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Central Nervous System - Study Notes

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PART 1 - OVERVIEW AND DIVISIONS

Structural Divisions

Functional Divisions

Exam tip: The CNS = brain + spinal cord only. Everything else (nerves, ganglia, plexuses) is PNS.

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PART 2 - BRAIN

Embryological Divisions (rostral → caudal)

Key term: "Brainstem" in common clinical usage = midbrain + pons + medulla (not diencephalon)

Cerebral Hemispheres

• Gray matter (outer cortex): contains neuronal cell bodies
• White matter (inner): axons forming tracts/pathways
• Ventricles: CSF-filled spaces

Lobes of the Cerebrum

• Frontal, Parietal, Temporal, Occipital (named for overlying skull bones)
• Insula (hidden within lateral sulcus)

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PART 3 - SPINAL CORD

General Anatomy

• Part of CNS within the superior 2/3 of the vertebral canal
• Roughly cylindrical; oval in cross-section; has a central canal
• Extends from foramen magnum → disc between L1/L2 in adults (as low as L2/L3; as high as T12)
• In neonates: extends to ~L3 (can reach L4)
• Conus medullaris = cone-shaped distal end
• Filum terminale (pial part): fine connective tissue filament extending inferiorly from conus

Enlargements

External Features (fissures/sulci)

• Anterior median fissure - runs full length anteriorly
• Posterior median sulcus - posterior surface
• Posterolateral sulcus (bilateral) - where posterior rootlets enter

Internal Organization

     GRAY MATTER (H-shaped cross-section)
     - Rich in nerve cell bodies
     - Forms longitudinal columns = horns
       • Anterior horn: motor neurons (LMN)
       • Posterior horn: sensory neurons
       • Lateral horn (T1-L2/L3; S2-S4): autonomic neurons

     WHITE MATTER (surrounds gray matter)
     - Rich in axon processes
     - Ascending tracts + Descending tracts

Key Spinal Tracts

Vasculature of Spinal Cord

• Two sources:
  1. Longitudinally oriented vessels arising superior to the cervical cord, descending on cord surface (anterior spinal artery from vertebral arteries; two posterior spinal arteries)
  2. Segmental vessels (radicular arteries from intercostal/lumbar arteries) at each level

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PART 4 - MENINGES

Mnemonic: DAPe - Dura, Arachnoid, Pia

Three Layers (external → internal)

Subarachnoid Space

• Between arachnoid and pia mater
• Contains CSF (cerebrospinal fluid)
• Continuous at foramen magnum between cranial and spinal compartments
• Arachnoid trabeculae span the space; large blood vessels suspended here
• Spinal subarachnoid space ends at ~S2 lower border
• Cord ends at L1/L2 but subarachnoid space extends to S2 → lumbar cistern contains cauda equina → safe site for LP

Dural Folds (cranial)

Tentorial notch: oval opening in tentorium through which the midbrain passes - site of uncal herniation in raised ICP

Spinal Meninges

• Spinal dura: extradural/epidural space between dura and vertebral canal; continues as sleeve-like sheaths around spinal nerves
• Spinal arachnoid: ends at S2; thin, not adherent to dura
• Spinal pia: vascular; extends into anterior median fissure; forms denticulate ligaments (transverse sheets stabilizing cord to dura)
• Filum terminale: pial part (conus → S2) + dural part (S2 → coccyx)

Lumbar Puncture (LP)

• Performed at L3/L4 or L4/L5 interspace (below conus medullaris)
• Cord ends at L1/L2 → no risk of cord injury
• Needle traverses: skin → supraspinous lig. → interspinous lig. → ligamentum flavum → epidural space → dura → arachnoid → subarachnoid space
• CSF: clear, colorless; ~0.5 L produced/day by choroid plexus in adults

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PART 5 - VENTRICULAR SYSTEM AND CSF

Ventricles

CSF Production

• Produced by choroid plexus (modified ependymal cells; lining ventricles)
• ~0.5 L/day in adults
• Functions: cushioning, waste removal, buoyancy for brain

