn activates the sympathetic nervous system. The sympathetic nervous system stimulates smooth muscles and glands via neural connections. The result is an increase in heart rate, increase in respiration, increase perspiration, decreased motility of the large intestines, increased tension of the muscles, increased pupillary dilation, and increased blood pressure.
The hormonal response comes from the activation of the HPA system. The amygdala senses fear, then the amygdala stimulates the hypothalamus to release corticotropin releasing factor (CRF), which stimulates the pituitary to release adrenocorticotropic hormone (ACTH). ACTH then travels via the bloodstream to the adrenal cortex, which is in the adrenal glands on top of the kidneys. The adrenal cortex then releases cortisol into the blood, which is a “stress” hormone, and is responsible for increasing glucose in the blood and suppressing the immune system. In addition, the adrenal medulla, also located in the adrenal glands, is activated by the sympathetic nervous system, and the adrenal medulla releases adrenaline (epinephrine) and noradrenaline (norepinephrine) into the bloodstream, which are also “stress” hormones. Adrenaline and noradrenaline also participate in the overall fight or flight response, where the body becomes faster, stronger, and more alert. Of note is that noradrenaline is responsible for the increased alertness and vigilance of the fight or flight response.
So the two components of the fight or flight response are the neural response, which communicates with smooth muscles and glands via neural connections, and the hormonal response, which releases the “stress” hormones adrenaline, noradrenaline, and cortisol into the bloodstream. These “stress” hormones travel to other parts of the body that are not innervated by the sympathetic nervous system. So these two components make the body faster, stronger, and vigilant for danger.