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Mental disorders

Anxiety Disorders

Alzheimer’s-type Dementia

When someone suffers a particularly traumatic experience, it bombards their amygdala with electrical and chemical signals that condition its circuits. Thus, for example, the sounds, images, and smells of the battlefield become associated in the circuits of a soldier's amygdala with the traumatic emotional experience of having friends killed in combat.

Even years later, some of these same stimuli, or others associated with them, may suffice to trigger an intense fear reaction, by reactivating these strongly conditioned circuits in the amygdala.

Anxiety, like fear and other emotions, reflects the activity of specific, identifiable circuits in the brain. Early on, researchers studying the brain circuits involved in anxiety realized that the amygdala was a key structure for understanding this phenomenon. The amygdala is a complex structure in the brain and is composed of several interconnected nuclei.

The sensory stimuli associated with pain follow one of two pain pathways to the brain, then converge at the lateral nucleus of the amygdala. Because of its position at the "gateway" to the amygdala, this nucleus seems to play an important role in the process of fear conditioning, which underlies many anxiety disorders (follow the Experiment module link to the left).

The nerve cells in the lateral nucleus of the amygdala project their axons to its other nuclei, thus forming various circuits that ultimately converge on the central nucleus. This nucleus acts as the output pathway from the amygdala, which in turns activates the hypothalamus, the locus coeruleus, and other parts of the brain that are responsible for the characteristic symptoms of anxiety.

During the fear-conditioning process, certain internal circuits in the amygdala may be strengthened to create a memory trace of the conditioned fear. Such traces appear to be very persistent, and perhaps even permanent. If these potentiated circuits in the amygdala are strengthened enough, they could even become the source of a specific phobia.

Many researchers think that fear conditioning records memories in these strengthened circuits of the amygdala permanently. Hence these researchers believe that the efforts involved in psychotherapy achieve their positive results not by removing the fearful memories encoded in the amygdala's circuits, but rather by strengthening other neural pathways, particularly ones originating in the cortex, that calm these circuits.

This is why a phobia that seemed to have been successfully cured through therapy can re-emerge suddenly during a particularly stressful event. The calming influence of the cortex no longer suffices, and the conditioned fear in the circuits of the amygdala expresses itself once again.

Unconscious fears that result from early conditioning seem to be etched in the amygdala's circuits permanently. The persistence of these fears throughout our lives would represent an advantage if we were living in a stable environment like our ancestors did, because it would save us from having to constantly relearn that a given situation was dangerous.

But in our complex modern world, where change is an integral part of our daily lives and the contexts in which we operate are no longer the same, persistent fears can become a handicap instead. Thus we pay a heavy price for the formidable efficiency of the fear pathways in our nervous systems.

Experiment : Identifying the Brain Structures Involved in Conditioned Fear

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