Overview

The limbic system links the cortex with the hypothalamus. It gives sensory experience its emotional significance and stores it as memory, and it expresses emotion through autonomic, endocrine and behavioural responses. Its core components are:
• the limbic lobe (cingulate and parahippocampal gyri);
• the hippocampal formation (hippocampus, dentate gyrus, subiculum, entorhinal cortex);
• the amygdala;
• the septal nuclei;
• the mammillary bodies of the hypothalamus and the anterior nucleus of the thalamus;
• the nucleus accumbens (ventral striatum) and the orbitofrontal cortex;
• the interconnecting tracts: the fornix, mammillothalamic tract, stria terminalis and cingulum.

The Papez circuit (1937) is the classic memory loop: hippocampus → fornix → mammillary bodies → mammillothalamic tract → anterior thalamic nucleus → cingulate gyrus → cingulum → parahippocampal/entorhinal cortex → hippocampus. A lesion anywhere along it can impair episodic memory. A separate reward circuit runs from dopaminergic neurons of the ventral tegmental area to the nucleus accumbens and prefrontal cortex (the mesolimbic and mesocortical pathways). It signals reward prediction and drives motivated behaviour and addiction. The amygdala and its outputs to the hypothalamus and brainstem form the fear and defence circuit.

Parts & subdivisions

Limbic lobe
Cingulate and parahippocampal gyri.
Hippocampal formation
Episodic and spatial memory consolidation.
Amygdala
Emotional salience, fear conditioning, emotional memory.
Hypothalamus and mammillary bodies
Autonomic and endocrine output of emotion; the mammillary bodies relay hippocampal output via the fornix.
Anterior nucleus of the thalamus
Relay of the Papez circuit to the cingulate cortex.
Septal nuclei
Below the septum pellucidum; reward and pleasure; cholinergic input to the hippocampus (theta rhythm).
Nucleus accumbens (ventral striatum)
Reward, motivation and reinforcement; target of the mesolimbic dopamine pathway and of drugs of abuse.
Fornix
C-shaped efferent tract from the hippocampus to the mammillary bodies and septal nuclei; arches beneath the corpus callosum.

Function

Emotion: generation, recognition and expression
Declarative memory formation (hippocampus, Papez circuit)
Motivation and reward (mesolimbic dopamine)
Autonomic and endocrine responses to emotion (via the hypothalamus)
Olfactory–emotional association
Drives such as feeding, defence and sexual behaviour

Clinical significance

Wernicke–Korsakoff syndrome

Thiamine (B1) deficiency, typically in alcohol use disorder, damages the mammillary bodies, the medial thalamus and the periaqueductal grey. Acute Wernicke encephalopathy presents with confusion, ophthalmoplegia (especially lateral rectus and nystagmus) and ataxia, and is treated urgently with IV thiamine before glucose. Untreated it leads to chronic Korsakoff syndrome: anterograde and retrograde amnesia with confabulation.

Klüver–Bucy syndrome

Bilateral destruction of the anterior temporal lobes and amygdalae, e.g. by HSV encephalitis or trauma, causes placidity with loss of fear, hyperorality, hypersexuality, hypermetamorphosis and visual agnosia.

Autoimmune limbic encephalitis

Subacute memory loss, psychiatric change and seizures. It may be anti-NMDA-receptor encephalitis (young women, often with an ovarian teratoma) or anti-LGI1 encephalitis (faciobrachial dystonic seizures, hyponatraemia), or paraneoplastic (anti-Hu with small-cell lung cancer). The first two respond to immunotherapy.

Mnemonics

Limbic functions — the "5 Fs"

Feeding, Fleeing, Fighting, Feeling and sex (Fornication) — plus memory.

Papez circuit

"Hippo Forgot My Aunt's Cat Purring" — Hippocampus → Fornix → Mammillary body → Anterior thalamic nucleus → Cingulate gyrus → Parahippocampal gyrus → back to Hippocampus.

Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.