Ambiguity of the word

Stress is a term that is commonly used today but has become increasingly difficult to define. It shares, to some extent, common meanings in both the biological and psychological sciences.
The term stress had none of its contemporary connotations before the 1920s. It is a form of the Middle English destresse, derived via Old French from the Latin stringere, "to draw tight."[1] It had long been in use in physics to refer to the internal distribution of a force exerted on a material body, resulting in strain. In the 1920s and 1930s, the term was occasionally being used in biological and psychological circles to refer to a mental strain, unwelcome happening, or, more medically, a harmful environmental agent that could cause illness. Walter Cannon used it in 1926 to refer to external factors that disrupted what he called homeostasis.[2]
Homeostasis is a concept central to the idea of stress. In biology, most biochemical processes strive to maintain equilibrium, a steady state that exists more as an ideal and less as an achievable condition. Environmental factors, internal or external stimuli, continually disrupt homeostasis; an organism’s present condition is a state in constant flux wavering about a homeostatic point that is that organism’s optimal condition for living. Factors causing an organism’s condition to waver away from homeostasis can be interpreted as stress. A life-threatening situation such as a physical insult or prolonged starvation can greatly disrupt homeostasis. On the other hand, an organism’s effortful attempt at restoring conditions back to or near homeostasis, oftentimes consuming energy and natural resources, can also be interpreted as stress. In such instances, an organism’s fight-or-flight response recruits the body’s energy stores and focuses attention to overcome the challenge at hand. The ambiguity in defining this phenomenon was first recognized by Hans Selye in 1926 who loosely described stress as something that "…in addition to being itself, was also the cause of itself, and the result of itself."[3] First to use the term in a biological context, Selye continued to define stress as "the non-specific response of the body to any demand placed upon it." Present-day neuroscientists including Bruce McEwen and Jaap Koolhaas believe that stress, based on years of empirical research, "should be restricted to conditions where an environmental demand exceeds the natural regulatory capacity of an organism."[4] Despite the numerous definitions given to stress, homeostasis appears to lie at its core.
Biology has progressed in this field greatly, elucidating complex biochemical mechanisms that appear to underlie diverse aspects of stress, shining a necessary light on its clinical relevance and significance. Despite this, science still runs into the problem of not being able to settle or agree on conceptual and operational definitions of stress. Because stress is ultimately perceived as a subjective experience, it follows that its definition perhaps ought to remain fluid. For a concept so ambiguous and difficult to define, stress nevertheless plays an obvious and predominant role in the everyday lives of humans and nature alike.
[edit]Biological background
Biology primarily attempts to explain major concepts of stress in a stimulus-response manner, much like how a psychobiological sensory system operates. The central nervous system (brain and spinal cord) plays a crucial role in the body’s stress-related mechanisms. Whether these mechanisms ought to be interpreted as the body’s response to a stressor or embody the act of stress itself is part of the ambiguity in defining what exactly stress is. Nevertheless, the central nervous system works closely with the body’s endocrine system to regulate these mechanisms. One branch of the central nervous system, the sympathetic nervous system, becomes primarily active during a stress response, regulating many of the body’s physiological functions in ways that ought to make an organism more adaptive to its environment. Below is a brief biological background of the neuroanatomy and neurochemistry and how they relate to stress.
[edit]Neuroanatomy

[edit]Brain
The brain plays a critical role in the body’s perception of and response to stress. However, pinpointing exactly which regions of the brain are responsible for particular aspects of a stress response is difficult and often unclear. Understanding that the brain works in more of a network-like fashion carrying information about a stressful situation across regions of the brain (from cortical sensory areas to more basal structures and vice versa) can help explain how stress and its negative consequences are heavily rooted in neural communication dysfunction. In spite of this, several important brain structures implicated in playing key roles in stress response pathways are described below.

Human brain (hypothalamus=red, amygdala=green, hippocampus/fornix=blue, pons=gold, pituitary gland=pink)
[edit]Hypothalamus
The hypothalamus is a small portion of the brain located "below the thalamus" and above the brainstem. One of its most important functions is to help link together the body’s nervous and endocrine systems. This structure has many bidirectional neural inputs and outputs from and to various other brain regions. These connections help regulate the hypothalamus’ ability to secrete hormones into the body’s blood stream, having far-reaching and long-lasting effects on physiological processes such as metabolism. During a stress response, the hypothalamus secretes various hormones, namely corticotropin-releasing hormone, which stimulates the body’s pituitary gland and initiates a heavily regulated stress response pathway.
[edit]Amygdala
The amygdala is a small, "almond"-shaped structure located bilaterally, deep within the medial temporal lobes of the brain and is a part of the brain’s limbic system, with projections to and from the hypothalamus, hippocampus, and locus coeruleus, among other areas. Thought to play a role in the processing of emotions, the amygdala has been implicated in modulating stress response mechanisms, particularly when feelings of anxiety or fear is involved.
[edit]Hippocampus
The hippocampus is a structure located bilaterally, deep within the medial temporal lobes of the brain, just lateral to each amygdala, and is a part of the brain’s limbic system. The hippocampus is thought to play an important role in memory formation. There are numerous connections to the hippocampus from the cerebral cortex, hypothalamus, and amygdala, among other regions. During stress, the hippocampus is particularly important, in that cognitive processes such as prior memories can have a great influence on enhancing, suppressing, or even independently generating a stress response. The hippocampus is also an area in the brain that is susceptible to damage brought upon by chronic stress.
[edit]Locus coeruleus
The locus coeruleus is an area located in the pons of the brainstem that is the principal site of the synthesis of the neurotransmitter norepinephrine, which plays an important role in the sympathetic nervous system’s fight-or-flight response to stress. This area receives input from the hypothalamus, amygdala, and raphe nucleus among other regions and projects widely across the brain as well as to the spinal cord.
[edit]Raphe nucleus
The raphe nucleus is an area located in the pons of the brainstem that is the principal site of the synthesis of the neurotransmitter serotonin, which plays an important role in mood regulation, particularly when stress is associated with depression and anxiety. Projections extend from this region to widespread areas across the brain, namely the hypothalamus, and are thought to modulate an organism's circadian rhythm and sensation of pain among other processes.

Human spinal cord
[edit]Spinal cord
The spinal cord plays a critical role in transferring stress response neural impulses from the brain to the rest of the body. In addition to the neuroendocrine blood hormone signaling system initiated by the hypothalamus, the spinal cord communicates with the rest of the body by innervating the peripheral nervous system. Certain nerves that belong to the sympathetic branch of the central nervous system exit the spinal cord and stimulate peripheral nerves, which in turn engage the body’s major organs and muscles in a fight-or-flight manner.
[edit]Pituitary gland
The pituitary gland is a small organ that is located at the base of the brain just under the hypothalamus. This gland releases various hormones that play significant roles in regulating homeostasis. During a stress response, the pituitary gland releases hormones into the blood stream, namely adrenocorticotropic hormone, which modulates a heavily regulated stress response system

Adrenal gland
[edit]Adrenal gland
The adrenal gland is a major organ of the endocrine system that is located directly on top of the kidneys and is chiefly responsible for the synthesis of stress hormones that are released into the blood stream during a stress response. Cortisol is the major stress hormone released by the adrenal gland.
In addition to the locus coeruleus existing as a source of the neurotransmitter norepinephrine within the central nervous system, the adrenal gland can also release norepinephrine during a stress response into the body’s blood stream, at which point norepinephrine acts as a hormone in the endocrine system.
[edit]Neurochemistry