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The Nervous System — Overview & Spinal Cord Anatomy

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1. Overview of the Nervous System

Anatomical Division

Functional Division

Cells of the Nervous System

• Cell body (perikaryon) — contains nucleus + Nissl bodies
• Axon — carries impulses away from cell body (usually one, usually long)
• Dendrites — carry impulses toward cell body (many, short)

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2. Spinal Cord — Macrostructure

Location & Extent

• Starts at the foramen magnum (continuous with medulla oblongata)
• Ends at the level of the L1–L2 intervertebral disc in adults → this distal tip is the conus medullaris
• In neonates, it reaches down to ~L3
• A fine connective tissue thread, the filum terminale (pia part), continues from the conus inferiorly and anchors at the coccyx

Viva tip: The cord ends at L1–L2 but the dural sac (subarachnoid space) extends to S2. This is why lumbar puncture is done at L3–L4 or L4–L5 — you tap CSF safely below the cord.

Shape & Enlargements

External Surface Landmarks

• Anterior median fissure — deep groove along the front surface (midline)
• Posterior median sulcus — shallow groove along the back (midline)
• Posterolateral sulci (one on each side) — where posterior (sensory) rootlets enter the cord

Spinal Nerve Roots

• 31 pairs of spinal nerves emerge along the cord (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal)
• Anterior root = motor (efferent)
• Posterior root = sensory (afferent) — has the posterior root ganglion (dorsal root ganglion)
• Below the conus medullaris the roots descend as the cauda equina ("horse's tail") before exiting at their levels

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3. Internal Structure — Gray and White Matter

Overall Layout

  White matter
    Gray matter  (H-shaped)
      Central canal
    Gray matter
  White matter

Gray Matter

• Rich in neuron cell bodies
• In cross-section looks like the letter "H" or a butterfly
• Divided into horns (columns):

Viva pearl: A patient with a lesion destroying anterior horn cells (e.g., poliomyelitis, motor neuron disease) has lower motor neuron signs — flaccid paralysis, wasting, fasciculations, absent reflexes.

White Matter

• Rich in myelinated axons arranged into tracts (fasciculi)
• Divided into three paired funiculi (columns):

Key ascending tracts (sensory, going UP to brain):

Key descending tracts (motor, going DOWN from brain):

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4. Spinal Cord Membranes (Meninges)

Dura Mater ("tough mother")

• Outermost, thickest, toughest layer — leathery, fibrous
• The spinal dura is continuous with the inner (meningeal) layer of the cranial dura
• Extends from foramen magnum to the level of S2 vertebra
• Not attached to bone; separated from the vertebral canal wall by the epidural (extradural) space, which contains fat and the internal vertebral venous plexus
• Potential subdural space lies just deep to it

Arachnoid Mater ("spider web mother")

• Middle layer; thin and translucent, like a spider web
• In the spine, it is NOT tightly attached to the dura (unlike the brain) — potential subdural space between them
• Separated from pia by the subarachnoid space
• The subarachnoid space extends to S2 (below the cord which ends at L1–L2) → this enlarged pool of CSF below the cord is the lumbar cistern — the target for lumbar puncture

Pia Mater ("gentle/tender mother")

• Innermost, thinnest, most delicate — directly adherent to the spinal cord surface
• Highly vascular
• Forms the denticulate ligaments — lateral extensions of pia between the nerve roots that pierce the arachnoid and attach to the dura, anchoring the cord in the center of the subarachnoid space like a hammock
• At the conus medullaris, pia continues as the filum terminale → pierces the dural sac at S2 → attaches to the back of the coccyx as the coccygeal ligament

Spaces — Summary

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Quick-Fire Viva Points

• Conus medullaris = tapered tip of cord at L1–L2
• Cauda equina = bundle of lumbar/sacral roots below conus (looks like horse's tail)
• Filum terminale = pia extension anchoring cord to coccyx
• Lumbar cistern = subarachnoid CSF pool L2–S2 → safe for LP at L3/L4 or L4/L5
• Gray matter = H-shaped (anterior horn = motor, posterior horn = sensory, lateral horn = autonomic)
• White matter = funiculi (anterior, lateral, posterior) containing ascending and descending tracts
• Dorsal columns = fine touch/vibration/proprioception; Spinothalamic = pain/temperature
• Epidural space = outside dura (real space) vs. subdural = potential space
• Denticulate ligament = pia anchors cord sideways to dura

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