Stress Response Mechanisms PDF

STRESS!  Chapter 26: Stress Recall: Homeostasis – the state in which all systems are in balance at a particular ideal “set- point”. Allostasis – the overall process of adaptive change necessary to maintain survival and well-being Stress – the sum of biological reaction...

STRESS!  Chapter 26: Stress Recall: Homeostasis – the state in which all systems are in balance at a particular ideal “set- point”. Allostasis – the overall process of adaptive change necessary to maintain survival and well-being Stress – the sum of biological reactions produced when an organism’s homeostasis is disrupted Stressors – agents or conditions that are capable of producing stress and endangering homeostasis, and initiating allostasis in order to return to a stable state General Adaptation Syndrome (GAS) In the 1920s, Hans Selye theorized the General Adaptation Syndrome (GAS) Three physiologic states of GAS 1. Alarm stage 2. Resistance stage 3. Exhaustion stage GAS: Alarm Stage Known as the “fight-or-flight response”  provides a surge of energy and physical alterations to either run from or confront danger Hypothalamus senses a need to activate the GAS in response to a stressor that puts homeostasis at risk (either internal or external, physical or emotional, positive or negative…) Hypothalamus then mediates the activation of the sympathetic nervous system (SNS) which then stimulates the adrenal medulla Catecholamines are released which enable the body to fight or flee Alarm stage continued The hypothalamus also secretes corticotropic-releasing hormone (CRH) which stimulates the anterior pituitary gland to secrete adrenocorticotropic hormone (ACTH)  acts on the adrenal cortex to release lots of glucocorticoids, especially cortisol The above cascade is termed the “hypothalamic-pituitary-adrenal (HPA) axis” Once the pituitary gland is active, the alarm stage will occur in full force; it cannot be deactivated  If the alarm stage continued for long periods of time, the body would experience permanent damage SNS, adrenal medulla and cortex are functioning full force when the body moves into the resistance stage GAS: Resistance Stage Resistance Stage involves the allostatic return to homeostasis Levels of SNS activity, catecholamine and glucocorticoid secretion are normalized If the stressors are adequately addressed and resolved, the body returns to a steady state Thurs, the body has “adapted” or “increased resistance” to the stressor GAS: Exhaustion Exhaustion occurs when the body is no longer able to return to homeostasis When adaptive energy stores are depleted, the body is no longer able to adapt Pathologic triad  hypertrophy of adrenal glands, atrophy of lymphoid tissues, and bleeding GI ulcers Death can occur if homeostasis is not returned PROBLEM!! GAS theory fails to explain that stress is frequently psychosocial or psychological rather than physical Concept of allostasis is able to include psychosocial stress-related illness ALLOSTASIS = the process of achieving stability, or homeostasis through physiological and/or behavioural change Allostatic load – collective effect of an allostatic state on the organism; the state during which the mechanisms that facilitate the adjustments to life’s demands are chronically over- or underactivated resulting in wear and tear Allostatic overload – occurs if the allostatic load is chronic or excessive; reflects the “cost” to the body’s organs or tissues for an allostatic response that is excessive or ineffectively regulated  Remember: stressors initiate allostasis  can be external (air pollution, car accident) or internal (low blood sugar, feelings of threat), physical (burn, injury) or chemical (car exhaust) or biological (viruses) or social (bad relationship) or cultural (behavioural norms) or psychological (feelings of sadness) Emotional stressors can be present or anticipated Stressors can be low intensity, but can have significant impact over time Risk factors- not stressors but rather conditions or situations that increase the likelihood of encountering or experiencing a stressor (ex. eating while driving increases the risk of getting into a car accident) Neurohormones of stress and adaptation Catecholamines (neurotransmitters)  norepinephrine (NE) and epinephrine (Epi) Released from some neurons of the CNS in response to stressors NE is released by sympathetic neurons near target/effector organs and tissues NE and Epi are released from the adrenal medulla These catecholamines are released into the bloodstream and enhance/support the sympathetic nervous system Norepinephrine and epinephrine Norepinephrine is the primary constrictor of smooth muscle in all blood vessels; therefore it regulates blood flow through tissues/organs and maintains blood pressure Reduces GI secretion and motility, and dilates the iris and relaxes ciliary muscles of the eye to increase night and far vision Epinephrine increases heart rate, myocardial contractility and venous return to the heart  raises cardiac output and blood pressure Also increases glycogenolysis and release of glucose from liver; increases blood flow to brain to heighten mental attention and alertness More neurohormones Adrenocortical steroids  cortisol and aldosterone Have regulatory roles in cardiovascular system and fluid volume, contribute to metabolism, immunity and inflammatory response, brain function and reproduction Glucocorticoids are lipid-soluble hormones Onset is slower than catecholamines, but their effect lasts longer Glucocorticoids also support the catecholamines by promoting epinephrine synthesis by the adrenal medulla and helping to control blood pressure and cardiac output Cortisol Primary glucocorticoid is cortisol and is secreted by the adrenal cortex in response to ACTH from the anterior pituitary Cortisol increases rate of protein synthesis in the liver; but has a catabolic effect on muscle, lymphoid, adipose tissues, skin and bone (proteins are broken into amino acids to ensure availability for the liver) Stimulates gluconeogenesis in the liver (ensures glucose availability for body tissues, especially nerve cells Also promotes appetite Suppresses the acute-phase response to infection and inflammation Aldosterone Primary mineralocorticoid is aldosterone and is secreted by the adrenal cortex in response to stimulation of the SNS Usually is released in response to fluid volume depletion  acts on kidneys to enhance reabsorption of sodium, therefore increasing ECF volume and increasing blood pressure More neurohormones Stress naturally inhibits pain through the release of small peptides called endorphins and enkephalins Endogenous opioids are produced in the CNS and are released in response to stressors, by certain foods, laughter, massage and acupuncture Endorphins raise the pain threshold and produce sedation and euphoria More neurohormones Sex hormones effect stress and vice versa For example, cortisol has inhibiting effects on the female reproductive system by suppressing release of luteinizing hormone, estradiol and progesterone Androgens, such as testosterone and dehydroepiandrosterone(DHEA) inhibit the catabolic effects on the bone and decrease the depressive tendencies associated with glucocorticoids Stress often lowers the levels of testosterone  In combination with the hormone vasopressin, testosterone enhances blood pressure and heart rate  fight or flight response Estrogen combined with the hormone oxytocin produces a calming effect during times of stress Oxytocin is also produced during childbirth, lactation and orgasm (in men too!) and promotes bonding and social attachment  thought to lower the stress response, calm and reduce perceived anxiety  Other hormones like growth hormone are released during intensely stressful physical or psychological stimuli, like strenuous exercise or extreme fear Growth hormone is released from the anterior pituitary gland and increases protein synthesis and fat mobilization and decreases carbohydrate utilization; also enhances immune function Prolactin is also released from the anterior pituitary gland in response to stress, sexual activity and breast feeding  function similar to growth hormone including immune regulation Allostatic overload When adaptive mechanisms are inadequate , allostatic overload occurs. There are many physiologic and psychological effects of excessive stress