Overview & Definitions

• Basal ganglia — collection of subcortical nuclei deep to cerebral cortex, lateral to the thalamus
• Primary role: motor control, procedural learning, habit formation, emotion, executive function
• The basal ganglia do NOT initiate movement — they modulate cortically initiated motor plans

Component Structures

Spatial Relationships & Blood Supply

Spatial Organization (Medial → Lateral)

• Caudate nucleus — C-shaped, follows lateral ventricle (head, body, tail); head bulges into frontal horn
• Internal capsule — between caudate/thalamus (medially) and lentiform nucleus (laterally)
• Globus pallidus — medial portion of lentiform nucleus
• Putamen — lateral portion of lentiform nucleus
• External capsule — lateral to putamen
• Claustrum — thin gray matter between external and extreme capsules
• Extreme capsule — between claustrum and insular cortex

Blood Supply

• Lenticulostriate arteries (lateral branches of MCA M1) → putamen, globus pallidus, caudate head, internal capsule — most common site of hypertensive hemorrhage
• Recurrent artery of Heubner (from ACA) → caudate head, anterior limb of internal capsule, anterior putamen
• Anterior choroidal artery (from ICA) → GPi, posterior limb of internal capsule, optic tract

Direct Pathway (GO Pathway) — Facilitates Movement

• Cortex → excites Striatum (glutamate)
• Striatum → inhibits GPi/SNpr (GABA, substance P, dynorphin)
• GPi/SNpr releases inhibition on Thalamus (less GABA output)
• Thalamus → excites Cortex (glutamate)
• Net effect: disinhibition of thalamus → increased cortical drive → movement facilitated
• Dopamine effect: D1 receptors on direct pathway neurons → dopamine excites the direct pathway → promotes movement
• Mnemonic: "D1 = Direct = Do it"

Indirect Pathway (STOP Pathway) — Inhibits Movement

• Cortex → excites Striatum (glutamate)
• Striatum → inhibits GPe (GABA, enkephalin)
• GPe releases inhibition on STN (less GABA output)
• STN → excites GPi/SNpr (glutamate)
• GPi/SNpr → inhibits Thalamus (increased GABA output)
• Net effect: increased thalamic inhibition → less cortical drive → movement suppressed
• Dopamine effect: D2 receptors on indirect pathway neurons → dopamine inhibits the indirect pathway → reduces suppression → net facilitation of movement
• Mnemonic: "D2 = inDirect = Don't do it (inhibits the inhibitor)"

Hyperdirect Pathway

• Route: Cortex → STN → GPi (bypasses striatum entirely)
• Function: rapid, global suppression of motor programs — acts as an "emergency brake"
• Speed: fastest of the three pathways because it skips the striatal relay
• Clinical relevance: implicated in impulse control, response inhibition, and OCD pathophysiology

Neurotransmitter Summary

Direct vs. Indirect Pathway Comparison

Hypokinetic vs. Hyperkinetic Framework

Parkinson Disease

• Pathology: loss of dopaminergic neurons in SNpc; Lewy bodies (alpha-synuclein aggregates)
• Mechanism: dopamine loss → underactive direct pathway + overactive indirect pathway → increased GPi output → excessive thalamic inhibition → bradykinesia
• Cardinal features (TRAP):
  • Tremor — resting, "pill-rolling," 4–6 Hz, asymmetric onset
  • Rigidity — cogwheel (rigidity + superimposed tremor) or lead-pipe
  • Akinesia / bradykinesia — decrement on repetitive movements, required for diagnosis
  • Postural instability — late feature, poor prognostic sign
• Other motor features: masked facies, hypophonia, micrographia, shuffling gait, reduced arm swing, festination, en bloc turning
• Non-motor features: anosmia (early), REM sleep behavior disorder, constipation, depression, dementia (late), orthostatic hypotension
• Treatment targets: levodopa/carbidopa (dopamine precursor), dopamine agonists (pramipexole, ropinirole), MAO-B inhibitors (rasagiline, selegiline), COMT inhibitors (entacapone), DBS of STN or GPi

