Amygdala

The amygdala (plural: amygdalae) is a very well studied part of the limbic system and forms part of the mesial temporal lobe. 

Gross anatomy

The amygdala is a complex grey matter structure located anterior and superior to the temporal horn of the lateral ventricle and head of the hippocampus. The anterior extension of the amygdala is identified by the entorhinal sulcus. 

Both the amygdala and the hippocampus are derived from the telencephalon. In 1997, Gloor classified the amygdala into three large groups of nuclei ^2,3:

• basolateral
• corticomedial (phylogenetically the oldest ⁴)
• central group

Connections

Output fibres

The output fibres of the amygdala are mainly through two efferent fibre systems:

• stria terminalis
• ventral amygdalofugal pathway

The stria terminalis has a semicircular course in the body of the lateral ventricle. The level of the anterior commissure is a very important anatomical landmark because the stria terminalis splits into three parts:

• precommissural part
• commissural part
• postcommissural part

The precommissural fibres end in the septal region, and the postcommissural fibres end in the nucleus of stria terminalis. The commissural fibres connect the contralateral amygdala via the anterior commissure.

The ventral amygdalofugal pathway arises from the dorsomedial aspect of the amygdala, and courses medially through substantia innominata, anterior perforated substance, and fibres spread into the cerebral cortex, brainstem, mediodorsal thalamus, and hypothalamus. Direct efferent fibres project to the entorhinal cortex as well as the hippocampus ².

Input fibres

The input fibres to the amygdala are mainly represented by sensory fibres, which transfer information from numerous subcortical regions such as the hypothalamus, basal ganglia, septal region, and autonomic centres in the brainstem. Both the ventral amygdalofugal pathway and the stria terminals carry afferent and efferent fibres together. Some input fibres project to the amygdala along the olfactory stria, and directly from the temporal lobe.

Reciprocal fibres

The basal nucleus of Meynert, and the nucleus of the diagonal band project reciprocally to the amygdala along the ventral amygdalofugal pathway ^1,2.

Relations

• superiorly: partly continuous with the inferomedial aspect of the claustrum
• laterally: optic tract
• posteriorly: hippocampus, connected to the tail of the caudate nucleus

History and etymology

Amygdala is derived from the Greek amugdalē, meaning almond, as it is almond-shaped.