Stress represents any stimulus or event that requires internal or external adjustment by an organism and is the origin of the fight or flight reaction within human beings (Goldstein & McEwen, 2002). Hans Selye’s work helped to identify the cascade of responses in the central and peripheral nervous system and was the first to consider the difference between acute stress, that which results in an immediate fight or flight reaction, and chronic stress, which he referred to as the General Adaptation Syndrome. When under the influence of a chronic stressor, three phases follow: Alarm (assessment and accumulation of resources), Resistance (fight or flight), and Exhaustion (Tian & Yip, 2018). Acute stressors produce a similar reaction to chronic stressors, but in a limited time-frame, as the stressor is present during a discrete period of time (Goldstein & McEwen, 2002). However, acute stressors can give rise to chronic stressors, say in the event of a lingering reaction to the initial event (e.g., post-traumatic stress disorder, adaptation to a chronic medical condition discovered during an acute event, or the recent COVID-19 long-haulers who are left fighting residual symptoms months after the acute illness; McEwen, 1998). Understanding the body’s reaction to stress may help us to identify ways to facilitate resilience in the aftermath of stress, whether acute or chronic.
The nervous system is divided into central and peripheral branches. The central nervous system includes the brain and spinal cord while the peripheral nervous system encompasses everything else (e.g., peripheral nerves, communication with internal and sensory organs, glands). One way to think about the basic divisions is that the central nervous system is the CEO and the peripheral nervous system is a set of managers who carry out the plan initiated by their boss. The peripheral nervous system is further sub-divided into the somatic branch, which is responsible for sensory data and movement, and the autonomic branch, which is responsible for the activity of organs and glands through the sympathetic and parasympathetic divisions (Goldstein & McEwen, 2002). The sympathetic division is responsible for the organ and glandular readiness for fight or flight when under distress—Think of it as sympathizing with your distress (Goldstein & McEwen, 2002). When the sympathetic division is activated, everything that is not needed for fighting or flighting is shut down (e.g., digestion, salivation, bladder, sexual) and systems that are needed are readied (e.g., hear and respiration rate increases, pupils dilate, glucose released, stress hormones released; Goldstein & McEwen, 2002). The parasympathetic division helps to return the organs and glands to a pre-stress state of homeostasis—Think of it as a hug from a parent meant to help you calm down. Is it too corny to say parentsympathetic? When the threat is mitigated, the parasympathetic division returns the body to homeostasis—hear rate declines, pupils constrict, digestion returns, stress hormones cease, bladder and airways constrict, etc. (Goldstein & McEwen, 2002).
The excitation of the sympathetic division begins with the central nervous system. Data from the environment is transduced into the neurochemical and electrical impulses of the brain. Perhaps due to the slight delay between reality and perception, our brains process information on 2 pathways—the high road and the low road (Peyk et al., 2009). All sensory data passes through the thalamus, informally known as the Grand Central Station of sensory processing. From there, the high road processes sensory data in each of the specialized areas of the cortex (the wrinkly gray stuff you think of as the brain) for each of the senses: Vision is processed at the back of the skull in the occipital lobe, hearing in the temporal lobe, near your ear, sense of touch in the sensory cortex in your parietal lobe, and smell/ taste in part of the frontal lobe just behind your eyes. If a threat is determined to be present, the data is sent to the amygdala in the emotion center of the brain called the limbic system. The amygdala is responsible for fear, anger, and feelings of satiation or homeostasis (Ressler, 2010). Think of the amygdala as the alarm system of the central nervous system. The low road is responsible for your more immediate, reactionary responses and employs the components of your brain associated with less evolved beings. Low road processing occurs when data is sent directly from the thalamus to the amygdala, which then signals the hypothalamus, which initiates the fight or flight reaction in the autonomic nervous system, causing the release of hormones to aid in the fighting or flighting that may be necessary to survive.
The amygdala is closely situated with the hippocampus, known as the gateway to memory, which becomes impaired when exposed to an overly active amygdala (Kim et al., 2015). Chronic stress leaves the amygdala overly active and the hippocampus suffers loss in the volume of hippocampal neurons (Kim et al., 2015). This could help to explain difficulties with concentrating and remembering when under both acute and chronic stress, as problems with thinking are noted in many mental health conditions (e.g., depression, anxiety, and PTSD). When a threat is perceived, the amygdala also sends along the data through the anterior cingulate cortex, which serves as a screen for false alarms—that is, if the high road processing produces a different result. If the data is deemed true, the data is forwarded to the frontal lobes where plans and decisions are made for action. This may be why there is an initial moment of freeze before fighting or flighting ensues. In cases of chronic stress or conditions induced by traumatic stressors (e.g., depression, anxiety, PTSD), the amygdala remains in a heightened state of alert, exhausting resources and impairing cognitive and emotional functioning (Wilson et al., 2019).
Because exposure to stress can alter the way in which your brain functions, it is recommended that you work with a highly trained, licensed provider on your stress-management plan, whether for prevention or intervention. While the internet is full of pseudo-professionals with certificates that sound fancy, they are no replacement for the 7 years of training (on average) that Psychologists complete and the 2 years completed by licensed mental health professionals with Master's degrees.
Goldstein, D. S., & McEwen, B. (2002). Allostasis, homeostats, and the nature of stress. Stress (Amsterdam, Netherlands), 5(1), 55–58. https://doi.org/10.1080/102538902900012345
Kim, E. J., Pellman, B., & Kim, J. J. (2015). Stress effects on the hippocampus: a critical review. Learning & memory (Cold Spring Harbor, N.Y.), 22(9), 411–416. https://doi.org/10.1101/lm.037291.114
McEwen B. S. (1998). Protective and damaging effects of stress mediators. The New England journal of medicine, 338(3), 171–179. https://doi.org/10.1056/NEJM199801153380307
Peyk, P., Schupp, H. T., Keil, A., Elbert, T., & Junghöfer, M. (2009). Parallel processing of affective visual stimuli. Psychophysiology, 46(1), 200–208. https://doi.org/10.1111/j.1469-8986.2008.00755.x
Ressler K. J. (2010). Amygdala activity, fear, and anxiety: modulation by stress. Biological psychiatry, 67(12), 1117–1119. https://doi.org/10.1016/j.biopsych.2010.04.027
Tan, S. Y., & Yip, A. (2018). Hans Selye (1907-1982): Founder of the stress theory. Singapore medical journal, 59(4), 170–171. https://doi.org/10.11622/smedj.2018043
Wilson, M. A., Liberzon, I., Lindsey, M. L., Lokshina, Y., Risbrough, V. B., Sah, R., Wood, S. K., Williamson, J. B., & Spinale, F. G. (2019). Common pathways and communication between the brain and heart: connecting post-traumatic stress disorder and heart failure. Stress (Amsterdam, Netherlands), 22(5), 530–547. https://doi.org/10.1080/10253890.2019.1621283