Amygdala
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Direct answer
Sitting anterior to the hippocampus in the rostral part of the temporal lobe, close to the uncus and the roof of the inferior horn of the lateral ventricle, the amygdala is the brain's evaluative core for emotion, especially fear and aggression. Its basolateral group of nuclei receives rich cortical and sensory input, while the corticomedial group (including the central nucleus) receives direct olfactory fibres and drives output — chiefly through the stria terminalis, which arcs along the thalamostriate groove beside the caudate nucleus to the hypothalamus and bed nucleus of the stria terminalis, and through the ventral amygdalofugal pathway to the hypothalamus, thalamus and brainstem. Bilateral destruction of the amygdala and temporal neocortex produces the placid, orally exploratory Klüver–Bucy syndrome.
What you must remember
- Position: a walnut-sized mass in the anterior pole of the temporal lobe, anterior to and partly continuous with the hippocampus; it forms part of the uncus, the structure that herniates through the tentorial hiatus.
- Groups: a large basolateral group (lateral, basal, accessory basal nuclei) receiving polymodal sensory and cortical input, and a smaller corticomedial group (cortical, medial, central nuclei) with olfactory connections.
- Main efferents: the stria terminalis — the long curved highway in the groove between the caudate nucleus and thalamus — and the shorter ventral amygdalofugal pathway; both target the hypothalamus, septal area and brainstem autonomic centres.
- Afferents: direct fibres from the olfactory bulb to the corticomedial group (the amygdala is one of the very few telencephalic stations receiving direct sensory input), plus taste and association-cortex input.
- Functions: assigning emotional valence to experience, fear conditioning, modulating memory consolidation through the hippocampus, and activating the autonomic and endocrine stress responses via the hypothalamus.
- Klüver–Bucy syndrome: bilateral temporal lobe damage — docility, psychic blindness (visual agnosia), hyperorality, hypermetamorphosis, altered food preference and hypersexuality.
- Clinical links: uncinate fits begin with an aura of foul smell or taste because the epileptiform discharge starts near the amygdala and uncus; amygdala enlargement and dysfunction are described in autism spectrum disorder.
- Downstream anatomy: central nucleus output to the periaqueductal grey and locus coeruleus organises fear behaviours — freezing, autonomic arousal, startle — without any cortical participation.
How fear takes the low road
Why does a hiker jump back from a garden hose before knowing it is not a snake? The amygdala's wiring explains the sequence. Visual information from the thalamus reaches the basolateral amygdala by two routes: a rapid thalamo-amygdaloid "low road" carrying a coarse, barely processed image, and a slower route through the visual cortex delivering accurate identification. The low road lets the amygdala trigger the centrally organised fear response — via the central nucleus to the hypothalamus (adrenaline, cortisol through the HPA axis), periaqueductal grey (freezing), locus coeruleus (alertness) and autonomic brainstem centres (tachycardia, sweating) — within milliseconds, while the cortex is still deciding whether the object is a reptile. When the cortex finally rules "hose", cortical inhibition quiets the response. Classical fear conditioning — pairing a tone with shock — is encoded in the basolateral group, and its persistence after cortical damage explains phobic and post-traumatic reactions that rational argument cannot switch off. Klüver and Bucy's monkeys showed the opposite experiment: with both amygdalae and temporal neocortex removed, animals approached snakes, mouthed objects and lost fear entirely — the mirror image of the hypervigilant amygdala.
Where students slip
The standard confusion is efferent highways: the fornix belongs to the hippocampus, the stria terminalis to the amygdala — mixing them up in a diagram is the most penalised error. The second is forgetting that the amygdala receives direct olfactory input, bypassing the thalamus — the reason smell evokes emotion faster than any other sense. Examiners also probe the Klüver–Bucy list: candidates who remember hyperorality and docility but drop psychic blindness and hypermetamorphosis lose the classification mark.
Frequently asked questions
Where is the amygdala located, and what is its relation to the uncus?
In the rostral temporal lobe, anterior to the hippocampus, partly forming the uncus. Its medial position makes it relevant in uncal herniation, when the uncus is forced through the tentorial hiatus.
Which pathways carry the amygdala's main output?
The stria terminalis, arching along the groove between the caudate nucleus and thalamus to the hypothalamus and bed nucleus of the stria terminalis, and the ventral amygdalofugal pathway passing directly to the hypothalamus, septal region and brainstem.
What is Klüver–Bucy syndrome?
Placidity, psychic blindness (inability to recognise objects by sight despite intact vision), hyperorality, hypermetamorphosis, altered dietary habits and hypersexuality after bilateral amygdala and temporal lobe destruction. It demonstrates the amygdala's role in emotional appraisal.
Why do odours trigger emotions and memories so strongly?
Olfactory fibres reach the corticomedial amygdala directly from the olfactory bulb without a thalamic relay, placing smell one synapse from the emotional motor of the brain. The amygdala's links with the hippocampus then couple the odour to its memory.
What is an uncinate fit?
A temporal lobe seizure originating near the uncus and amygdala, ushered in by an aura of disagreeable odour or taste. The name recalls the uncus, around which the epileptiform discharge begins.