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PART 6 - BRAINSTEM

Overview

Midbrain (Mesencephalon)

• Tectum (roof): superior and inferior colliculi
  • Superior colliculi = visual reflex (pupillary light reflex coordination, visual tracking)
  • Inferior colliculi = auditory relay
• Tegmentum (covering):
  • Red nucleus - motor coordination
  • Substantia nigra - dopaminergic; movement modulation (lost in Parkinson's)
  • Oculomotor nucleus (CN III), Edinger-Westphal nucleus (parasympathetics → pupil)
  • Trochlear nucleus (CN IV) at caudal midbrain
  • Periaqueductal gray (PAG) - pain modulation
  • Mesencephalic nucleus of CN V

Pons (Metencephalon portion)

• Pontine nuclei: relay cortical signals to cerebellum
• Pontocerebellar fibers run transversely to contralateral cerebellum
• Corticospinal fibers run longitudinally (will form pyramids in medulla)
• Medial lemniscus - horizontal at pons; carries fine touch/proprioception
• Spinothalamic tract - lateral to medial lemniscus
• Medial longitudinal fasciculus (MLF) - coordinates eye movements
• CN nuclei present: CN V (trigeminal - motor + principal sensory), CN VI (abducens), CN VII (facial), CN VIII (vestibulocochlear)

Medulla Oblongata (Myelencephalon)

• Most caudal brainstem; ends at foramen magnum
• Pyramids = corticospinal fibers anteriorly
• Pyramidal decussation at caudal medulla (~85% fibers cross → lateral corticospinal tract)
• Inferior olivary nucleus = relay for cerebellar coordination
• CN nuclei: CN VIII-XII
• Vital centers: cardiovascular, respiratory, swallowing, vomiting

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PART 7 - CEREBELLUM

Structure

• 2 lateral hemispheres + midline vermis
• Located in posterior cranial fossa, below tentorium cerebelli
• Receives massive input; fine-tunes and coordinates movements

Connections

Functions

• Coordination of voluntary movement
• Balance and posture (receives vestibular input)
• Motor learning
• Smooth, precise movements

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PART 8 - BASAL NUCLEI (Basal Ganglia)

Corpus Striatum

• Caudate nucleus + Putamen = striatum (phylogenetically newer - neostriatum)
• Putamen + Globus pallidus = lentiform nucleus
• Caudate + Lentiform nucleus = corpus striatum
• Subthalamic nucleus and Substantia nigra are functionally related

Internal Capsule

• White matter tract between thalamus/caudate (medially) and lentiform nucleus (laterally)
• Contains corticospinal, corticobulbar, and thalamocortical fibers
• Anterior limb, genu, posterior limb

Function

• Modulate voluntary movement via direct pathway (facilitatory) and indirect pathway (inhibitory)
• Parkinson's disease = loss of dopaminergic substantia nigra neurons → reduced direct pathway activation

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PART 9 - THALAMUS

• Major sensory relay station
• Located bilaterally, surrounds 3rd ventricle
• Almost all sensory information (except olfaction) relays through thalamus before reaching cortex
• Key nuclei: VPL (body sensation), VPM (face), LGN (vision), MGN (auditory), pulvinar (visual association), CM (motor)

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PART 10 - HYPOTHALAMUS

Location

• Below thalamus; surrounds lower part of 3rd ventricle
• Connected to pituitary by infundibulum

Functional Zones

Functions (Mnemonic: FLAT PEG)

• Food intake (satiety/hunger)
• Limbic connections (emotion, behavior)
• Autonomic regulation
• Temperature regulation
• Pituitary control (endocrine)
• Emotional responses
• Gonadal (reproductive) function

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PART 11 - BLOOD SUPPLY OF THE BRAIN

Two Sources

1. Internal carotid arteries (anterior circulation) - enter via carotid canals
2. Vertebral arteries (posterior circulation) - enter via foramen magnum; fuse below pons to form basilar artery

