⚡ Neurons & Myelination
Structure & transport
- Nissl substance (RER) – absent in axon hillock
- Fast anterograde: kinesin, 200‑400 mm/day (vesicles, mitochondria)
- Retrograde: dynein (trophic factors, pathogens: rabies, herpes, tetanus)
- Microtubule toxins: colchicine, vinblastine → neuropathy
Glial cells
- CNS: astrocytes (GFAP, BBB), oligodendrocytes (myelin, 1:many), microglia (immune), ependyma (CSF)
- PNS: Schwann cells (1:1 myelin, regeneration)
- MS: CNS demyelination, oligoclonal bands
- GBS: PNS demyelination, albuminocytologic dissociation
🌱 Autonomic & Development
Neurulation
- Neural crest → PNS, adrenal medulla, melanocytes, Schwann cells, pia/arachnoid
- Alar plate (sensory) · Basal plate (motor)
- AFP ↑ in open NTD, ↓ in Down syndrome
Malformations
- Anencephaly – anterior neuropore
- Spina bifida occulta – tuft of hair, AFP normal
- Arnold‑Chiari II – myelomeningocele + hydrocephalus
- Dandy‑Walker – vermis agenesis, 4th ventricle cyst
ANS overview
- Preganglionic: ACh (nicotinic)
- Postganglionic: para = ACh (muscarinic); sym = NE (α/β) except sweat (ACh)
- Adrenal medulla: modified postganglionic → E/NE
- Horner: T1 sympathetic → ptosis, miosis, anhidrosis
🦴 Spinal Cord & Tracts
UMN vs LMN
- UMN: hypertonia, hyperreflexia, Babinski +
- LMN: hypotonia, areflexia, fasciculations, atrophy
- Decorticate: arms flexed (above midbrain)
- Decerebrate: arms extended (below midbrain)
Sensory pathways
- DCML: proprioception, vibration, fine touch – cross in medulla
- Spinothalamic: pain, temperature – cross in spinal cord
- Romberg: dorsal column lesion → sway with eyes closed
- Brown‑Séquard: ipsilateral DCML + UMN, contralateral spinothalamic
🧩 Brainstem · Cranial Nerves
Midbrain (III, IV)
- CN III – down‑and‑out, ptosis, dilated pupil
- CN IV – superior oblique; head tilt away
- Weber: CN III + contralateral hemiparesis
- Parinaud: upgaze palsy, light‑near dissociation
Pons (V, VI, VII, VIII)
- CN V – mastication, facial sensation
- CN VI – lateral rectus (internal strabismus)
- CN VII – facial muscles; Bell's palsy (upper & lower face)
- MLF lesion → INO (adduction failure, convergence spared)
Medulla (IX, X, XII)
- CN X – uvula deviates away
- CN XII – tongue deviates toward lesion
- Wallenberg: ipsilateral facial pain/temp loss, contralateral body, Horner, dysphagia
⚖️ Vestibular & Brainstem Syndromes
VOR & caloric
- Warm water → fast phase toward irrigated ear
- Cold water → fast phase away
- Absent caloric response → brainstem dysfunction
Syndromes
- Medial medullary: contralateral hemiparesis + tongue deviation + loss of proprioception
- Lateral pontine (AICA): facial palsy + hearing loss + Horner + ataxia
- Pontocerebellar angle: CN VII + VIII (no long tract signs)
🌀 Cerebellum
Organization
- Vermis – axial/proximal
- Intermediate – distal limbs
- Lateral – motor planning
- Flocculonodular – balance, VOR
Lesion signs (ipsilateral)
- Intention tremor, dysmetria, dysdiadochokinesia
- Scanning speech, hypotonia
- Anterior vermis – gait ataxia (alcohol)
- Purkinje = sole output (GABA, inhibitory)
🔗 Basal Ganglia
Direct (promotes)
- Cortex → striatum (GABA) → GPi → thalamus → cortex ↑
- D1 excites direct
- D2 inhibits indirect
Indirect (suppresses)
- Cortex → striatum → GPe → STN → GPi → thalamus ↓
- ACh drives indirect
- Parkinson: hypokinetic (Dopamine ↓, ACh relative ↑)
- Huntington: hyperkinetic (chorea)
🔺 Diencephalon
Thalamus
- VPL – body sensation
- VPM – face sensation
- LGB – vision
- MGB – hearing
- VA/VL – motor relay
Hypothalamus
- Lateral – feeding (lesion → starvation)
