13.2 Stress responses and allostasis

Stress responses and allostasis are crucial for survival, helping organisms adapt to challenges. The body activates complex systems to maintain balance, including the HPA axis and sympathetic nervous system, which release hormones like cortisol and adrenaline.

Chronic stress can lead to allostatic overload, causing health problems. Understanding these processes is key to grasping how animals regulate their physiology and adapt to environmental changes, a central theme in this unit on integrating physiological systems.

Stress and Allostasis

Defining Stress and Allostasis

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• Stress refers to a state of threatened homeostasis caused by intrinsic or extrinsic adverse forces (stressors)
• Stressor is a stimulus or event that triggers a stress response in an organism
  • Can be physical (injury, infection) or psychological (anxiety, fear)
• Allostasis is the process of achieving stability through physiological or behavioral change in response to stressors
  • Maintains homeostasis by adjusting parameters of the internal milieu by matching them appropriately to environmental demands
• Allostatic load refers to the cumulative physiological consequences of chronic exposure to stress
  • Repeated or prolonged activation of allostatic systems can lead to wear and tear on the body and brain (allostatic overload)

Consequences of Chronic Stress

• Chronic stress and allostatic overload can have detrimental effects on health
  • Increases risk of cardiovascular disease (hypertension, atherosclerosis)
  • Suppresses immune function, increasing susceptibility to infections and cancer
  • Contributes to metabolic disorders (obesity, insulin resistance, type 2 diabetes)
  • Affects brain structure and function, leading to cognitive impairment and mental health issues (depression, anxiety disorders)

Neuroendocrine Stress Response

Hypothalamic-Pituitary-Adrenal (HPA) Axis

• The HPA axis is a major neuroendocrine system involved in the stress response
  • Hypothalamus releases corticotropin-releasing hormone (CRH), which stimulates the anterior pituitary gland
  • Anterior pituitary secretes adrenocorticotropic hormone (ACTH) into the bloodstream
  • ACTH stimulates the adrenal cortex to produce and release glucocorticoids, primarily cortisol
• Cortisol is a steroid hormone that plays a key role in the stress response
  • Increases blood glucose levels by promoting gluconeogenesis and glycogenolysis
  • Suppresses immune function and inflammation
  • Enhances cardiovascular function (increases heart rate and blood pressure)
  • Modulates brain function and behavior (enhances alertness, attention, and memory formation)
• Glucocorticoids, including cortisol, exert their effects by binding to glucocorticoid receptors in target tissues
  • Regulate gene expression and modulate cellular processes
  • Negative feedback inhibits further release of CRH and ACTH, preventing excessive glucocorticoid production

Sympathetic Nervous System Stress Response

Catecholamines and the Fight-or-Flight Response

• The sympathetic nervous system (SNS) is activated during stress, leading to the release of catecholamines
• Epinephrine (adrenaline) is released from the adrenal medulla in response to SNS activation
  • Increases heart rate and contractility, dilates bronchioles, and constricts blood vessels in the skin and gut
  • Mobilizes glucose and fatty acids from storage for energy production
• Norepinephrine (noradrenaline) is released from sympathetic nerve terminals and the adrenal medulla
  • Similar effects to epinephrine, but primarily acts as a neurotransmitter in the brain and peripheral nervous system
  • Enhances arousal, alertness, and attention
• The fight-or-flight response is a physiological reaction that occurs in response to a perceived threat or stressor
  • Prepares the body for action by increasing heart rate, blood pressure, and blood glucose levels
  • Diverts blood flow to skeletal muscles and the brain, while reducing blood flow to the digestive system and other non-essential organs
  • Enhances sensory perception, reaction time, and cognitive function to facilitate rapid decision-making and action

Interactions between the HPA Axis and SNS

• The HPA axis and SNS work together to coordinate the stress response
  • CRH and norepinephrine stimulate each other's release, creating a positive feedback loop
  • Glucocorticoids enhance the effects of catecholamines on target tissues
  • Chronic activation of both systems can lead to allostatic overload and adverse health consequences