Homeostasis and the Stress Response

The Brain-Body stays in balance (HOMEOSTASIS) through its complex regulatory systems. 

- is a very complex process carried out, from second-to-second, throughout the entire Brain-Body system
- the result of many layers of "nested" Regulatory CYCLES > SENSING and RESPONDING to signals from inside and outside the Brain-Body.

A key factor is the Acute Stress Response - a major INBORN REGULATORY process - triggered by threat.

Key steps in the Acute Stress Response:
   o   A burst of sympathetic (activating) hormones is released (adrenaline/epinephrine, norepinephrine)
   o   Bodily functions "speed up" - heart rate, breathing, blood pressure, cardiac output, dilation of pupils, muscle tension
   o   Release of cortisol and other biochemicals

In the past, the STRESS RESPONSE  was
- Thought of as "just a Reflex" - an automatic, unconscious response
- Visualized as being contained in the HPA Axis (Hypothalamic-Pituitary-Adrenal), and
- Described as having 2 paths:  FIGHT  or   FLIGHT

Now, we know that the STRESS RESPONSE is much more complex in at least 5 important ways
    1.  Central role of the BRAIN and the integration of sensory input in determining the presence of threat - and therefore triggering the Stress Response.  The HPA Axis model tends to leave out these connections.
    2.  STRESS RESPONSE involves much more than release of adrenaline (sympathetic hormones) and glucocorticoids.  Stress triggers signaling and "crosstalk" (two-way communication) between the Brain and cardiovascular, immune, and other systems.
    3.   STRESS RESPONSE has 3 potential paths
                o   FIGHT, 
                o   FLIGHT, or   
                o   FREEZE/FAINT   (in people, FREEZING often is manifested as DISSOCIATION)
    4.  BRAIN BODY patterns (emotional-behavioral-physiologic) of how we response to Stress  often develop early in life in our relationship with our primary caregivers.
    5.  STRESS RESPONSE is sensitized with sustained stress (COMPLEX TRAUMA) and often triggers severe system-wide inflammation and other disease-promoting pathways.


McEwen (2007) Physiology and neurobiology of stress and adaptation:  central role of the brain.  Physiol Rev.  CLASSIC Resource.  Describes why the brain and sensory organs are key to the response to stress - and why the STRESS RESPONSE is both adaptive and maladaptive.  Under chronic complex stress, the brain and body are remodeled - resulting in patterns we diagnose as disease.

Chrousos (2009) Stress and disorders of the stress system.  Nature Reviews Endocrinology.  Stress occurs when the Brain-Body balance (homeostasis) is threatened.  Sustained stress affects thinking (cognition, executive function), reward-fear, wake-sleep, and all parts of the endocrine, GI, cardiac, respiratory, metabolic and immune systems.  Excess activation of the stress system can impair development, body composition, and lead to a host of behavioral or physical illness.

Klatzkin (2019) Physiologic response to acute stress and the drive to eatAppetite.  The stress-eating relationship is mediated by the STRESS RESPONSE.  Understanding the physiologic mechanisms is critical to prevent and treat obesity.