Limbic system and hypothalamus
Limbic system, hypothalamus and reticular formation: functions and lesions
Limbic system, hypothalamus and reticular formation for FCPS Part 1: Papez circuit, amygdala, hypothalamic nuclei, temperature and arousal. Covers Klüver–Bucy, Wernicke–Korsakoff, fever, the pituitary portal system, narcolepsy and coma.
Limbic system: parts and the Papez circuit
- The limbic system links emotion, memory, motivation and autonomic responses. Its parts are the hippocampus, amygdala, cingulate and parahippocampal gyri, fornix, mammillary bodies, anterior thalamic nucleus and septal nuclei.
| Step | From | Through | To |
|---|---|---|---|
| 1 | Hippocampus | Fornix | Mammillary bodies |
| 2 | Mammillary bodies | Mammillothalamic tract | Anterior thalamic nucleus |
| 3 | Anterior thalamic nucleus | Thalamocortical fibres | Cingulate gyrus |
| 4 | Cingulate gyrus | Cingulum | Parahippocampal gyrus (entorhinal cortex), back to hippocampus |
- Damage anywhere in this loop impairs the laying down of new memories.
| Structure | Function | Lesion |
|---|---|---|
| Hippocampus | Converts short-term to long-term declarative memory (long-term potentiation) | Anterograde amnesia (cannot form new memories); old memories and skills largely preserved; early site of Alzheimer disease; CA1 very sensitive to hypoxia |
| Amygdala | Fear, emotional colouring of memory, aggression | Bilateral: Klüver–Bucy syndrome: docility (loss of fear), hyperorality, hypersexuality, visual agnosia |
| Mammillary bodies | Memory circuit relay | Wernicke–Korsakoff (thiamine deficiency) |
| Cingulate gyrus | Emotion, motivation, attention | Apathy; bilateral damage gives akinetic mutism |
| Septal nuclei | Reward, pleasure | Rage behaviour in animals |
- Wernicke encephalopathy (acute): confusion, ophthalmoplegia (nystagmus, VI palsy) and ataxia, with haemorrhages in the mammillary bodies and around the third ventricle and aqueduct. Give thiamine before glucose. Korsakoff syndrome (chronic): profound anterograde and some retrograde amnesia with confabulation; dorsomedial thalamus and mammillary bodies.
- The limbic structure most concerned with emotion (particularly fear) is the amygdala; the hypothalamus produces the bodily expression of emotion. Removing the cortex above an intact hypothalamus causes "sham rage" in animals.
Hypothalamus: nuclei and functions
| Nucleus or area | Function | Effect of lesion |
|---|---|---|
| Supraoptic and paraventricular | Make ADH and oxytocin, transported down axons to the posterior pituitary; paraventricular also makes CRH and TRH | Central diabetes insipidus |
| Ventromedial | Satiety centre; inhibits the feeding centre; leptin and insulin act here and in the arcuate nucleus | Hyperphagia and obesity; rage |
| Lateral | Feeding (hunger) centre; orexin neurones for wakefulness | Refusal to eat, weight loss |
| Anterior (preoptic) | Heat loss: sweating, cutaneous vasodilatation; also parasympathetic responses | Hyperthermia |
| Posterior | Heat gain: shivering, vasoconstriction, sympathetic activation | Poikilothermia |
| Suprachiasmatic | Circadian clock, driven by light through the retinohypothalamic tract; controls pineal melatonin | Loss of sleep–wake rhythm |
| Arcuate | GHRH, GnRH, dopamine (prolactin inhibition), appetite neurones | Hypogonadism, raised prolactin |
| OVLT and subfornical organ | Thirst and ADH release via osmoreceptors and angiotensin II | Loss of thirst |
- Hunger, thirst and temperature are all controlled by the hypothalamus, not the thalamus or basal ganglia.
- Obesity in people is usually due to leptin resistance, not leptin deficiency.
Temperature and fever
- The preoptic anterior hypothalamus compares core temperature (from its own warm-sensitive neurones and skin receptors) with a set point.
- Too hot: anterior hypothalamus drives sweating and vasodilatation. Too cold: posterior hypothalamus drives shivering, vasoconstriction and piloerection, and raises metabolic rate through thyroid and adrenaline.
- Fever: endogenous pyrogens (IL-1, IL-6, TNF-alpha) act on the OVLT, which lies outside the blood–brain barrier, inducing prostaglandin E2 that raises the set point. The body then shivers until the new set point is reached. Aspirin and paracetamol block cyclo-oxygenase and lower the set point. In heat stroke the set point is normal but heat loss fails, so antipyretics do not help.
