Amygdaloid body

The amygdaloid body (amygdala) is an ovoid collection of grey-matter nuclei located deep within the anterior part of the uncus in the medial temporal lobe. It is closely related to the hippocampus and other structures of the limbic system and has extensive connections with the cerebral cortex, basal forebrain, thalamus, hypothalamus and brainstem.

Location and relations

The amygdaloid body lies anterior to the head of the hippocampus. Its posterior surface contributes to the anterolateral wall of the temporal horn of the lateral ventricle, and the uncal recess separates it from the hippocampal head. Medially, it is related to the semilunar and ambiens gyri, while posteriorly it is related to the intralimbic gyrus and piriform cortex. Anteriorly, it lies within the white matter of the temporal pole. Superiorly, it adjoins the white matter of the basal forebrain and may continue cranially with the globus pallidus.

Nuclear groups

The amygdaloid body is not a single nucleus but a complex of several nuclei, which are organized into four main groups:

• Basolateral group: lateral, basolateral and basomedial nuclei.

• Centromedial group: central and medial nuclei. The central nucleus lies more anteriorly and inferolaterally, whereas the medial nucleus lies more posteriorly and superomedially, close to the stria terminalis.

• Cortical group: several small nuclei situated mainly in the superomedial part of the complex.

• Transitional areas: surrounding grey matter that gradually merges with neighbouring structures. The amygdalo-piriform area, for example, lies between the amygdaloid body and the piriform cortex.

Major connections

The amygdaloid body communicates with many parts of the brain through several white-matter pathways. Two particularly important output pathways are the stria terminalis and the ventral amygdalofugal pathway:

• The stria terminalis arises from the centromedial part of the amygdaloid body and follows a C-shaped course toward the septal and hypothalamic regions.

• The ventral amygdalofugal pathway arises mainly from the basolateral part and provides more direct connections toward the septum, hypothalamus and thalamus.

• The amygdaloid body is also linked to the hippocampus by the amygdalo-hippocampal bundle and to the temporal pole by the amygdalo-temporal fascicle.

• Additional connections occur through the uncinate fascicle with frontal regions, through the cingulum with other limbic areas and the basal forebrain, through the lateral olfactory stria with olfactory-related and insular regions, and through pathways associated with the medial forebrain bundle toward the brainstem.

Functional and clinical correlates

This extensive connectivity allows the amygdaloid body to integrate information across the limbic, cortical, autonomic and neuroendocrine systems. Weiss et al. (2021) particularly highlight the participation of amygdaloid pathways in the generation and spread of seizures, making the region relevant to epilepsy surgery. Its connections are also of interest in functional neurosurgery for several neuropsychiatric disorders, where components of the amygdaloid network have been investigated as targets for deep brain stimulation.