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                     Physiology of Stress                              

Stress is the state resulting from pressure applied to an organism or a structure. Pressure can be caused in humans by external or internal demands called stressors.

The result is a chain of nervous or hormonal reactions destined to trigger the “fight or flight” survival mechanism.

Stressors can be external and originate from work, social, environmental, financial or family demands that are difficult to meet. They can be internal, initiated by self-obligations, guilt, anxiety, self-criticism, conflicting values, illnesses, psychological strain.

Stress is very positive when it pushes people to more creativity and solutions by marshalling their resources; it also can bring an athlete to victory, and such stress is called “eustress”.

When stress is a sustained condition it is a permanent state of the “fight or flight” reaction that can have very harmful consequences on the body.

Everyone has his own way of reacting to stress with a mixture of the following symptoms, some being predominant in a given individual:

• Cardiac acceleration      

• Rise of blood pressure      

• Faster breathing      

• Release of hormones:  ACTH, cortisol, adrenaline, ocytocin, vasopressin      

• Muscle fatigue and tension, especially in the back (lumbar region)  and neck      

• Redirection of blood flow: from brain and viscera to legs and arms to fight or run      

• Hypersensitivity to noise, smells, contact, emotional irritability, anxiety, depression

• Abundant sweat      

• Hormonal imbalance causing a dampening of the immune system and a proneness to get contaminated by germs, to be victim to cancer or an auto-immune form of illness

• Stress triggers a cascade of hormonal secretions and complex nervous mechanisms involving different brain structures connected to the pituitary (hypophyse) and adrenal glands.

Structures involved in the stress response 

A: Nervous mechanisms

At the cortical brain level (conscious intelligent brain):      

• Sensory areas of the cortex receiving information from the thalamus      

• The temporal lobe for the recall of  past experiences (implicit memory)

At the limbic brain level (emotional or visceral brain) (composed of thalamus, hypothalamus, amygdala, hippocampus, involved in primitive urges and drives such as sexual arousal, anger, aggression, reward etc.) :

• The thalamus:  (processing center for incoming messages from the sensory organs -  olfactive, visual, auditory, kinaesthetic - that are then dispatched to specific areas of the cortex  or conscious brain

• The amygdala: (command center receiving rough and fast information from the thalamus for immediate action)      

• The hippocampus: (memory integration center - receives from the cortex, in a delayed way, information essential to the processing of new conscious memories about recent events [explicit memory])      

• The hypothalamus: sensitive to emotional stressors, fires the hormones or neurotransmitters necessary for the “fight or flight” response through activation of the autonomic nervous system (ANS)      

• The hypothalamus, through connections to the locus coeruleus, can also stimulate the secretion of norepinephrine (noradrenalin), an essential neurotransmitter in the central nervous system (CNS)      

• The hypothalamus can activate metabolism by stimulation of the thyroid to produce thyroxin

At this level stress is triggered by hormones (cortisol, adrenalin, noradrenalin from the adrenal gland) causing the release of neurotransmitters that activate or block the transmission of nerve influx in the synapses.

Neurotransmitters can be classified as:

• Inducing good feelings - endorphins (opioïds), exorphins (opioïds derived from food or drugs), noradrenalin (norepinephrin), dopamine, acetylcholine, phenylethylmine

• Inhibiting pain or anxiety - enkephalins, GABA (Gamma Amino Butyric Acid)

• Also having a hormonal activity - serotonin, melatonin, oxytocin

B: Hormonal secretions

At the hormonal level:

The pituitary gland (hypophyse): situated below the hypothalamus in the middle base of the brain, it is divided into anterior and posterior pituitary:

Ø       The anterior part of the pituitary is controlled by the hypothalamus through the release of an endocrine (released into the blood) hormone CRH (corticotrophin releasing hormone), that in its turn provokes the release of ACTH (adrenocorticotrophic hormone), another hormone active on the adrenal gland situated at the top of the kidney where three different hormones can be released.

Ø       The posterior part of the pituitary is directly under nervous control from the hypothalamus and, when activated, releases vasopressine (the antidiuretic hormone that controls the resorption of water) and oxytocin (the “love and social behaviour hormone”).

Ø       The adrenal gland: divided into three layers, it secretes three different types of hormones called sexual hormones, glucocorticoids and mineralocorticoids  which can rise to dangerous levels under stressful conditions:

• The outer zone secretes aldosterone (an antidiuretic-retaining salt hormone), regulating blood pressure - an excess of aldosterone causes hypertension and a rise of potassium level in the blood with considerable risk for the heart.

• The middle zone secretes cortisol which is released into the bloodstream to stimulate the catabolism of lipids and proteins and thus produce energy by:

Ø    activating gluconeogenese and dampening insulin to provide sugar to muscles

Ø       improving the contractility of the heart and peripheric microcirculation

• The inner zone secretes sexual hormones: (decreased levels under stress) DHEA, testosterone, oestrogens.

 Consequences of prolonged stress

Stress causes the production and release of cortisol and adrenaline by the adrenal gland. This is an adaptive reaction during the initial period of stress in order to mobilize energy to deal with a critical situation. During this early stage of stress a negative feedback process is also exerted by cortisol on the secretion of the corticotrophin releasing factor at the hypothalamus level, causing - in the end - a lowering of the circulating cortisol.

However, when stress becomes chronic this negative feedback mechanism loses its efficiency and dire consequences appear with high levels of cortisol:      

• Hypertension, coronary disease      

• Diabetes, obesity, muscle loss, osteoporosis      

• Depletion of the immune system, cause of cancer and contagious diseases      

• Brain damage by atrophy of the hippocampus, harming the free recall of information with impairment of the judging and decision-making processes. This alteration can be reversed at the onset but addressing the problem too late can lead to irremediable brain atrophy.