Hypothalamus and pituitary
| Link | Anterior pituitary | Posterior pituitary |
|---|---|---|
| Connection | Hypophyseal portal veins: releasing hormones enter capillaries at the median eminence and travel to a second capillary bed in the adenohypophysis | Axons of the supraoptico-hypophyseal tract; no portal vessels |
| Origin | Rathke's pouch (oral ectoderm) | Neuroectoderm (downgrowth of diencephalon) |
| Hormones | GH, prolactin, ACTH, TSH, FSH, LH (made in the gland) | ADH, oxytocin (made in the hypothalamus, stored here) |
- Dopamine is the main prolactin-inhibiting factor. Stalk compression, hypothalamic lesions or D2-blocking drugs raise prolactin; bromocriptine and cabergoline (D2 agonists) lower it.
- The neuroendocrine response to stress and surgery is coordinated by the hypothalamus: CRH and ACTH raise cortisol, sympathetic output raises catecholamines, and ADH rises.
- Craniopharyngioma (Rathke's pouch remnant, calcified, often in children) causes diabetes insipidus, growth failure, obesity and bitemporal hemianopia.
Reticular formation and arousal
- A network of neurones in the core of the brainstem.
- The ascending reticular activating system (upper pons and midbrain, relaying through thalamic intralaminar nuclei) keeps the cortex awake; a lesion here, or bilateral diffuse cortical damage, causes coma.
- Contains respiratory and cardiovascular centres, the vomiting centre, and monoamine nuclei: raphe nuclei (serotonin), locus coeruleus (noradrenaline), and nearby dopaminergic nuclei.
- Reticulospinal tracts modulate tone and posture.
Sleep
- Orexin (hypocretin) from the lateral hypothalamus stabilises wakefulness; loss of these neurones causes narcolepsy with cataplexy (sudden loss of tone with emotion, sleep paralysis, hypnagogic hallucinations, early REM).
- The ventrolateral preoptic nucleus promotes NREM sleep; REM sleep is generated in the pons, with muscle atonia and rapid eye movements.
- Melatonin from the pineal rises in darkness under suprachiasmatic control.
Remember Ventromedial lesion makes the animal eat (obesity); lateral lesion makes it refuse food; anterior loses heat, posterior keeps it.
Exam trap Bilateral amygdala damage, not hippocampal damage, causes Klüver–Bucy syndrome; hippocampal damage causes anterograde amnesia.
Exam trap The posterior pituitary has no portal supply; releasing hormones reach only the anterior pituitary through portal veins.
How FCPS asks about this
- Docility, hyperorality, hypersexuality and visual agnosia: bilateral amygdala damage.
- Destroy the lateral hypothalamus: refusal to eat and weight loss.
- Nucleus that inhibits the feeding centre: ventromedial (satiety centre).
- Stimulation of the posterior hypothalamus: shivering and heat conservation; anterior causes sweating and vasodilatation.
- Hippocampal lesion: anterograde amnesia.
- Alcoholic with confabulation and amnesia: Korsakoff syndrome, mammillary bodies, thiamine deficiency.
- Structure receiving hypothalamic signals through portal vessels: anterior pituitary.
Test yourself on this topic
5 of the 9 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
A man has brief episodes of an unpleasant smell of burning rubber followed by lip-smacking and impaired awareness. Where is the seizure focus?
Answer: Medial temporal lobe
Olfactory pathways project, without a thalamic relay, to the primary olfactory cortex in the uncus and piriform cortex, the amygdala and the entorhinal cortex of the medial temporal lobe. Focal seizures arising here can begin with olfactory or gustatory auras (unpleasant smells such as burning rubber), epigastric rising sensation, déjà vu or fear, followed by impaired awareness and automatisms such as lip-smacking (temporal lobe epilepsy, often from hippocampal sclerosis or tumour). MRI is needed to exclude a structural lesion. Occipital seizures cause visual phenomena; motor cortex seizures cause jerking that may march (Jacksonian).
Uncinate fits
- Olfactory cortex: uncus, piriform, amygdala
- No thalamic relay for smell
- Aura: unpleasant smell, déjà vu, epigastric rising
- Then automatisms (lip-smacking), impaired awareness
- Cause: hippocampal sclerosis, tumour → MRI
Remember A phantom burning smell points to the temporal lobe.
Why the other options are wrong
- Occipital lobe Visual aura.
- Primary motor cortex Clonic jerking.
- Cerebellum Not a seizure focus.
- Spinal cord Not a seizure focus.
After surgery for a craniopharyngioma, a patient develops persistent high fever without infection. Damage to which region best explains this?
