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Anxiety is the normal emotional response to a threatening situation. It has motor accompaniments in flight and freezing, that avoid or minimise the threat, and sympathetic autonomic system activity. Anxiety states are defined as the presence of anxiety in a situation or to a degree where it becomes maladaptive. Simple states are agoraphobia, a fear of crowds, claustrophobia, fear of enclosed spaces eg lifts, and stage fright. Very common are fears of spiders and snakes or small rodents, which can be interpreted as useful on an evolutionary timescale but can become disabling in degree. Such fear reactions can be learned and unlearned according to Pavlovian and operant principles which have been applied with considerable success in standardised treatment regimes. There is a substantial body of work on animal models, and the general principles apply across mammalian species.
Although anxiety is commonly a feature of psychotic illness (manic-depressive or schizophrenic) anxiety states do not generally include psychotic symptoms. There is some overlap with obsessive compulsive illness. Anxiety states may sometimes have an abrupt onset in life-threatening situations, the traumatic stress disorders. These too have counterparts in the experimental animal literature.
Anxiety is characterized by the physiological components of activation of the sympathetic autonomic system, and behavioral inclination to flight ie avoidance. Thus an increase in heart rate, blood pressure, sweat secretion, together with activation of skeletal and inhibition of gastro-intestinal activity and secretion. These are the normal and adaptive responses to threat.
Anxiety states represent the maladaptive expression of this response constellation. Thus fear reactions to snakes, spiders, heights and the dark can be understood as evolutionary adaptations, perhaps genetically based. Expression of such responses in situations in which they interfere with the individual's rational sequence of behavior is classified as a phobia. More abstract fears, for example of small rooms, going in lifts, of open spaces or crowds, are less easily understood as products of evolution, but can also be considered as conditioned to specific stimuli according to Pavlovian principles. In so far as specific stimuli or situations can be identified behavioral treatments based on such principles enjoy significant success.
Phobias and other specific anxiety states are easily understood as anomalies of physiological mechanisms that cross species boundaries. More difficult to comprehend in this way are generalized anxiety states, where the level of vigilance to threat is raised apparently indiscriminately. The recent literature indicates that such states often coincide with features of depression.
Of all psychiatric syndromes anxiety states are easiest to relate to primate and mammalian models. The literature on the conditioning of fear and avoidance constitutes a substantial fraction of experimental psychology. Questions remain as to whether anxiety states, particularly of a generlized type, occur spontaneously in non-human populations, and whether the dimensions of anxiety proneness, and its heritability are the same in humans and other animals. Does the introduction of language entail a new principle of inter-individual variation with respect to the conditioning of fear responses?
Anxiety states are present in all human populations, at probably similar prevalences.
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