The fight or flight reaction centres around the sympathetic nervous system (SNS) activation due to perceived stress or danger. The primary emotional experience triggering a fight or flight response is fear, which is a normal emotion in response to a perceived danger. Examples of possible threats that may trigger fear include fire, smoke alarm, collapsing building etc. When the body perceives an incoming threat, information is relayed to the hypothalamus, which then stimulates the SNS to send signals to the adrenal glands. In response, the adrenal glands secrete the hormone adrenaline to drive critical physiological processes for a fight or flight reaction.
Adrenaline causes effects through its action on the alpha and beta receptors using second messengers coupled to G proteins. The hormone acts through alpha-1 receptors to bring about effects in the vascular smooth muscles. The effects of adrenaline on the heart and respiratory system are via B-1 (Beta) receptors and via beta-2 to cause bronchodilation. Adrenaline increases the heart rate allowing more blood to be supplied to muscles and other vital organs in the body. The hormone also causes a rise in the respiratory rate making a person breather faster to provide more oxygen to the body.
Besides, there is dilation of small lung airways to fill as much oxygen adequate for muscles to be able to fight or flee from danger. More oxygen can also be sent to the brain. Thus, an individual becomes more alert. Also, other senses such as sight and hearing vital to flee from danger become sharper. There is the constriction of blood vessels to redirect blood to significant muscles and vital organs, including the lungs and heart. Finally, adrenaline triggers glucose and fat release from storage sites for the production of energy to supply the body in readiness to fight or fly.
The sympathetic nervous system’s response is usually a sudden one, and thus an individual may not have the time to think about his or her next action. Once the existing stress resolves, the body goes back to the normal relaxed state, aided by the opposing effects of the peripheral nervous system (PNS). Hormonal responses during a fight or flight response also heighten a person’s emotions, which may be overwhelming. For instance, if a car accident, an emotional fight response would be to quarrel another driver even if the other person was not responsible for the accident. One may also become extremely afraid and want to escape from the scene, which in this case is the flight response.
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