10 November 2010

What Are You Afraid Of, Senorita Amygdala?

Scientists are beginning to learn how the brain learns to be afraid, and how learned fears are activated at the level of individual nuclei and cells in the amygdala.
In this week's issue of the journal Nature, a research team led by scientists at the California Institute of Technology (Caltech) has taken an important step toward understanding just how this kickoff occurs by beginning to dissect the neural circuitry of fear. In their paper, these scientists—led by David J. Anderson, the Benzer Professor of Biology at Caltech and a Howard Hughes Medical Institute investigator—describe a microcircuit in the amygdala that controls, or "gates," the outflow of fear from that region of the brain.

The microcircuit in question, Anderson explains, contains two subtypes of neurons that are antagonistic—have opposing functions—and that control the level of fear output from the amygdala by acting like a seesaw. _Eurekalert
Antagonistic neuronal types allow for finer control of output -- which allows us to preserve some of our dignity when surprised, at least.

A team of European scientists has looked at different parts of the central nucleus of the amygdala, and have discovered that the lateral subdivision of the central nucleus is involved in learning a fear response. The medial subdivision of the central nucleus is involved in the behavioural manifestation of a fear response.
Now, research involving several Swiss and German teams and a researcher from Inserm Unit 862, “Neurocentre Magendie”, in Bordeaux, has been able to identify, for the first time, distinct neuronal circuits within the central nucleus of the amygdala which are specifically involved in acquisition and control of behavioural fear responses. Details of these results are published in this week’s edition of the journal Nature.

...In that second step, real-time recording of the activity of the neurons in the lateral and medial subdivisions of the central amygdala, using unique electrophysiological techniques, made it possible for the researchers to identify the specific neurons, within the structures, which were involved in conditioning and behavioural manifestation of fear responses.

These neurons are inhibitor cells belonging to very organized and strongly interconnected neuronal circuits. Modification of the activity of these circuits enables the relevant behavioural fear response to be selected as a function of the environmental situation. _SB
Fear is at the center of primate learning, particularly the earlier stages. Much of the late early, middle, and latter parts of our lives involves the "papering over" of our raw learned fear responses, in order to avoid the unpleasantness of feeling afraid, and to allow us to act in groups without triggering contagious fear response chain reactions. Anxiety emerges when we are not sure what it is that we are afraid of at the moment.

There are good reasons why benzodiazepines, alcohol, and other anxiolytics account for so much economic activity.

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