Circle of Willis (Cerebral Arterial Circle)

• Anterior communicating artery - connects left and right anterior cerebral arteries
• Two posterior communicating arteries - connect internal carotid → posterior cerebral arteries

Arterial Territories

Arterial Supply to Dura

• Anterior meningeal arteries - branches of ethmoidal arteries (anterior cranial fossa)
• Middle meningeal artery - branch of maxillary artery; enters via foramen spinosum; largest meningeal artery; at risk at pterion (thinnest skull point) → extradural/epidural hematoma
• Posterior meningeal artery - posterior cranial fossa

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PART 12 - CLINICAL CORRELATES (High-yield for exams)

Stroke (CVA)

• Definition: Interruption of blood flow to brain/brainstem → impaired neurological function >24 hours
• TIA: same presentation but resolves <24 hours ("mini-stroke")
• Types: Ischemic (thrombotic or embolic - most common) vs. Hemorrhagic
• Symptoms: Hemiparesis/hemisensory loss, visual field deficits, dysarthria, ataxia, decreased consciousness
• Treatment: Thrombolytics within 3-4.5 hours of onset (only ischemic); mechanical thrombectomy also possible
• Imaging: Urgent CT brain (identify hemorrhagic before giving thrombolytics)

Uncal Herniation

• Raised ICP → uncus (medial temporal lobe) herniates through tentorial notch
• Compresses CN III → ipsilateral dilated/fixed pupil (blown pupil) + contralateral hemiparesis (corticospinal compression)

Epidural Hematoma

• Arterial bleed (middle meningeal artery) → blood between skull and periosteal dura
• Classic: trauma to pterion → "lucid interval" then rapid deterioration
• CT: biconvex (lens-shaped) hyperdense collection

Subdural Hematoma

• Venous bleed (bridging veins in subdural space)
• CT: crescent-shaped hyperdense (acute) or hypodense (chronic) collection

Subarachnoid Hemorrhage (SAH)

• Blood in subarachnoid space - commonly from berry aneurysm at Circle of Willis
• "Thunderclap headache" - worst headache of life
• CT: hyperdense blood in subarachnoid cisterns; if negative → LP (xanthochromia)

Lumbar Puncture

• Level: L3/L4 or L4/L5 (safest - below cord)
• Cauda equina syndrome: compression of cauda equina → bowel/bladder dysfunction, saddle anesthesia, lower limb weakness

Meningitis

• Inflammation of meninges → headache, neck stiffness (meningism), photophobia, fever
• Kernig's and Brudzinski's signs positive
• LP essential for diagnosis (CSF analysis)

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QUICK REVISION TABLE - Key Landmarks

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MEMORY AIDS

• Midbrain: CN III, IV
• Pons: CN V, VI, VII, VIII
• Medulla: CN IX, X, XI, XII

• Pain/temperature (spinothalamic): crosses within spinal cord (1-2 levels above entry)
• Fine touch/proprioception (dorsal columns): crosses at medulla

• Corticospinal: crosses at caudal medulla (pyramidal decussation) → that's why contralateral deficit after cortical stroke

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Central Nervous System - Complete Study Notes

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PART 1 - OVERVIEW AND DIVISIONS

Structural Divisions

Functional Divisions

Exam tip: CNS = brain + spinal cord ONLY. Everything else (nerves, ganglia, plexuses) = PNS.