Huntington Disease

• Genetics: CAG trinucleotide repeat expansion in huntingtin gene (chromosome 4p16.3); autosomal dominant; anticipation (earlier onset with successive generations)
• Pathology: loss of striatal medium spiny neurons — indirect pathway neurons (enkephalin+) degenerate first
• Mechanism:
  • Early: indirect pathway loss → decreased GPi output → chorea (hyperkinetic)
  • Late: both pathways lost → rigidity and bradykinesia (Westphal variant if juvenile onset)
• Clinical triad: chorea + psychiatric symptoms (depression, irritability, psychosis) + cognitive decline (subcortical dementia)
• Imaging: caudate atrophy → "box-car" ventricles (enlarged frontal horns); also putaminal and cortical atrophy
• Treatment of chorea: VMAT2 inhibitors (tetrabenazine, deutetrabenazine, valbenazine) — deplete presynaptic dopamine

Hemiballismus

• Definition: violent, large-amplitude, flinging movements of proximal limb (unilateral)
• Lesion: contralateral subthalamic nucleus (STN)
• Mechanism: STN normally excites GPi (glutamate) → STN lesion → reduced GPi output → thalamic disinhibition → excessive movement
• Etiology: lacunar stroke (most common), nonketotic hyperglycemia (second most common — T1 hyperintensity on MRI), tumor, demyelination
• Prognosis: often self-limited; treatment with dopamine receptor blockers or tetrabenazine if persistent

Wilson Disease

• Genetics: autosomal recessive; mutations in ATP7B gene (chromosome 13) → impaired biliary copper excretion
• Pathology: copper accumulation in putamen (primary BG target), globus pallidus, liver, cornea
• Neurological features: wing-beating tremor, dystonia, parkinsonism, dysarthria, drooling, psychiatric symptoms
• Key findings: Kayser-Fleischer rings (corneal copper deposits), low ceruloplasmin, elevated 24-hour urine copper
• MRI: "face of the giant panda" sign (midbrain); T2 hyperintensity in putamen and caudate
• Treatment: copper chelation (penicillamine, trientine), zinc supplementation (blocks gut absorption)

Dystonia

• Definition: sustained or intermittent muscle contractions → twisting, repetitive movements, abnormal postures
• Pathophysiology: loss of surround inhibition in BG circuits; abnormal sensorimotor integration
• Classification: focal (cervical dystonia, blepharospasm, writer's cramp), segmental, generalized (DYT1/TOR1A mutation)
• Treatment: botulinum toxin (focal), anticholinergics (trihexyphenidyl, especially young patients), GPi DBS (generalized)

Disorder-to-Structure Comparison Table

General Organization

• Location: paired ovoid structures forming the lateral walls of the 3rd ventricle
• Function: "gateway to the cortex" — relay and processing station for virtually all sensory, motor, and limbic information (exception: olfaction bypasses thalamus)
• Internal medullary lamina: Y-shaped white matter band dividing thalamus into anterior, medial, and lateral nuclear groups
• Intralaminar nuclei: embedded within the lamina (centromedian [CM], parafascicular [PF]) → arousal, attention, pain
• Reticular nucleus: thin shell around lateral thalamus; does NOT project to cortex — only inhibitory output that gates thalamic relay

Blood Supply

Thalamic Relay Nuclei (Specific Nuclei)

Association & Limbic Nuclei

Nonspecific & Modulatory Nuclei

Dejerine-Roussy Syndrome (Thalamic Pain Syndrome)

• Lesion: VPL/VPM region (posterolateral thalamic infarct, usually thalamogeniculate artery territory)
• Acute phase:
  • Contralateral hemianesthesia (all modalities)
  • Mild contralateral hemiparesis (may occur if internal capsule is involved)
  • Hemiataxia (proprioceptive loss)
• Chronic phase (weeks to months later):
  • Central post-stroke pain (CPSP) — severe, burning, poorly localized contralateral pain
  • Allodynia — pain from normally innocuous stimuli (light touch)
  • Hyperpathia — exaggerated, prolonged pain response
  • Spontaneous paroxysmal pain episodes
• Treatment: tricyclics (amitriptyline), gabapentin, pregabalin, lamotrigine; often refractory

