ABSTRACT
The nature o f stress Much of the material from which this chapter is derived comes from the work of Fraser (1983a) published as a monogram by the International Labour Office, Geneva, outlining the interrelationships among stress, work, and job satisfaction. Because of its connotations and the diversity of its usage, stress has always been a difficult term to define. To some, stress describes the state of a physical body which has been subjected to pressures or forces close to or beyond its tolerance; to others, the term describes the phenomena which produce these pressures of forces. Stress can be seen as a physical entity associated with physical changes; or it is subjective and associated with psychological and emotional conditions. To some, stress and strain are synonymous; to others, they are descriptive of cause and effect.Regardless of whether one refers to the condition in man as stress or strain, there is a tendency among many to consider it as pathological human response to psychological, social, occupational and/or environmental pressures. This, however, is not so. Hans Selye, the founder of stress physiology, has stated (Selye, 1974): ‘Contrary to widespread belief, stress is not simply nervous tension nor the result of damage. Above all, stress is not necessarily something to be avoided. It is associated with the expression of all our innate drives. Stress ensues as long as a demand is made in any part of the body. Indeed, complete freedom from stress is death!’ Consistent with this viewpoint, although perhaps inconsistent in terminology, the International Symposium on Society, Stress and Disease, sponsored by the World Health Organization and the University of Uppsala at Stockholm, 1972, adopted as a definition the statement that: ‘Stress is the non-specific response of the organism to any demand made of it’, thus placing themselves in the camp that considers that stress is the response and not the causative state.While perhaps the point is academic, pursuit of which may lead to
fruitless semantic discussion, the viewpoint that stress is the resulting and not the causative state is in direct opposition to the view of the physical scientists and engineers who originally defined the term. In fact, even Selye himself has publicly stated (Selye, 1973) that where he originally used the term stress he should have used strain. Perhaps, however, the term in its above-defined meaning is now too well established in psychophysiology and psychosomatic medicine to be changed. In physical terms, however stress exists when a force is applied to distort a body. The effect is manifest as elastic or non-elastic distortion and is measurable as strain. The relationship between stress and strain can be plotted in the form of a curve, as shown in Figure 7.1.The foregoing curve demonstrates that, with the initial application of stress to an elastic object, some unmeasurable change will occur in the object, probably not rectilinear in its relationship to the causative stress. As the stress continues there is a measurable strain directly proportional to the stress. This is the region of elastic distortion, and if the stress applied to this level is removed then the strain will also disappear. In other words, at this level conditions are reversible. There is some point, however, at which the stress is such that there is no longer a direct proportional relationship between stress and strain. The stressed object changes its character and the resulting strain is not reversible. An elastic band, for example, will stretch beyond recovery.There is a psycho-physiological analogue of this stress/strain relationship. The initial portion of the curve represents a non-measurable perhaps cellular or biochemical, response to the mildest of human stress. As the stress increases, either in intensity or duration, various physiological and psychological changes can be observed in relation to the stress, which
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are reversible when the stress is removed. With still further increase in stress we enter the realm of pathology, beyond the level of adaptation, where the changes are not reversible, and some trauma, mental or physical, results.By this analogy, stress describes the stimulus state, and strain the response, although the difference is more one of usage than meaning. Homeostasis and feedback control Regardless of the semantics involved, to examine the psycho-physiological mechanisms of human stress it is necessary first to examine the mechanisms responsible for control of the human organism in the relatively unstressed state.As noted earlier, man is a complex of systems and sub-systems. Coordinated control is exerted over this complex to maintain it in a state of dynamic equilibrium to which is given the name homeostasis (Greek: ‘steady state’). The major purpose of human physiological function is to maintain homeostasis, or more specifically, to maintain the internal environment of the body in a state of chemical and thermal stability in the face of constant change, or threat of change.Some of this control is inherent in the individual cell, which is itself , a complex system, the functions of which can be modified by specialized chemicals, or horm ones, secreted by glands of the endocrine system . Overall control and co-ordination of the entire system is exerted via the central nervous system, through the network of the autonom ic nervous system.The principles of control are the same as for any other system of interactive variables. Preset standards are established by the autonomic nervous system at levels within which a variable may function. The resulting function is monitored by a sensory mechanism, again by way of the autonomic nervous system, to various co-ordinating centres deep in the brain. In these centres incoming information is compared with the preset standards. If significant variation is found, instructions are issued via the nervous system and glands to effect appropriate change in function. There is thus a closed-loop operation with an input, some means of processing, and an output, which in turn affects the input. The loop, and other interacting loops, are activated to reach and maintain equilibrium. Thus, there are acceptable limits within which the body function may be permitted to vary. Stress, however, provides the stimulus for change in those limits, or, in other words, for adaptation. N eu ro -en d o crine control system
The ultimate co-ordination and control of involuntary bodily functions comes under the direction of the complex of nerve centres, nerve fibres, and glands known collectively as the neuro-endocrine system . This system
controls and/or co-ordinates all metabolism, other than that inherent in the individual cell, and all bodily function including the co-ordination of some of the function that is conducted voluntarily, for example, ensuring an appropriate blood supply for muscles engaged in voluntary movement. It is therefore responsible for maintaining homeostasis in the face of internal and external demand, including the demands of stress.The system in turn comprises two sub-systems, namely, the autonomic nervous system and the endocrine system. A u tonom ic nervous system
The autonomic nervous system is a functional system of interlinked nerve cell clusters, or centres, in the base of the brain, with an input and output network of nerve fibres leading to and from these centres and bearing information about, or instructions concerning, such matters as heart rate, blood pressure, breathing rate, muscle tone, sweat production, urinary output, body temperature, and so on. There are, for example, respiratory centres, vascular (vasomotor) centres, thermal regulation centres, even entities like cough centres, along with their associated networks.The autonomic nervous system can be further divided into two parts, namely the sym pathetic nervous system and the parasym pathetic nervous system. In general, but not always, the sympathetic nervous system serves to increase a given function, while the parasympathetic nervous system serves to decrease it. In practice they work together in opposition in a kind of ‘push-pull’ manner.The sympathetic nervous system derives from centres in the brain and sends (or receives) nerve fibres to the spinal cord and thence into the chest. There they form two chains of nerves which proceed downwards, one on each side of the spinal column. These chains send out nerve fibres drawing from the different centres in the brain which unite in a multiplicity of relays called ganglions. Fibres from the ganglions (postganglionic fibres) pass to the various body organs or parts. On stimulation these fibres release a chemical called norepinephrine (sometimes known by the proprietary name of noradrenaline) at the end of the fibre. This norepinephrine then acts either to stimulate another fibre at a nerve junction, or synapse, for onward transmission, or acts directly on the organ itself to increase its activity. The effects normally last only a few seconds or less. If the norepinephrine were permitted to remain at the site the effects would go on until the material was exhausted. Normally this is not desirable, and consequently the norepinephrine is destroyed as soon as it is formed. This is acccomplished by the presence of an enzyme, or chemical catalyst, called orthom ethyltransferase. If the initial stimulus is sufficiently strong, however, it also stimulates one of the glands of the endocrine system, namely the adrenal gland (see later), to produce epinephrine (adrenaline) the effects of which last for minutes.