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PART 2 - BRAIN (EMBRYOLOGICAL DIVISIONS)

"Brainstem" in clinical usage = midbrain + pons + medulla (not diencephalon)

• Outer gray matter = neuronal cell bodies (cortex)
• Inner white matter = axon tracts/pathways
• Ventricles = CSF-filled internal spaces

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PART 3 - SPINAL CORD

Extent and Landmarks

• Occupies superior 2/3 of vertebral canal
• Adult: ends at disc between L1/L2 (range: T12 to L2/L3)
• Neonate: ends at ~L3 (can reach L4)
• Conus medullaris = cone-shaped distal end
• Filum terminale (pial part): fine pial filament from conus inferiorly

Enlargements

External Markings

• Anterior median fissure (full length, anterior)
• Posterior median sulcus (posterior)
• Posterolateral sulcus (bilateral) - where posterior rootlets enter

Internal Organization

GRAY MATTER - H-shaped, central
  Anterior horn: LMN (lower motor neurons)
  Posterior horn: sensory
  Lateral horn: autonomic (T1-L2/3 = sympathetic; S2-4 = parasympathetic)

WHITE MATTER - surrounds gray matter
  Ascending tracts (sensory) + Descending tracts (motor)

Spinal Tracts - High Yield

Vasculature

• Anterior spinal artery (from vertebral arteries) - anterior 2/3 of cord
• Two posterior spinal arteries - posterior 1/3
• Segmental radicular arteries (from intercostals/lumbars) reinforce at each level
• Artery of Adamkiewicz = largest anterior segmental artery (usually at T9-T12 on left)

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PART 4 - MENINGES

Mnemonic: D-A-P (Dura - Arachnoid - Pia, outer to inner)

Three Layers

• Contains CSF
• Arachnoid trabeculae connect arachnoid to pia; large vessels suspended here
• Spinal subarachnoid space ends at ~lower border of S2
• Lumbar cistern = below L1/L2 conus; contains cauda equina; LP done here safely

Cranial Dural Folds

Spinal Meninges

• Spinal dura: outer epidural fat/veins; forms nerve root sleeves
• Spinal arachnoid: ends at S2
• Denticulate ligaments: lateral extensions of pia mater anchoring cord to dura bilaterally (stabilize cord)
• Filum terminale: pial part (conus → S2) + dural part (S2 → coccyx)

Arterial Supply to Dura

• Anterior meningeal arteries = branches of ethmoidal arteries
• Middle meningeal artery = branch of maxillary artery; enters via foramen spinosum; largest; runs over pterion
• Posterior meningeal artery = posterior fossa

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PART 5 - VENTRICULAR SYSTEM AND CSF

Ventricular Flow (must know)

Lateral ventricles (in hemispheres; C-shaped)
        ↓ Interventricular foramen (of Monro)
Third ventricle (between thalami)
        ↓ Cerebral aqueduct (of Sylvius) — through midbrain
Fourth ventricle (pons/medulla anteriorly, cerebellum posteriorly)
        ↓ Foramen of Luschka (×2, lateral) + Foramen of Magendie (midline)
Subarachnoid space
        ↓ Arachnoid granulations
Venous sinuses (reabsorption)

CSF Facts

• Produced by choroid plexus (modified ependymal cells lining ventricles)
• ~0.5 L/day produced in adults
• Functions: cushioning brain, waste removal, buoyancy
• Hydrocephalus: obstruction of flow (obstructive/non-communicating) or impaired reabsorption (communicating)

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PART 6 - BRAINSTEM

Cranial Nerves by Level

• Midbrain: CN III (oculomotor), CN IV (trochlear - exits posteriorly, only CN to do so)
• Pons: CN V (trigeminal), CN VI (abducens), CN VII (facial), CN VIII (vestibulocochlear)
• Medulla: CN IX (glossopharyngeal), CN X (vagus), CN XI (accessory), CN XII (hypoglossal)

Mnemonic: "My Pet Monkey Very Frivolously Ate Giant Vegetables" = Midbrain (III,IV), Pons (V,VI,VII,VIII), Medulla (IX,X,XI,XII)

Midbrain

• Tectum (roof): 4 colliculi = corpora quadrigemina
  • Superior colliculi = visual reflexes (pupillary light reflex, visual tracking)
  • Inferior colliculi = auditory relay
• Tegmentum: Red nucleus, Substantia nigra (dopaminergic; lost in Parkinson's), Oculomotor nucleus (CN III), Edinger-Westphal nucleus (pupil constriction/accommodation), Periaqueductal gray (PAG - pain modulation)

