Amygdaloid body

At a glance

English

Amygdaloid body

Latin (TA2)

Corpus amygdaloideum

System

Brain and spinal cord

Region

Head

Side

Paired, left and right

About the amygdaloid body

The amygdaloid body, or amygdala, takes its name from the Greek word for almond, which it resembles in shape and size. One lies in each cerebral hemisphere, deep within the medial part of the temporal lobe, beneath the uncus and just in front of the tip of the temporal horn of the lateral ventricle and the head of the hippocampus. It is usually grouped with the limbic system, and some of its nuclei have also been classed with the basal ganglia.

It is not a single nucleus but a collection of grey-matter groups with different connections. The largest is the basolateral complex, made up of lateral, basal and accessory basal nuclei, which behaves much like cortex and receives information from all the senses. A centromedial group, comprising the central and medial nuclei, is the main output. A cortical nucleus lies near the surface, and small clusters of intercalated cells sit between the groups. The medial and cortical parts take in smell signals from the olfactory tract.

Its outflow leaves mainly through two routes, the stria terminalis and the ventral amygdalofugal pathway. These reach the hypothalamus, the bed nucleus of the stria terminalis, the mediodorsal thalamus, the nucleus accumbens and brainstem centres such as the locus coeruleus and the motor nuclei of the trigeminal and facial nerves. Through these the amygdala links the meaning of a stimulus to bodily responses: freezing or escape, changes in heart rate and blood pressure, facial expression and release of stress hormones. During fear conditioning, sensory signals arrive in the lateral nucleus, become associated with an unpleasant outcome, probably by long-term potentiation, and are relayed to the central nucleus to trigger the response. The amygdala also strengthens the storage of emotionally charged memories by the hippocampus and takes part in reward learning and social judgement, such as reading faces.

Blood reaches it chiefly from the anterior choroidal artery, with branches of the middle cerebral artery. Bilateral damage, as in the rare Urbach-Wiethe disease, removes ordinary fear responses,, and wider anterior temporal lesions produce the Klüver-Bucy syndrome of tameness and indiscriminate behaviour. Altered amygdala size or activity is reported in anxiety disorders, depression, post-traumatic stress disorder and bipolar disorder, and the region is a common starting point of temporal lobe seizures.

Function and supply

Function

Assigns emotional significance to sensory information, drives fear and defensive responses through autonomic, endocrine and motor outputs, strengthens emotional memory, and contributes to reward learning, smell-related behaviour and social judgement.

Blood supply

Mainly the anterior choroidal artery, with branches of the middle cerebral artery; venous blood drains to the basal vein.