- Ventromedial – satiety (lesion → obesity)
- Supraoptic/PVN – ADH (lesion → diabetes insipidus)
- Anterior – heat dissipation; Posterior – heat conservation
Epithalamus / Subthalamus
- Pineal – melatonin; Parinaud syndrome
- Subthalamic nucleus – hemiballismus
🧠 Cortex & Arteries
Lobar syndromes
- Frontal: Broca aphasia, apraxia, contralateral paresis
- Parietal: Gerstmann (dominant), neglect (non‑dominant)
- Temporal: Wernicke aphasia, Meyer loop → superior quadrantanopia
- Occipital: hemianopia with macular sparing
Arterial territories
- ACA: leg > arm; apraxia (callosal)
- MCA: face & arm; aphasia (dominant); neglect
- PCA: hemianopia with macular sparing; alexia without agraphia
- Watershed: border zones (hypotension)
🫧 Limbic & Olfactory
Memory & emotion
- Hippocampus: memory consolidation (bilateral → anterograde amnesia)
- Amygdala: emotion, fear conditioning
- Klüver‑Bucy: bilateral amygdala → placidity, hypersexuality
- Papez circuit: hippocampus → fornix → mammillary bodies → anterior thalamus → cingulate
Olfactory & disorders
- CN I – no thalamic relay; direct to temporal lobe
- Alzheimer: hippocampus, basal nucleus of Meynert (cholinergic)
- Korsakoff: mammillary bodies & dorsomedial thalamus (thiamine)
- Anosmia: cribriform plate fracture
👁️ Visual Pathways
Lesion localization
- Optic nerve: monocular blindness
- Chiasm: bitemporal hemianopia (pituitary)
- Tract: contralateral homonymous hemianopia
- Meyer loop: superior quadrantanopia ("pie in sky")
- Parietal radiations: inferior quadrantanopia
- Visual cortex: hemianopia with macular sparing
Reflexes & pupils
- Light reflex: CN II → pretectal → EW → CN III → sphincter
- Argyll Robertson: accommodation intact, light absent (neurosyphilis)
- Marcus Gunn: afferent defect (optic neuritis)
- Horner: miosis, ptosis, anhidrosis
- Adie: tonic pupil, slow accommodation
💧 Ventricular system & CSF
Flow & hydrocephalus
- Flow: lateral → Monro → 3rd → aqueduct → 4th → Luschka/Magendie → subarachnoid
- Non‑communicating: obstruction (aqueductal stenosis)
- Communicating: resorption defect (post‑hemorrhage, meningitis)
- NPH: dementia, gait apraxia, incontinence
CSF composition
- Production: choroid plexus (400‑500 mL/day)
- Glucose: ~60% of serum
- Cells: 0‑4 lymphocytes/µL
- Xanthochromia: prior hemorrhage
⭐ USMLE Step 1 · High‑Yield Pearls
- ▹ Parkinson: resting pill‑rolling tremor, bradykinesia (dopamine ↓, ACh ↑)
- ▹ Huntington: chorea, autosomal dominant, CAG repeat (caudate atrophy)
- ▹ Wallenberg: ipsilateral facial pain/temp loss, contralateral body, Horner, dysphagia
- ▹ Brown‑Séquard: ipsilateral UMN + DCML, contralateral spinothalamic
- ▹ Subacute combined degeneration: B12 deficiency → dorsal columns + corticospinal
- ▹ Internuclear ophthalmoplegia (INO): MLF lesion, adduction failure, convergence spared
- ▹ Macular sparing: PCA occlusion (collateral from MCA)
- ▹ Korsakoff: thiamine deficiency, mammillary bodies, confabulation
- ▹ Klüver‑Bucy: bilateral amygdala → placidity, hypersexuality, hyperorality
💊 Ocular & Neuro Pharmacology
Eye receptors
- Sphincter (M3): miosis; blocked → mydriasis (atropine)
- Dilator (α): mydriasis; blocked → miosis
- Ciliary (M3): accommodation; cycloplegia with anticholinergics
- β blockers: reduce aqueous secretion (glaucoma)
Key drugs
- L‑DOPA + carbidopa – Parkinson
- Anticholinergics (benztropine) – reduce ACh excess
- Penicillamine – Wilson disease (copper chelator)
- Haloperidol – Huntington, Tourette
- IVIG / plasmapheresis – Guillain‑Barré
⚕️ USMLE Step 1 · Nervous System Anatomy Revision ·