Neuraxial Anatomy

Vertebral Column & Ligaments - Bone Zone & Ties

• Vertebral Column:
  • Composed of 33 vertebrae (C7, T12, L5, S5, Co4).
  • Curves: Primary (Kyphotic - Thoracic, Sacral); Secondary (Lordotic - Cervical, Lumbar).
  • Vertebral canal: Houses spinal cord & meninges.
  • Intervertebral foramina: Spinal nerve passage.
• Key Spinal Ligaments (encountered posterior to anterior in midline approach):
  • Supraspinous: Connects tips of spinous processes.
  • Interspinous: Between adjacent spinous processes.
  • Ligamentum Flavum: Connects laminae; yellow, elastic. Strongest.

  ⭐ Ligamentum flavum is the thickest spinal ligament, offering distinct resistance during epidural needle insertion.

  • Posterior Longitudinal Ligament (PLL): On posterior vertebral bodies.
  • Anterior Longitudinal Ligament (ALL): On anterior vertebral bodies.

Spinal Cord & Meninges - Cord Central & Wraps

• Spinal Cord:
  • From medulla. Segments: C8, T12, L5, S5, Co1.
  • Enlargements: Cervical (brachial plexus), Lumbar (lumbosacral plexus).
  • Conus Medullaris (termination): Adults L1-L2, Neonates L3.
  • Filum Terminale (pia): Anchors cord.
  • Cauda Equina: Lumbosacral nerve roots below conus.
• Meninges (Outer → Inner): 📌 DAP (Dura, Arachnoid, Pia)
  • Dura Mater: Tough outer layer; dural sac ends at S2.
    • Epidural Space: Between dura & vertebrae; fat, venous plexuses.
  • Arachnoid Mater: Delicate middle layer.
    • Subarachnoid Space (SAS): Between arachnoid & pia; CSF, site for spinal anesthesia.
  • Pia Mater: Innermost layer, adheres to cord; forms denticulate ligaments & filum terminale.

, cauda equina, and meningeal layers (dura, arachnoid, pia))

⭐ In adults, the spinal cord typically terminates at the lower border of L1 or upper border of L2 vertebra (conus medullaris), while the dural sac extends to S2.

Epidural Space - The Epi Envelope

• Potential space: External to dura mater; internal to ligamentum flavum & vertebral periosteum.
• Boundaries:
  • Superiorly: Foramen magnum (dura fuses with periosteum).
  • Inferiorly: Sacral hiatus (sacrococcygeal membrane).
  • Anteriorly: Posterior longitudinal ligament.
  • Posteriorly: Ligamentum flavum, vertebral laminae.
  • Laterally: Pedicles, intervertebral foramina.
• Contents:
  • Loose areolar (adipose) tissue
  • Internal vertebral venous plexus (Batson's plexus)
  • Arteries (segmental spinal branches)
  • Lymphatics
  • Spinal nerve roots (after dural exit)
• Clinically significant for epidural anesthesia/analgesia. Pressure is typically subatmospheric (negative), but can be positive.

⭐ The epidural space contains Batson's plexus, a network of valveless veins that can become engorged during pregnancy or with increased intra-abdominal pressure, increasing the risk of intravascular catheter placement during epidural procedures and facilitating hematogenous spread of infection/metastases to vertebrae/CNS.

Subarachnoid Space - CSF Sea Deep

• Location: Between arachnoid and pia mater.
• Contents:
  • Cerebrospinal Fluid (CSF) - the "CSF Sea"
  • Spinal nerve roots (cauda equina)
  • Spinal blood vessels
• Extent:
  • Cranial: Continuous with intracranial subarachnoid space.
  • Caudal limit: S2 vertebral level (dural sac).
• Clinical Significance:
  • Spinal anesthesia target (intrathecal).
  • Lumbar Puncture (LP) for CSF sampling.

⭐ The volume of cerebrospinal fluid (CSF) in the spinal subarachnoid space is approximately 25-35 mL in adults.

Dermatomes & Vascular Supply - Nerve Maps & Pipes

• Key Dermatomes (Block Height Assessment):
  • Nipple: T4
  • Xiphoid process: T6
  • Umbilicus: T10 (📌 "Tummy-TEN")
  • Inguinal ligament: L1
  • Little toe: S1
• Spinal Cord Arterial Supply:
  • Anterior Spinal Artery (ASA): Supplies anterior 2/3rds.
  • Posterior Spinal Arteries (PSAs) (2): Supply posterior 1/3rd.
  • Segmental medullary arteries reinforce supply.

⭐ The Artery of Adamkiewicz (great anterior radicular artery) is the major blood supply to the anterior lower two-thirds of the spinal cord, typically arising from a left intercostal artery between T9 and L2.

• Venous Drainage: Internal vertebral venous plexus (Batson's plexus) - valveless.

High‑Yield Points - ⚡ Biggest Takeaways

• Spinal cord termination: L1-L2 in adults, extends to L3 in neonates.
• Dural sac and subarachnoid space typically terminate at the S2 vertebral level.
• Ligamentum flavum: thickest ligament, first significant resistance felt.
• Epidural space: a potential space containing fat, veins, and lymphatics.
• Tuffier's line (intercristal line): marks L4 spinous process or L4-L5 interspace.
• Cerebrospinal Fluid (CSF): volume ~150 mL; specific gravity 1.003-1.009.
• Sacral hiatus: formed by non-fusion of S4/S5 laminae, used for caudal epidural block.