Stress: Dynamic state within the body resulting from the interaction of the body with the stressor.
Stressors: Stimuli that impose strain on body homeostasis.
Types of stressors:
- orthostasis (upright standing position)
- loud noise
- air polution
- bright light
- peer pressure
- fitting in
Body’s Response to Stress
- Nocireceptor: pain
- Thermo: temperature
- Mechano: physical
- Baro: blood pressure
- Chemo: glucose, acid, O2
- In Special senses: sight, smell
Ascending sensory pathways:
Action potential go to..
- -> hypothalamus (neuroendocrine response)
- -> somatic sensory cortex (perception/stress)
- -> motor cortex (voluntary action response)
- -> limbic cortex (emotions)
- Increase in CRH – ACTH
- Increase in GHRH, Decrease in somatostatin– Growth Hormone
- Decrease in PIH – Increase in Prolactin
- Decrease in TRH – TSH
- Decrease in GnRH – Gonadotropin
Increase ACTH, GH & Prolaction
Mental stress: ACTH is high.
Physical stress: Cold pressor test (immersion of hand in cold water for 3 minutes). Blood pressure & Heart rate Increased.
Flight Fight Reaction (hypothalamo-sympatho-adrenalmedullary axis activation)
Stressor: External stress
Afferent Sensory Pathway: Limbic cortex (perception of stress)
Efferent Sensory Pathway:
- sympathetic nervous system –>Hypothalamus (release noradrenaline)
- Adrenal medulla (release A, NA, dopamine & opioid peptides) prolong effects of hypothalamus (EMERGENCY FUNCTION)
Target organs: Skeletal muscles
- Increased energy demands by skeletal muscles
- more fuel & O2 needed for aerobic production of ATP
- More blood flow to skeletal msucles (supply o2, fuels, elimination of products of metabolism & heat)
Effects of sympatho-adrenal axis activation (fight flight/LION rule):
- increase HR & contractility (increase stroke volume)
- vasoconstriction (increase TPR)
- vasoconstriction in skin & splanchnic regions (diversion of blood flow to skeletal muscles, heart muscles & brain)
- Venoconstriction & skeletal muscle pump (increase venous return)
- Accumulation of vasodilator metabolites in skeletal muscles (vasodilation in skeletal muscles)
- bronchodilation (decrease airway resistance –> increase tidal volume)
- increase O2 supply –> increase aerobic metabolism –> increase ATP production for skeletal muscle activity.
Increase fuel supply for ATP production
- muscle glycogenolysis –> increase glucose for local use (muscle)
- liver glycogenolysis –> increase plasma glucose
- lipolysis –> increase plasma FA
- Decrease GIT activity –> less energy demand
- Renal vasoconstriction
- Activation of brainstem reticular activating system
Pupil dilation (mydriasis)
- Stimulate dilator pupillae
Decrease bleeding/heat loss
- Skin vasoconstriction
- Release of opioid peptides by adrenal medulla
- decrease pain transmission & perception (stress analgesia)
General Adaptation Syndrome (Hypothalamo-pituitary-adrenocortical axis activation)
Based on 1st graph,
Alarm state/Compensation phase:
- When the stressor is noticed, there will be onset of shock and decreased resistance.
- During this time, both sympatho-adrenal & hypothalamo-pituitary-adrenocortical axis are activated.
- Resistance to the stressor increased as animal adapted & coped with it
- Lasts for as long as the animal can support this heightened resistance.
- Resistance has exhausted
- emergency function of sympatho-adrenal axis
Noradrenalin & Adrenaline
- equally potent in increasing alertness (adrenaline evokes more anxiety & fear)
- Known familiar stress –> more Noradrenaline
- Unknown stress – > more Adrenaline
- Stimulate protein catabolism –> liver uptake of amino acids –> conversion to glucose (gluconeogenesis)
- **Anti-insulin action on muscle (own local supply) & adipose tissue (inhibit uptake of glucose)
- Increase glucose supply to brain
- TG breakdown –> glycerol + FA (fuel for muscle activity)
- Increase plasma AA, glucose, glycerol & FA
- Permissive effects (gives permission) for:
-Catecholamines (vasoconstriction, bronchodilation, lipolysis)
- Antiallergic effect
- Increase RBC count
- Increase neutrophil count
Due to the permissive actions for catecholamines & glucagon, secretion of ACTH & glucocorticoids (cortisol) are essential for survival when stress is severe.
- Angiotensin 2: generalised vasoconstriction
- Aldosterone: Na+ & water reabsorption in distal nephron (increase blood volume & BP) li>
- Increase ACTH secretion
- Increase water reabsorption (increase blood volume & BP)
- Generalised vasoconstriction
- Liver glycogenolysis (in large amounts)
- Lipolysis (FA for muscle activity)
- Anti-insulin action (decrease glucose uptake in peripheral tissues, direct to brain)
Effect of stress on performance
Therefore, moderate stress is advantageous for optimum performance.
Effect of PROLONGED stress on health
- Repeated episodes can cause cardiovascular disease.
- Glucocorticoids in large amounts can suppress the immune system and increase protein catabolisme (muscle loss)