CSF, Meninges & Spinal Cord Comprehensive Notes

Meninges of the Brain vs. Spinal Cord

• Three layers surround CNS structures:
  • Dura mater
  • Brain: TWO fused sheets: periosteal layer (continuous with cranial bones) + meningeal layer → exceptionally tough.
  • Spinal cord: ONLY the meningeal layer; periosteal layer is absent → slightly less rigid.
  • Arachnoid mater
  • Loose-fitting, web-like.
  • Creates potential spaces both above (sub-dural) and below (sub-arachnoid).
  • Houses arachnoid villi (a.k.a. arachnoid granulations) around the brain—one-way valves that return CSF to venous blood (sagittal sinus). Not prominent around the spinal cord.
  • Pia mater
  • Delicate, thin, adherent to cortex/spinal surface; almost imperceptible to touch.

Cerebrospinal Fluid (CSF): Production, Circulation, Re-absorption

• Production
  • Site: Choroid plexus (capillary + ependymal cell complexes) in four ventricles:
  • One in each lateral ventricle (2)
  • Roof of third ventricle
  • Roof of fourth ventricle
  • Capillaries are continuous (tight junctions) → part of the Blood–Brain Barrier (BBB).
  • Ependymal cells possess tight junctions at plexus sites → CSF formed by controlled filtration & secretion (glucose, O₂, ions, minimal protein).
• Flow pathway
  1. Lateral ventricles
  2. \rightarrow Interventricular (foramen of Monro)
  3. Third ventricle
  4. \rightarrow Cerebral aqueduct
  5. Fourth ventricle
  6. Exit options from 4th ventricle:
  • Lateral apertures (2)
  • Median aperture (1, dorsal)
  • Down the central canal of spinal cord (diminishes with age)
  1. Sub-arachnoid space surrounding brain & cord
  2. Re-absorbed via arachnoid villi \rightarrow superior sagittal sinus \rightarrow venous circulation.
• One-way nature: villi/granulations prevent back-flow; maintain pressure equilibrium.

Functional Significance of CSF

• Provides buoyancy: brain “floats,” reducing effective weight & pressure on inferior cortex—"high tide raises all boats" analogy.
• Shock absorber for brain & cord.
• Vehicle for nutrient/waste exchange where BBB limits direct blood access.

Blood–Brain Barrier (BBB)

• Continuous capillaries + tight junctions throughout CNS, modulated by astrocytes (can relax/ tighten junctions).
• Restricts pathogens & many drugs; complicates CNS pharmacology.

Clinical Correlates: CSF & Trauma

• Concussion
  • Primary impact \rightarrow neuronal damage;
  • Secondary swelling/edema increases intracranial pressure (ICP) globally.
  • Severe cases may require decompressive craniectomy (bone flap often stored in patient’s abdominal wall).
• Lumbar puncture (spinal tap)
  • Performed below L3 (commonly L4–L5) into sub-arachnoid space: avoids cord (which ends ≈L1–L2).
  • Collects CSF for infection, hemorrhage, pressure studies.
  • Risks: nerve irritation (cauda equina), infection.

Spinal Cord: Gross Anatomy

• Extends from foramen magnum \rightarrow conus medullaris (~L1–L2).
• Filum terminale (pia extension) anchors cord to coccyx; maintains tension.
• Cauda equina: bundle of lumbar, sacral, coccygeal roots descending below conus.
• Enlargements
  • Cervical (upper limb innervation)
  • Lumbar (lower limb innervation).

Vertebral Relationships & Disc Pathology

• Spinal nerves exit intervertebral foramina between bodies/discs.
• Herniated (bulging) disc can compress a neighboring spinal nerve → dermatomal pain/weakness.

