Core idea: The limbic system links emotion, memory, motivation, and autonomic responses. It is central for learning, fear, and reward – all very testable on boards.
| Structure | Location | Key Functions |
|---|---|---|
| Hippocampus | Medial temporal lobe (floor of temporal horn) | Declarative memory (episodic & semantic); memory consolidation |
| Amygdala | Anterior medial temporal lobe | Fear, threat detection, emotional learning, aggression |
| Cingulate Gyrus | Medial surface of frontal/parietal lobes above corpus callosum | Emotion, pain affect, motivation, attention (ACC) |
| Parahippocampal Gyrus | Medial temporal surface, surrounding hippocampus | Contextual memory, navigation, gateway into hippocampus |
| Mammillary Bodies | Inferior hypothalamus (posterior) | Relay for memory circuits (Papez); damaged in Wernicke-Korsakoff |
| Septal Nuclei | Basal forebrain | Reward, pleasure, cholinergic projections to hippocampus |
| Nucleus Accumbens | Ventral striatum (at junction of caudate & putamen) | Reward, motivation, addiction (mesolimbic dopamine) |
| Orbitofrontal & Medial PFC | Ventral & medial frontal lobes | Emotion regulation, social behavior, decision-making |
Limbic “big 3” for boards = hippocampus (memory), amygdala (fear/emotion), cingulate gyrus (motivation/pain affect).
Core role: Consolidation of declarative (explicit) memory.
Pathway (classic board sequence):
Lesion sites & consequences:
Key pathway:
Functions:
Clinical relevance:
Inputs:
Outputs:
Clinical: Central in anxiety disorders, PTSD, and emotional memory of traumatic events.
Think “Papez = memory loop”, “mesolimbic = reward/addiction”, “amygdala loop = fear/autonomic”. Many vignette stems are just dressed-up versions of these three circuits.
| Memory Type | Description | Main Structures |
|---|---|---|
| Declarative (Explicit) | Conscious memory for facts & events (episodic, semantic) | Hippocampus, medial temporal lobe, diencephalon (thalamus, mammillary bodies) |
| Non-declarative (Implicit) | Skills, habits, priming, conditioning (unconscious) | Basal ganglia, cerebellum, neocortex, amygdala (fear conditioning) |
| Working Memory | Short-term holding & manipulation (seconds) | Dorsolateral prefrontal cortex |
Internal structure:
Connections:
Highly vulnerable to:
Anterograde vs Retrograde:
Key clinical scenarios:
Pure anterograde amnesia with preserved remote memory and normal language = usually bilateral hippocampal/medial temporal damage.
Functions:
Key inputs: Sensory cortex, thalamus, hippocampus, prefrontal cortex
Key outputs: Hypothalamus (autonomic/endocrine), brainstem (freezing/startle), basal forebrain & cortex (emotional experience)
Side note: Amygdala activity is often increased in anxiety disorders and PTSD.
Klüver–Bucy Syndrome (bilateral anterior temporal/amygdala):
Urbach–Wiethe disease (rare):
Temporal lobe epilepsy: Emotional auras (fear), autonomic changes, déjà vu, rising epigastric sensation often reflect amygdala/hippocampal involvement.
Behavioral triad “hyperorality + hypersexuality + placidity” in a temporal lesion vignette → think bilateral amygdala / Klüver–Bucy.
| Syndrome | Primary Lesion Site | Clinical Features |
|---|---|---|
| Mesial Temporal Sclerosis | Hippocampus (usually unilateral, medial temporal) | Temporal lobe epilepsy with déjà vu, rising epigastric aura, automatisms, memory complaints |
| H.M.-type Amnesia | Bilateral medial temporal lobes (hippocampi) | Severe anterograde amnesia, some retrograde loss, preserved procedural memory & IQ |
| Wernicke–Korsakoff Syndrome | Mammillary bodies, medial thalamus, periaqueductal gray |
Wernicke triad: confusion, ophthalmoplegia, ataxia (acute, reversible) Korsakoff: profound anterograde amnesia, confabulation (chronic, often irreversible) |
| Klüver–Bucy Syndrome | Bilateral anterior temporal lobes/amygdala | Hyperorality, hypersexuality, placidity, visual agnosia, hypermetamorphosis |
| Limbic Encephalitis | Medial temporal lobes (often bilateral) |
Subacute confusion, seizures, mood changes, prominent anterograde memory loss. Autoimmune/paraneoplastic (LGI1, CASPR2, Hu, Ma2, etc.) |
| HSV Encephalitis | Medial temporal & orbitofrontal cortex | Fever, headache, focal seizures, behavioral changes, memory deficits; MRI temporal lobes |
| Anterior Cingulate Lesion | Medial frontal/ACC | Abulia, apathy, reduced motivation, akinetic mutism if severe |
Limbic lesions often present with a triad of: new-onset seizures, memory impairment, and behavior/personality change. Think medial temporal/limbic process (tumor, limbic encephalitis, HSV).
| Clinical Finding | Likely Structure |
|---|---|
| Anterograde amnesia after bilateral temporal injury | Hippocampi (medial temporal lobes) |
| Confabulation + chronic memory loss in alcoholic patient | Mammillary bodies & medial thalamus (Korsakoff) |
| Déjà vu, rising epigastric sensation, fear aura before seizure | Mesial temporal (hippocampus + amygdala) |
| Hyperorality + hypersexuality + placidity | Bilateral amygdala/anterior temporal (Klüver–Bucy) |
| Apathy/abulia with intact motor strength | Anterior cingulate / medial frontal limbic cortex |
| Addiction, drug craving, reward-seeking | Mesolimbic dopamine (VTA → nucleus accumbens) |
If the vignette mentions: “medial temporal FLAIR signal + seizures + memory loss” – your first thought should be limbic encephalitis.
