AMYGDALA
The amygdala is an almond-shaped complex located in the medial temporal lobe, and contains approximately 13 nuclei. The three main regions are the corticomedial nuclei, basolateral nuclei (both receive afferents and project axons to target structures), and central nucleus (which provides mainly efferent projections to the brainstem). Afferent connections to the amygdala originate from cortical and thalamic areas, and hypothalamic and brainstem areas. Its function is to provide emotional relevance to external and internal sensory information and to provide a behavioral and emotional response, particularly a fearful and aversive response, to a sensory input.
The majority of afferent information
arises from the glutamatergic projections arising from pyramidal neurons in
layer V of the cortex. These projections largely travel ipsilaterally via the
extreme capsule. Information from sensory association areas and memory-related
structures, such as the hippocampus, are relayed via cortical and thalamic
inputs. Autonomic and behavioral inputs arise from the hypothalamus and
brainstem.
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| Plate 2-11 |
Afferents to the corticomedial
nuclei arrive primarily from subcortical limbic sources, including the
olfactory bulb, septal nuclei, and hypothalamic nuclei (ventrome-dial [VM],
lateral hypothalamic area [LHA]); the thalamus (intralaminar nuclei); the stria
terminalis; and excessive numbers of autonomic nuclei and monoamine nuclei of
the brainstem. Afferents to the basolateral nuclei arrive mainly from the
cortical areas, including extensive sensory association cortices, the
prefrontal cortex, the cingulate cortex, and the subiculum.
In the late 1930s Klüver and Bucy
described a behavioral syndrome characterized by hypersexuality, hyperorality,
excessive exploration of visual stimuli (hypermetamorphosis),
visual agnosia, apathy, and withdrawal. They linked this behavior to bilateral
amygdaloid complex lesions. This syndrome has been described in patients with
neurodegenerative diseases, such as Alzheimer disease and even frontotemporal
dementia. Because the amygdala processes sensory information for emotional
relevance, it is not surprising that atypical emotional responses, such as
anger, aggressive behavior, and even apathy, can evolve
in these patients. Damage to the hypothalamic connectivity of the amygdala is
responsible for hyperphagia, hypersexuality, and overeating/obesity. Likewise,
alterations of the visual association cortical afferent projections to the
amygdala result in hypermetamorphosis and visual agnosia (patients cannot recognize
facial expressions that indicate fear).