Cross-Section Anatomy (Typical Segment)

• Gray matter (H-shape)
  • Dorsal horns: sensory processing.
  • Ventral horns: somatic motor neuron somata (lower motor neurons).
  • Lateral horns (T1–L2): visceral motor (sympathetic).
  • Gray commissure with central canal.
• White matter (funiculi/columns)
  • Dorsal columns → DCML pathway (touch, proprioception).
  • Lateral & ventral columns → various ascending (spinothalamic, spinocerebellar) and descending (corticospinal) tracts.
• Roots & Ganglia
  • Dorsal root: sensory, houses dorsal root ganglion (pseudo-unipolar somata).
  • Ventral root: motor (somatic + autonomic).
  • Merge \rightarrow spinal nerve (mixed).

Spinal Nerves & Dermatomes

• Total pairs: 31 = 8\;\text{C} + 12\;\text{T} + 5\;\text{L} + 5\;\text{S} + 1\;\text{Co}.
• Dermatome map: cutaneous territories of individual spinal nerves.
  • Cervical (green in diagram): posterior head, neck, arms.
  • Thoracic (tan): torso bands.
  • Lumbar (blue/teal): anterior legs.
  • Sacral (yellow/red): posterior legs, perineum.
• Clinical use: locate cord/root lesions by sensory loss pattern.

Levels of Spinal Cord Injury (SCI)

• Rule: functions at level of lesion & below are lost.
  • C4 injury → quadriplegia, impaired shoulder elevation.
  • C6 injury → retains shoulder/partial arm, loses hand & all lower.
  • T6 injury → paraplegia; trunk control above mid-thorax intact.
  • L1 injury → paraplegia limited to lower limbs.
• Higher cervical (≥C3) compromises phrenic nerve → ventilatory dependence (e.g., Christopher Reeve, C3).

Central vs. Peripheral Lesions

• Peripheral (root or nerve)
  • Affects only one dermatome/myotome band.
  • Dorsal root lesion → pure sensory loss; ventral root lesion → pure motor loss.
• Central (cord)
  • Wider deficits: bilateral, multi-segment, often mixed sensory & motor.

Flaccid vs. Spastic Paralysis

Reflex Arc (Stretch Example)

1. Muscle spindle stretched → afferent AP via dorsal root.
2. Synapse (± interneuron) in dorsal/ventral horn.
3. Efferent LMN exits ventral root → same muscle contracts.

• Basis for clinical tendon reflex testing; absent reflex ⇒ LMN lesion, exaggerated ⇒ UMN lesion.

Nerve Regeneration vs. CNS Regeneration

• Peripheral: Schwann cells create regeneration tube; axons regrow \approx 1–3 mm/day if alignment, blood supply adequate → explains partial recovery after limb replantation.
• Central: Oligodendrocytes & CNS milieu release inhibitory molecules; crushed, not clean-cut injuries; limited regeneration.
• Active research: neutralizing inhibitors, stem cells, scaffolds.

Numbers & Equations to Remember

• CSF volume ≈ 150\;\text{mL}, replaced \sim every 8 h (≈ 500–700\;\text{mL/day} produced).
• Spinal nerve tally: 31 pairs.
• Spinal cord length: foramen magnum → \text{L}1/\text{L}2 (≈ 42–45\;\text{cm} in adults).
• Disc herniation commonly at \text{L}4–\text{L}5 or \text{L}5–\text{S}1.

Ethical / Practical Notes

• Historical dissections were clandestine (17th c. “grave-robbing”).
• Modern surgery: bone flap storage mishaps highlight procedural vigilance.
• Epidural anesthesia (labor): drug placed in epidural space (between dura & vertebral canal periosteum) → blocks roots without piercing dura.

Exam Tips Summarized

• Know meninges order & differences (brain vs. cord).
• Be able to trace CSF path & name apertures.
• Distinguish dorsal vs. ventral roots in images → identify ganglion.
• Match cord injury level to functional deficit (C vs. T vs. L).
• Use 31 = 8 + 12 + 5 + 5 + 1 for nerve counts.
• Differentiate flaccid vs. spastic paralysis by lesion site & reflex profile.