Spinal Cord Anatomy & Clinical Correlates

Termination Point of the Spinal Cord

• Begins at the base of the brain but ends roughly halfway down the back.
  • Anatomically located at the level of L_1 (first lumbar vertebra).
  • Common misconception: many believe the cord descends the entire bony spine; in reality, only peripheral nerves continue below L_1.

Distal (Inferior) Anatomy of the Cord

• Conus Medullaris
  • Tapered, cone-shaped terminus of the cord.
  • Name: Latin; “conus” = cone, “medulla” = marrow.
• Cauda Equina (“horse’s tail”)
  • Bundle of spinal roots that fan out below the conus.
  • Purely peripheral nerves—no central cord tissue here.
• Stabilising Ligaments
  • Denticulate ligaments – lateral tooth-like pia extensions tether cord segmentally to vertebrae.
  • Filum Terminale – single midline “end string” anchoring conus to the coccyx.
  • Prevents cord from free-floating or twisting inside vertebral canal.

Cross-Sectional Anatomy

• • Gray Matter (central “butterfly”)
  • Contains neuronal cell bodies & interneurons.
  • White Matter (peripheral rim)
  • Contains myelinated axons bundled into tracts.
• Brain vs. Cord contrast:
  • Brain = gray outside / white inside.
  • Cord = gray inside / white outside.

Functional Roots & Nerves

• Ventral (Anterior) Root
  • 100 % motor (efferent) axons leave cord here.
• Dorsal (Posterior) Root
  • 100 % sensory (afferent) axons enter cord here.
  • Swelling = Dorsal Root Ganglion (DRG) housing cell bodies of pseudo-unipolar sensory neurons.
• Spinal Nerve (mixed)
  • Ventral + dorsal roots merge → single nerve containing both modalities.
• Rami Split
  • Posterior (dorsal) Ramus – smaller; supplies skin & muscles of the back.
  • Anterior (ventral) Ramus – larger; supplies anterolateral trunk, limbs; more axons because “more skin in front.”

Longitudinal Tract Organisation (“Neural Highways”)

• Ascending (sensory) tracts generally lie posteriorly.
• Descending (motor) tracts generally lie anteriorly.
• Visual mnemonic: car motor sits in the front → motor pathways in front of cord.

Neuron Chains per Modality

• Motor pathways: 2 neurons (upper motor neuron → lower motor neuron).
• Sensory pathways: 3 neurons (1° → 2° → 3°) to reach cortex.

Dermatomes – Cutaneous Map

• "Dermatome" = skin segment supplied by a single spinal nerve pair.
• Key landmarks to memorise:
  • T_4 – nipple line.
  • T_{10} – umbilicus.
  • Additional (preview): C4, C8, T1, T6, L_1.

Clinical Correlates: Spinal Cord Injury (SCI)

• Complete SCI – full transection → total loss of motor & sensory below lesion.
• Incomplete / Hemicord Patterns
  • Central cord, anterior cord, posterior cord, hemisection (Brown-Séquard) each produce different deficits.
• Quick reasoning with basic rules:
  • Posterior-cord damage → mainly sensory loss; motor spared.
  • Anterior-cord damage → mainly motor loss; sensory spared.
• Level-dependent Syndromes
  • C_4 & above → risk of respiratory failure (diaphragm paralysis).
  • C_8 & above → quadriplegia (all limbs paralyzed).
  • T_1 & below → paraplegia (legs only).
  • T_6 lesion → neurogenic shock (loss of sympathetic BP/HR control).
  • L1–L2 → bladder dysfunction (spastic ↔ flaccid bladder).
  • Rule: “and above” means higher lesions include every deficit below on the list.

Real-World & Ethical Notes

• Diving into shallow water is a common preventable cause of cervical SCI (example of Guy’s neck injury story).
• Healthcare providers must assess:
  • Sensory loss pattern (dermatomes) & motor loss pattern to localise lesion.
  • Respiratory status in high cervical injuries.
  • Autonomic stability (BP, HR) in thoracic injuries (>T_6).
• Patient-centred implications:
  • Loss of bladder/bowel control requires lifelong management; affects dignity & independence.
  • Ethical duty to provide counselling on risk-taking behaviors (e.g., diving, contact sports).

Preview of Upcoming Sessions

• Thursday: draw & analyse three canonical tracts – 2 sensory, 1 motor.
• Apply 2-neuron vs. 3-neuron rule to map lesions.
• Use dermatomes to diagnose SCI level & predict functions lost or spared.