As a mood or affect or emotional state, depression is part of normal human experience. Feelings of sadness and disappointment are within the vicissitudes of the normal human condition. The distinction between normal mood and abnormal depression is not always clear, although considerable research on diagnostic criteria is underway, and neither psychiatrists nor other clinicians agree on the precise line between normal and psychopathological affective phenomena. Diagnosis is especially difficult for the large number of patients with episodes of mood change in which a recent precipitating event appears significant, especially since clinicians tend to credit depressive reactions to such stressful events when they are apparent.
As a pathological symptom, depression often occurs in association with other psychiatric and medical illnesses, which makes precise diagnosis even harder. For example, in elderly people the differential diagnosis between early senile dementia and depression may be difficult. The term "secondary depression" has been proposed to encompass these symptomatic depression states. In clinical psychiatric practice, however, most depressive symptoms are "primary," that is, without apparent association with preexistent or concomitant illness.
Each year, between 4 and 8 percent of the population experiences a clinical depressive syndrome, a constellation of symptoms in which the mood disturbance is accompanied by sleep difficulty, change in appetite, retardation of thinking, and attitudes of hopelessness, helplessness, pessimism, and even suicidal tendencies. Moreover, since these symptoms often persist, the diagnostic criteria embodied in the third edition of the American Psychiatric Association's Diagnostic and Statistical Manual, known as DSM-III, require at least two weeks' duration of such symptoms, plus evidence that their intensity and pervasiveness have impaired the individual's usual social role performance and personal activities. (Brett Silverstein, Emily Blumenthal, 1997).
THE BIOLOGY OF DEPRESSION
The human brain is extremely sophisticated; indeed, it is far more complicated and versatile than even the most powerful modern computer. It contains in excess of 100 billion brain cells, known as neurones, each of which is connected to many other neurones. If you look at them under a microscope, neurones appear as thin wires connecting little blobs of brain tissue. However, even with strong magnification the neurone-to-neurone connections, known as synapses, are not apparent. Synapses can be electrical but the majority are chemical. A signal from one part of the brain travels to another-as a series of electrical impulses-along neurones. Where two neurones meet, the signal is carried across the synapse by the release of a tiny balloon-like packet of 'neurotransmitter', in which a message-carrying chemical is carefully packaged. Once released into the synapse this balloon immediately ruptures, releasing its chemical contents, which are then free to quickly migrate across the synapse