Pons

• Pontine nuclei relay cortex → cerebellum
• Pontocerebellar fibers run transversely to contralateral cerebellum
• Corticospinal fibers run longitudinally
• Medial lemniscus = horizontal "butterfly wings" at pons
• MLF (medial longitudinal fasciculus) = coordinates conjugate eye movements

Medulla

• Pyramids anteriorly = corticospinal tracts
• Pyramidal decussation at caudal medulla (~85% fibers cross) → forms lateral corticospinal tract
• Inferior olivary nucleus = relay for cerebellar coordination
• Vital centers: cardiovascular, respiratory, swallowing, vomiting (chemoreceptor trigger zone)

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PART 7 - CEREBELLUM

Structure

• 2 hemispheres + midline vermis
• Posterior cranial fossa, below tentorium cerebelli
• Cortex: 3 layers (molecular, Purkinje, granular)
• Deep nuclei: dentate (largest), emboliform, globose, fastigial

Peduncles (connections)

Functions

• Smooth, coordinated voluntary movement
• Balance and posture (receives vestibular input)
• Motor learning
• Cerebellar lesion signs: DANISH = Dysdiadochokinesia, Ataxia, Nystagmus, Intention tremor, Slurred speech (dysarthria), Hypotonia

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PART 8 - BASAL NUCLEI (Basal Ganglia)

Components

Striatum = Caudate + Putamen (receives input from cortex)
Lentiform nucleus = Putamen + Globus pallidus
Corpus striatum = Caudate + Lentiform nucleus
+ Subthalamic nucleus + Substantia nigra (functionally linked)

• Contains: corticospinal, corticobulbar, thalamocortical fibers

Pathways

• Direct pathway (facilitatory): Striatum → GPi/SNr → thalamus → cortex (movement facilitation)
• Indirect pathway (inhibitory): Striatum → GPe → subthalamic nucleus → GPi/SNr → thalamus (movement suppression)
• Dopamine from substantia nigra stimulates direct pathway, inhibits indirect = net facilitatory effect on movement

Disease Associations

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PART 9 - THALAMUS AND HYPOTHALAMUS

Thalamus

• Major sensory relay station
• Surrounds 3rd ventricle bilaterally
• Almost ALL sensory (except olfaction) relays through thalamus before reaching cortex

Hypothalamus

• Below thalamus; surrounds inferior 3rd ventricle
• Connected to pituitary via infundibulum

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PART 10 - BLOOD SUPPLY

Two Arterial Systems

1. Internal carotid arteries (anterior circulation) - enter via carotid canals
2. Vertebral arteries (posterior circulation) - enter via foramen magnum → fuse to form basilar artery below pons

Circle of Willis

• Anterior communicating artery - connects right and left ACAs
• Posterior communicating arteries (×2) - connect ICA → PCA each side

Arterial Territories

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PART 11 - CLINICAL CORRELATES (High-Yield)

Stroke

• Ischemic (thrombotic or embolic) vs. Hemorrhagic
• TIA = <24 hours; Stroke = >24 hours
• Thrombolytics within 3-4.5 hours (ischemic only)
• Urgent CT to exclude hemorrhage before thrombolytics

Hematomas

Herniation Syndromes

• Uncal herniation: Medial temporal lobe through tentorial notch → CN III compression → fixed dilated ipsilateral pupil + contralateral hemiplegia
• Tonsillar herniation: Cerebellar tonsils through foramen magnum → compress medulla → respiratory arrest

Lumbar Puncture

• Level: L3/L4 or L4/L5 (cord ends at L1/L2)
• CSF normal: clear, colorless, pressure 7-18 cmH₂O, protein 15-45 mg/dL, glucose 60-80% serum
• Xanthochromia = yellow CSF = blood breakdown products → SAH

Important Syndromes

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QUICK REFERENCE - KEY LANDMARKS TABLE

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