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How the Body Responds to Stress and Threat

Balance and Stress: The human body is designed to stay in balance, in spite of all the sources of stress and threat in the world. When we think of balance, we often think of stability or homeostasis, a relatively stable state of equilibrium. But even to approach homeostasis, the body often has to rely on allostasis—the use of change to achieve stability—much the way one would spread one’s arms and lean from side to side to balance on a narrow board. In general, the lower or more temporary the stress or threat, the easier it is to use allostasis to regulate and balance the body’s reactions (McEwen and Wingfield, 2003).

The autonomic nervous system—whose control of heart rate, breathing, metabolism, energy, perspiration, etc. works toward balance—has two “arms” that rise and fall in turn:

  • The sympathetic nervous system, the one that speeds up processes (heart rate, breathing, metabolism, energy, etc.) and tightens muscles
  • The parasympathetic nervous system, the one that slows down these processes and relaxes muscles

How We Work Toward Balance

The ideal: Homeostasis
(Stable Equilibrium)

The compromise: Allostasis
(Balance Through Change)

How do we do that: The Automatic Nervous System

Active Response to threat: The Sympathetic Nervous System
(Fight or Flight)

Passive Response to helplessness: The Parasympathetic Nervous System
(Rest, numb, and freeze)


The body’s responses to stress and danger tend to fall into these two categories, sympathetic and parasympathetic. They are meant to operate in gentle allostasis, with the sympathetic side rising to fight or flee from temporary threat, and the parasympathetic rising to keep us safe when we are helpless—and to quiet down the sympathetic and return us to homeostasis.

A useful tool for keeping these two terms straight might be to think of the sympathetic nervous system as being “sympathetic” toward our initial need to run from danger, and the parasympathetic as being the opposite and balancing reaction.

Brain Structures That Help Regulate Stress Reactions: It takes the coordinated work of many brain structures, systems, and chemicals to regulate all our stress reactions. These structures live on all levels of the brain, from the very primitive, emotional, “reptilian” brain stem; through the more recent limbic system or “mammalian brain”; to the higher and most recently developed cerebral cortex. And although the brain has two hemispheres (each a mirror image of the other), key structures and circuits in the right hemisphere are particularly important in responding to stress and threat (Schore, 2001; Siegel, 2001; van der Kolk and Fisler, 1995).


View a Diagram of the Anatomy of the Brain


The most powerful player in this drama is the amygdala, a small, almond-shaped structure in the limbic system. Buried deep in the brain, the amygdala is an ancient structure designed to keep us alive. It stores unconscious emotional memories and triggers the body’s responses to stress and threat. Under stress, the amygdala contacts the hypothalamus (control hub of many of the body’s chemical processes) and sets the autonomic nervous system in motion. Several organs respond by sending out powerful rushes of sympathetic (e.g., adrenaline, norepinephrine, dopamine) and parasympathetic (e.g., cortisol, endorphins, acetylcholine, oxytocin, GABA) stress chemicals.

Many of these chemicals (particularly adrenaline and norepinephrine) are meant to be used up in vigorous exercise as we take physical action to remove the threat or take care of the source of stress. And one of the brain’s many jobs is to keep these two chemical systems—sympathetic and parasympathetic—in balance. That balance helps keep us “resilient,” able to keep ourselves safe and bounce back after intense experiences (Schore, 2001; Scaer, 2005).

Receiving signals from the thalamus (the brain’s relay system) and the olfactory bulb (our sense of smell), the amygdala stores unconscious memories that are primitive fragments—pictures, sounds, scents, feelings. When it receives signals that remind it of past threats—even if these signals are very different from the earlier threats—the amygdala brings up those memories as if they were happening now. Then it sets off all the chemical fight/flight/freeze reactions. Its goal is to keep us alive and functioning (LeDoux, 1996).

Several other, more sophisticated structures are ready to help the primitive amygdala understand what is really happening and decide whether or not to flood our bodies with chemicals—and when to stop.

  • The hippocampus, right next to the amygdala, provides conscious, detailed memories that help us remember what has happened in the past and use those memories to understand what is happening now. The hippocampus orients us in time and space—the here and now.
  • The anterior (front) cingulate gyrus, above the amygdala, is a bridge between attention and emotion. It helps us perceive and understand the world of people, and it is one of the structures that tries to reason with the amygdala. It also asks the nearby prefrontal cortex for help when it senses that the amygdala may be overreacting.
  • The prefrontal cortex (PFC) is the highest, most sophisticated part of the brain, responsible for everything from logic to empathy. The PFC assesses the threat, weighs all the options and consequences, comes up with a plan, and works to calm the amygdala. The areas of the PFC that are most active in this are the right orbitofrontal (behind and above the eyes) and medial (middle) areas.

View a Model of the Brain's Response to Stress & Threat


The amygdala is a primitive structure, much like a guard dog whose understanding is sketchy but whose mission—our protection—is clear. The amygdala cannot see or hear subtle differences in the signals it receives from the thalamus, and it lacks the conscious memories that would help it put these signals in context—“Oh, that’s what it is!”—and interpret them correctly (LeDoux, 1996). Its memories are unconscious, primitive, and fragmented—Brown and Kulik (1977) called them “flashbulb memories” of sound, image, scent, and emotion. In its zeal to protect us, the amygdala reacts to incoming signals by pulling up whatever memories might be related to these signals. So a gunshot and a champagne cork are all the same to the amygdala.

The prefrontal cortex also receives signals from the thalamus, but these are more sophisticated than the ones the amygdala receives, and the PFC is better at decoding them (LeDoux, 1996). So even though it learns of potential threats after the amygdala has set the stress systems in motion, the PFC is still in a good position to provide more information and work toward regulating the amygdala’s ongoing response.


Next: Promoting Resilient Responses

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The material on all of the Clinical Pages is taken directly from the draft version of Finding Balance After the War Zone:  Considerations in the Treatment of Post-Deployment Stress Effects, a manual under development for the Great Lakes Addiction Technology Transfer Center and Human Priorities.  This draft is copyright © 2008, Pamela Woll.  Reprint permission is universally granted, but attribution is requested.
Click here for References and Other Resources.
Click here to link to a PDF file of the current version of the clinician’s manual draft.
Click here to link to a PDF file of the accompanying booklet for veterans.

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