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Emotions and the brain

Help Organization of intra-amygdaloid circuitries in the rat: an emerging framework for understanding functions of the amygdala Organization of intra-amygdaloid circuitries in the rat: an emerging framework for understanding functions of the amygdala Fear conditioning induces associative long-term potentiation in the amygdala
Emotion: Cellular Level Emotion: Region Level: Amygdala The Anatomy of Fear
Denis Paré, PhD
Original modules
Tool Module: Identifying Pathways in the Brain Identifying Pathways in the Brain
Experiment Module: Identifying the Brain Structures Involved in Conditioned Fear   Identifying the Brain Structures Involved in Conditioned Fear

Lesions to the central nucleus of the amygdala interfere with just about all of the manifestations of conditioned fear, including behavioural inhibition, autonomic nervous system responses, the suppression of pain, the release of stress hormones, and the potentiation of reflexes.

In addition, each of these responses is controlled by various bundles of nerve fibres projecting from the central nucleus. For example, the nerve fibres projecting to the periaqueductal grey matter interfere with behavioural inhibition, but not with blood pressure. Conversely, the fibres projecting to the lateral hypothalamus prevent blood pressure from increasing, but do not affect behavioural inhibition. Meanwhile, a lesion to the projection to the bed nucleus of the stria terminalis has no effect on blood pressure or behavioural inhibition, but does interfere with the release of stress hormones.


Because neuroanatomists distinguish many different nuclei within the amygdala, the term “amygdaloid complex” is often preferred to denote this structure within the brain. This term is all the more appropriate in that some of these nuclei can be further broken down into various divisions. The following table lists these nuclei and their divisions.

Source: Asla Pitkänen, Vesa Savander and Joseph E. LeDoux Trends Neurosci. (1997) 20, 517-523

Given all these nuclei and their many divisions, how does information circulate within the amygdaloid complex? Many studies have helped to reveal the wiring that enables the amygdala to detect potentially dangerous stimuli and orchestrate an appropriate physiological response.

First it was noted that the projections from the sensory regions of the brain enter the amygdala via the lateral nucleus, which constitutes the main gateway to the amygdala, though not the only one.

Some projections from various parts of the brain also converge at specific nuclei of the amygdaloid complex. For example, some projections from the entorhinal cortex terminate mainly in the basal nucleus, but also go to the central and lateral nuclei. The projections from the hypothalamus go to the central, medial, basal, and accessory basal nuclei.

Neuron in the lateral nucleus of the amygdala Source: Thomas Heinbockel, University of Arizona.


These observations suggest that the information that enters the amygdala becomes represented at multiple locations within it. Moreover, there seems to be a point-to-point correspondence in the way this happens, so that the spatial organization of the groups of neurons is preserved.

The integration of these various representations, which originate in systems responsible for such widely differing functions as long-term memory, internal balance, and auditory perception, therefore depends on the numerous internal connections among the various nuclei of the amygdala. The evidence does seem to indicate that these connections are sufficiently numerous, complex, and diversified to play this role.

Indeed, contrary to what was initially believed, the information flow within the amygdala is highly reciprocal. It does not simply travel in one direction, from the main entryway at the lateral nucleus to the exit from the central nucleus. Indeed, most of the main targets of the lateral nucleus send projections back to it as well.

The representations thus encoded in the amygdala and modulated by other brain structures ultimately converge at the output areas–primarily the central nucleus and the amygdalohippocampal area. This integration enables the brain to generate an activity pattern that can trigger the appropriate changes in the various structures responsible for the emotional reponse to the situation.

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