The Amygdalas

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The amygdalas are the basic functional brain centers for identifying potentially dangerous stimuli [4]. It is quite possible that the amygdala deals specifically with fear, while the bed nucleus of the stria terminalis (BNST), a nearby structure, deals with the processes of anxiety [22,23].

Le Doux [4] provided a description of the mechanism of activation of stress, and I shall refer to this at various points in this paper. Le Doux takes an approach intended to overcome the problem of scientific reliability, inevitable when trying to define emotional phenomena, by focusing on cognitive, emotional, and motivational aspects, which can be described in objective terms on the basis of the neurochemical processes underlying them.

In response to anxiogenic stimuli, the central nucleus of the amygdala solicits, either directly or indirectly through the BNST, the hypothalamic paraventricular nucleus (PVN) and the locus coeruleus. This latter is essential for the production of noradrenaline, which raises blood pressure, accelerates heart rate, and-with the nucleus accumbens-activates a motor reaction; the PVN then produces corticotropin-releasing factor (CRF), which stimulates the pituitary (PIT) to release adrenocorticotrop-ic hormone (ACTH), which is carried in the bloodstream to the adrenals. Cortisol, a hormone secreted by the capsules, then returns to the brain in the bloodstream, regulating pituitary stimulation. The body thus finds the sympathetic system activated and can prepare itself-or at least predispose itself-to attack or to flee.

Cortisol activates the hippocampus to record explicit memories that are particularly significant for survival, but when there is too much stimulation, or for too long a duration, glucose depletion occurs in the hip-

pocampal cells. These cells then become vulnerable to subsequent stimuli and may suffer atrophy or even die. This influences hippocampal neurogenesis, which is inhibited. These events help explain the memory defects in patients with PTSD or depression. Cortisol also causes damage to the prefrontal cortex, impeding decision-making processes.

The amygdala is part of a system of reciprocal regulation with the adrenals, as it is sensitive to the adrenaline and noradrenaline they produce, and governs the acquisition of explicit memories, generated in conditions of emotional activation. When the emotional stimulus is adequate, the memory is potentiated but it may have difficulty dealing with excessive stimuli, and the hippocampus finds it hard to establish the right context for an event that is emotionally important. If we add that these feedbacks also influence the processes of attention, we can see how important these neurobiological aspects are in describing automatic brain operations related to emotional states, in an area of unconscious functioning. Hippocampal impairment may lead to difficulty in putting fear and normal anxiety into their appropriate context, paving the way for dysfunction and pathological symptoms.

On the functional synergy between the amygdala, hippocampus, and cerebellum [24], it is worth remembering that the cerebellum and pons must be intact for the memory to consolidate itself, and to put anxiety-arousing situations into context, though we still do not know much about how these regions work. In reversible inactivation experiments, Attwell [25] reached conclusions similar to those of Sacchetti et al. [26] on the role of the cerebellar cortex in recording memory traces.

These findings underscore the importance of knowledge of brain processes and their implications, in seeking possible solutions to problems. In new or critical situations, conscious attention is aroused and the chain of associative memory is activated to find the best answers.

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