Paramedian Thalamic Stroke

• Territory: paramedian (thalamoperforating) arteries from P1 segment of PCA
• Nuclei affected: mediodorsal (MD), intralaminar nuclei
• Clinical features:
  • Decreased arousal / hypersomnolence (intralaminar nuclei damage)
  • Memory impairment (MD nucleus → prefrontal circuit disruption)
  • Vertical gaze palsy (involvement of riMLF and posterior commissure fibers)
  • Behavioral changes: apathy, confabulation, "thalamic dementia"
• Artery of Percheron variant: single arterial trunk from one P1 supplying bilateral paramedian thalami → occlusion causes bilateral thalamic infarction — "butterfly" pattern on MRI DWI

Thalamic Stroke Syndromes by Vascular Territory

Other Thalamic Syndromes

• Thalamic aphasia: fluent aphasia-like syndrome from dominant (usually left) pulvinar/posterior thalamic lesion; semantic paraphasias, reduced verbal output but preserved repetition
• Thalamic neglect: contralateral hemispatial neglect from right-sided thalamic strokes (pulvinar or anterior territory)
• Thalamic dementia: progressive cognitive decline from bilateral thalamic damage (vascular, prion disease [fatal familial insomnia], or tumor)
• Thalamic hand: dystonic posturing of contralateral hand (wrist flexion, MCP hyperextension, finger flexion) — "thalamic fist"

General Organization

• Location: forms the floor and inferior lateral walls of the 3rd ventricle; lies below the thalamus (separated by the hypothalamic sulcus)
• Boundaries:
  • Anterior: lamina terminalis, optic chiasm
  • Posterior: mammillary bodies
  • Superior: hypothalamic sulcus
  • Inferior: infundibulum (pituitary stalk), tuber cinereum, median eminence
• Function: master regulator of homeostasis — temperature, hunger/satiety, thirst, circadian rhythm, autonomic nervous system, pituitary hormonal control, emotion/behavior

Anatomical Divisions

Hypothalamic Nuclei & Functions

Major Hypothalamic Connections

• Fornix: hippocampus → mammillary bodies (memory circuit)
• Mammillothalamic tract: mammillary bodies → anterior thalamic nucleus (Papez circuit continuation)
• Medial forebrain bundle: connects septal nuclei, hypothalamus, and brainstem tegmentum; part of reward/pleasure pathway
• Hypothalamohypophyseal tract: supraoptic + paraventricular nuclei → posterior pituitary (transports ADH and oxytocin)
• Tuberoinfundibular tract: arcuate nucleus → median eminence (releases dopamine and releasing/inhibiting hormones into portal system)
• Dorsal longitudinal fasciculus: hypothalamus → brainstem autonomic nuclei (PAG, dorsal vagal nucleus, nucleus ambiguus)
• Stria terminalis: amygdala → hypothalamus (emotional and fear-related inputs)

Autonomic Control

• Anterior / medial hypothalamus: parasympathetic (rest and digest) — decreases heart rate, blood pressure; promotes digestion
• Posterior / lateral hypothalamus: sympathetic (fight or flight) — increases heart rate, blood pressure, pupillary dilation
• Descending pathways: via dorsal longitudinal fasciculus and reticulospinal tract to brainstem and spinal cord intermediolateral cell column

Pituitary Control

Posterior Pituitary (Neurohypophysis)

• Direct neural connection via hypothalamohypophyseal tract (axonal transport)
• ADH (vasopressin): from supraoptic & paraventricular nuclei → water reabsorption in collecting ducts
• Oxytocin: from paraventricular nucleus → uterine contraction, milk let-down