Answer: Anterior hypothalamus
The anterior hypothalamus (preoptic area) contains heat-sensitive neurones that trigger heat loss (sweating, vasodilatation). Lesions cause hyperthermia (central fever), which does not respond well to antipyretics. The posterior hypothalamus drives heat conservation and production (shivering, vasoconstriction); lesions there cause poikilothermia (body temperature drifts with the environment). Craniopharyngiomas and their surgery can also cause diabetes insipidus, hypopituitarism, visual field defects and obesity.
Hypothalamic temperature control
- Anterior: heat loss → lesion = hyperthermia
- Posterior: heat gain → lesion = poikilothermia
- Central fever: poor response to antipyretics
Remember Anterior cools, posterior warms.
Why the other options are wrong
- Posterior pituitary Causes diabetes insipidus, not fever.
- Cerebellum Coordination.
- Occipital cortex Vision.
- Medulla oblongata Cardiorespiratory centres.
A small haemorrhage in the upper brainstem leaves a patient in a coma with intact cortical hemispheres. Damage to which system explains the loss of consciousness?
Answer: Reticular formation
Consciousness depends on the ascending reticular activating system—neurons in the reticular formation of the upper pons and midbrain (with cholinergic, noradrenergic, serotonergic, histaminergic and dopaminergic projections) that project via the thalamus (intralaminar nuclei) and hypothalamus to the cortex—plus intact function of both cerebral hemispheres. A small lesion in the upper brainstem or bilateral thalamic damage can cause coma, whereas a large unilateral hemispheric lesion does not unless it compresses the brainstem (herniation). The reticular formation also contains cardiorespiratory centres in the medulla and regulates sleep–wake cycles, muscle tone and pain modulation.
Consciousness
- ARAS: reticular formation of upper pons and midbrain
- Projects via thalamus (intralaminar) to cortex
- Coma: brainstem/bilateral thalamic lesion or bilateral hemisphere dysfunction
- Unilateral hemisphere lesion alone: no coma unless herniation
Remember A tiny brainstem lesion can switch off the whole brain.
Why the other options are wrong
- Corticospinal tracts Motor weakness, not coma.
- Medial lemniscus Sensory loss.
- Cerebellar peduncles Ataxia.
- Olfactory tracts Smell.
Bilateral destruction of which structures causes Klüver-Bucy syndrome with docility, hyperorality, hypersexuality and visual agnosia?
Answer: Amygdalae
The amygdala attaches emotional significance to stimuli, especially fear. Bilateral medial temporal lesions involving the amygdalae (herpes simplex encephalitis, trauma) cause Klüver-Bucy syndrome: placidity and loss of fear, hyperorality (putting objects in the mouth), hypersexuality, hypermetamorphosis and visual agnosia. The hippocampus is needed to form new declarative memories (anterograde amnesia when damaged bilaterally); mammillary bodies are damaged in Wernicke-Korsakoff syndrome. The Papez circuit links hippocampus, fornix, mammillary bodies, anterior thalamus and cingulate gyrus.
Limbic system
- Amygdala: fear; bilateral loss → Klüver-Bucy
- Hippocampus: new memories (anterograde amnesia)
- Mammillary bodies: Korsakoff
- Cause: HSV encephalitis (temporal lobes)
Remember No amygdala, no fear.
Why the other options are wrong
- Mammillary bodies Korsakoff amnesia.
- Hippocampi only Anterograde amnesia.
- Frontal eye fields Eye movements.
- Occipital poles Cortical blindness.
Which hypothalamic nucleus receives input from the retina and sets the body's circadian rhythm?
Answer: Suprachiasmatic nucleus
The suprachiasmatic nucleus is the master clock. Retinal ganglion cells containing melanopsin signal light through the retinohypothalamic tract, and the nucleus controls melatonin release from the pineal gland via a sympathetic pathway. Melatonin rises in darkness. Jet lag and shift-work disorder reflect a mismatch between this clock and the environment.
Suprachiasmatic nucleus
- Circadian clock
- Input: retinohypothalamic tract
- Controls melatonin from pineal
- Melatonin rises in darkness
Remember Above the chiasm sits the clock.
Exam trap Melatonin is made in the pineal, not in the hypothalamus.
Why the other options are wrong
- Supraoptic nucleus Makes ADH.
- Ventromedial nucleus Satiety centre.
- Arcuate nucleus Makes GnRH and controls prolactin via dopamine.
- Mammillary body Memory (Papez circuit).
Where this sits in the CPSP syllabus
- Paper 1: Neuroanatomy, Limbic system, hypothalamus and reticular formation
- Medicine & Allied, Paper 2: Central nervous system, Limbic system, hypothalamus and reticular formation
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