- Common sites: L4–L5, L5–S1 (lumbar); C5–C6, C6–C7 (cervical).
- Sciatica: pain along the sciatic nerve distribution from lumbosacral nerve root compression.
- Posterolateral herniation spares the spinal cord but impinges on exiting nerve roots.
embryology Sclerotome cells (somitic mesoderm) migrate around the notochord and neural tube. Vertebral bodies form from the caudal half of one sclerotome and the cephalic half of the adjacent one — this resegmentation pattern gives the spine its segmental character.
🦴 Vertebral column
composition 33 vertebrae (7 C, 12 T, 5 L, 5 sacral fused, 3–4 coccygeal)
Typical vertebra: anterior body + posterior arch (2 pedicles, 2 laminae). The vertebral foramen encloses the spinal cord; intervertebral foramina (formed by adjacent vertebral notches) allow spinal nerve exit.
- Body – weight‑bearing
- Pedicles & laminae – form the arch
- Transverse & spinous processes – muscle/ligament attachment
Zygapophyseal joints (facet joints) — synovial articulations between superior and inferior articular processes; guide spinal movement.
📌 High yield: The spinal cord ends at L1–L2 (adult); the dural sac ends at S2 — key for lumbar puncture.
🔄 Intervertebral disk
Contribute ~25% of spinal column length. They are fibrocartilaginous joints (symphyses) between vertebral bodies.
- Annulus fibrosus – outer concentric rings of fibrocartilage; resist tension and limit motion.
- Nucleus pulposus – gelatinous core, remnant of notochord; acts as a hydraulic shock absorber.
⚠️ Disk herniation – most commonly posterolateral (posterior longitudinal ligament is weaker laterally). May compress nerve roots one level below the affected disk (e.g., L4/L5 disk → L5 root; C6/C7 disk → C8 root).
Ligaments: anterior longitudinal (prevents hyperextension), posterior longitudinal (limits flexion; located within vertebral canal).
🧠 Spinal meninges
Three connective tissue layers protect the spinal cord within the vertebral canal:
- Dura mater – tough, fibrous sac; ends at S2. Continuous with cranial dura at foramen magnum.
- Arachnoid – delicate, avascular; lines the dura. Subarachnoid space contains CSF.
- Pia mater – adherent to cord; forms denticulate ligaments (anchor to dura) and filum terminale (continuation beyond conus medullaris).
Epidural space – between bone and dura; contains fat and internal vertebral venous plexus (valveless, connects with pelvic/thoracic veins → potential route for metastasis).
Subarachnoid space – between arachnoid and pia; contains CSF; ends at S2.
🔬 Pearls: Conus medullaris (spinal cord termination) at L1–L2; dural sac and CSF end at S2. The filum terminale is part of the cauda equina.
⚡ Spinal nerves
31 pairs: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal.
- Dorsal root – sensory (pseudounipolar cell bodies in dorsal root ganglion).
- Ventral root – motor (cell bodies in ventral/lateral horn).
- Spinal nerve – mixed, formed by union of dorsal and ventral roots; exits via intervertebral foramen.
- Dorsal ramus – supplies deep back muscles and skin of the back.
- Ventral ramus – supplies anterolateral trunk and limbs (forms plexuses).
Exit pattern: C1–C7 exit above their corresponding pedicles; C8 exits below C7; T1 and below exit below the same-numbered pedicle.
💉 Lumbar puncture
Typically performed at L4–L5 interspace (iliac crest line marks L4). Needle passes through:
- Skin → superficial/deep fascia → supraspinous ligament → interspinous ligament → ligamentum flavum → epidural space → dura → arachnoid → subarachnoid space.
Clinical: Flexion of the spine widens interlaminar spaces. Used to obtain CSF or inject epidural anesthesia.
🌿 Autonomic nervous system
Two-neuron chain: preganglionic (CNS) → postganglionic (autonomic ganglion).
Sympathetic (thoracolumbar)
Preganglionic cell bodies in T1–L2 lateral horn. Postganglionic in paravertebral (chain) or prevertebral (collateral) ganglia.
T1–L2→
white rami→
sympathetic chain→
postganglionic
- Paravertebral – body wall, limbs, head, thoracic viscera.
- Prevertebral (celiac, superior mesenteric, inferior mesenteric) – abdominal/pelvic viscera.
Horner syndrome (ipsilateral ptosis, miosis, anhydrosis) – lesion in sympathetic pathway (T1–T2 or cervical chain).
Parasympathetic (craniosacral)
Preganglionic: brainstem (CN III, VII, IX, X) and S2–S4 (pelvic splanchnics). Postganglionic in terminal ganglia near or within target organs.
- Cranial – ciliary, pterygopalatine, submandibular, otic ganglia.
- Vagus (X) – thoracic and abdominal viscera (foregut, midgut).
- Pelvic splanchnics (S2–S4) – hindgut, bladder, reproductive organs.
| Division | Origin | Ganglion | Target |
|---|---|---|---|
| Sympathetic | T1–L2 | Paravertebral / prevertebral | Smooth muscle, glands, cardiac |
| Parasympathetic | CN III,VII,IX,X & S2–S4 | Terminal (near organ) | Glands, smooth muscle, viscera |
🧬 Clinical pearls & exam facts
- Conus medullaris – L1–L2; cauda equina below.
- Dural sac – ends at S2; subarachnoid space extends to S2.
- Epidural venous plexus – valveless → metastatic spread to vertebrae/brain.
- Herniated disk – most often posterolateral; nerve root affected is one level below the disk.
- Lumbar puncture – L4–L5; needle traverses ligamentum flavum, epidural, dura, arachnoid.
- Sympathetic vs parasympathetic – thoracolumbar vs craniosacral; 2-neuron pathway.
- Horner syndrome – loss of sympathetic innervation to face (ptosis, miosis, anhydrosis).
| Feature | Sympathetic | Parasympathetic |
|---|---|---|
| Origin | T1–L2 | CN III,VII,IX,X & S2–S4 |
| Ganglia | Paravertebral / prevertebral | Terminal (intramural or near organ) |
| Preganglionic length | short | long |
| Postganglionic length | long | short |
| Neurotransmitter (postganglionic) | norepinephrine (most) | acetylcholine |