Anterior Pituitary (Adenohypophysis)

• Controlled indirectly via hypophyseal portal system (releasing/inhibiting hormones)
• Releasing hormones: CRH (ACTH), TRH (TSH, prolactin), GnRH (LH, FSH), GHRH (GH)
• Inhibiting hormones: dopamine (inhibits prolactin — most important), somatostatin (inhibits GH, TSH)

Diabetes Insipidus (Central)

• Lesion: supraoptic/paraventricular nuclei or pituitary stalk → decreased ADH production or delivery
• Features: polyuria (massive dilute urine, low specific gravity), polydipsia, hypernatremia, serum hyperosmolality
• Etiologies: pituitary surgery, craniopharyngioma, Langerhans cell histiocytosis, sarcoidosis, traumatic brain injury, idiopathic
• Diagnosis: water deprivation test → urine fails to concentrate; responds to exogenous desmopressin (DDAVP)
• Triphasic response (post-surgical): DI (days 1–5) → SIADH (days 5–10, as stored ADH is released from dying neurons) → permanent DI (if >80% neurons destroyed)

SIADH (Syndrome of Inappropriate ADH Secretion)

• Mechanism: excessive ADH release → free water retention → dilutional hyponatremia
• Features: hyponatremia, low serum osmolality, inappropriately concentrated urine, euvolemic
• Neurological causes: subarachnoid hemorrhage, meningitis/encephalitis, traumatic brain injury, CNS tumors, Guillain-Barre syndrome
• Treatment: fluid restriction, salt tablets; severe/symptomatic → hypertonic saline (3%); chronic → vaptans (tolvaptan)
• Danger: rapid correction of hyponatremia → osmotic demyelination syndrome (ODS) — central pontine myelinolysis

Wernicke-Korsakoff Syndrome

• Pathology: thiamine (vitamin B1) deficiency → hemorrhagic necrosis of mammillary bodies, periaqueductal gray, medial thalamus (MD nucleus), and tectal plate
• Wernicke encephalopathy (acute triad):
  • Encephalopathy (confusion, decreased alertness)
  • Oculomotor dysfunction (nystagmus, CN VI palsy, conjugate gaze palsy)
  • Ataxia (gait > limb)
  • Classic triad present in only ~16% of cases — always maintain high clinical suspicion
• Korsakoff syndrome (chronic):
  • Anterograde amnesia (inability to form new memories) — mammillary body and MD thalamic damage
  • Confabulation (unintentional fabrication of memories)
  • Often irreversible once established
• Treatment: IV thiamine BEFORE glucose (glucose loading without thiamine can precipitate or worsen Wernicke encephalopathy)

Hypothalamic Obesity

• Lesion: ventromedial hypothalamus (satiety center destruction)
• Most common cause: craniopharyngioma (Rathke's pouch remnant) — compresses hypothalamus from below
• Features: intractable hyperphagia, rapid weight gain, often accompanied by hypopituitarism and visual field defects
• Other causes: hypothalamic glioma, surgical damage, radiation, infiltrative disease (sarcoidosis, LCH)

Other Hypothalamic Disorders

Anatomy

• Definition: white matter tract between caudate/thalamus (medially) and lentiform nucleus (laterally)
• Shape: V-shaped (with apex = genu) on axial imaging; opens laterally
• Key relationship: the most clinically important subcortical white matter structure — damage causes dense, contralateral neurological deficits

Segments & Contents

Somatotopic Organization of Posterior Limb

• Anterior portion: head/face & upper extremity (corticospinal fibers)
• Posterior portion: trunk and lower extremity
• Sensory radiations: posterior to motor fibers (VPL/VPM → S1)
• Because fibers are densely packed, even small lesions can produce profound deficits

Lacunar Stroke Syndromes of the Internal Capsule

Movement Disorder Localization Quick Reference

Hypothalamic Functions Quick Reference

Thalamic Nuclei Quick Reference

Red Flags — Subcortical Lesion Emergencies