Spinal Cord: Segments, Reflexes, and Clinical Syndromes

Overview

The spinal cord is a cylindrical bundle of nervous tissue extending from the foramen magnum (continuous with the medulla oblongata) to the L1-L2 vertebral level in adults. It is the body's main conduit for sensory and motor information between the brain and the periphery.

Length and Position

• Length: ~42-45 cm in adults
• Ends at: L1-L2 vertebral level in adults (L3 in neonates — important for lumbar puncture!)
• Enlargements:
  • Cervical enlargement (C5-T1): Supplies the upper limbs
  • Lumbosacral enlargement (L2-S3): Supplies the lower limbs

Spinal Cord Segments (31 total)

• Cervical: 8 segments (C1-C8)
• Thoracic: 12 segments (T1-T12)
• Lumbar: 5 segments (L1-L5)
• Sacral: 5 segments (S1-S5)
• Coccygeal: 1 segment (Co1)

Total: 31 pairs of spinal nerves

Important Note: Spinal Cord vs Vertebral Levels

The spinal cord is shorter than the vertebral column. Therefore, lower cord segments are located higher than their corresponding vertebrae:

• Spinal cord segment C7 → exits at C7 vertebra (above)
• Spinal cord segment T7 → exits at T7 vertebra (above)
• Below T10, there is a 2-3 segment discrepancy
• Lumbosacral cord segments (L2-S5) are at T12-L1 vertebral levels

Clinical relevance: A lesion at T10 vertebra affects the L1 cord segment (and below).

Internal Structure (Cross-section)

• Gray matter (H-shaped in center): Contains neuronal cell bodies
  • Dorsal (posterior) horn: Sensory input
  • Ventral (anterior) horn: Motor neurons (lower motor neurons)
  • Lateral horn: T1-L2 (sympathetic) and S2-S4 (parasympathetic)
• White matter (surrounding): Myelinated tracts
  • Ascending tracts: Sensory (spinothalamic, dorsal columns)
  • Descending tracts: Motor (corticospinal, vestibulospinal, reticulospinal)

Key Spinal Tracts

Ascending (Sensory)

1. Dorsal column-medial lemniscus:
   • Function: Fine touch, vibration, conscious proprioception
   • Pathway: Sensory receptor → dorsal root → dorsal column (ipsilateral) → medulla (cuneate and gracile nuclei) → decussates → medial lemniscus → thalamus → cortex
2. Spinothalamic tract:
   • Function: Pain, temperature, crude touch
   • Pathway: Receptor → dorsal horn → decussates within 1-2 levels → spinothalamic tract (contralateral) → thalamus → cortex
3. Spinocerebellar tracts:
   • Function: Unconscious proprioception
   • Pathway: Receptor → dorsal horn → cerebellum (does not reach cortex)

Descending (Motor)

1. Lateral corticospinal tract:
   • Function: Voluntary motor control of limbs
   • Pathway: Cortex → decussates at medullary pyramids → spinal cord → ventral horn
   • 85-90% of corticospinal fibers cross here
2. Anterior corticospinal tract: Uncrossed; ~10-15% of fibers; controls axial muscles

Spinal Reflexes

Monosynaptic Stretch Reflex

1. Patellar (knee jerk): L2-L4
2. Achilles (ankle jerk): S1-S2
3. Biceps: C5-C6
4. Triceps: C7-C8
5. Brachioradialis: C5-C6

Withdrawal (Polysynaptic)

• Nociceptive flexion reflex (pull away from pain)

Pathologic Reflexes (Babinski)

• Positive Babinski (extensor plantar): UMN lesion above S1
• Normal adult: Plantar flexion of toes

Meninges and CSF

• Dura mater: Tough outer layer
• Arachnoid mater: Middle; CSF in subarachnoid space
• Pia mater: Adherent to cord
• Lumbar cistern: Subarachnoid space below L2; site for lumbar puncture (between L3-L4 or L4-L5)

Cauda Equina

• Below L1-L2, the spinal cord ends (conus medullaris), and nerve roots descend to exit at lower vertebral foramina — resembling a horse's tail
• Cauda equina syndrome: Compression of these nerve roots
  • Saddle anesthesia (perineum, inner thighs)
  • Bowel/bladder dysfunction (urinary retention)
  • Lower limb weakness
  • Sexual dysfunction
  • Loss of anal sphincter tone
  • Surgical emergency

Brown-Séquard Syndrome (Spinal Cord Hemisection)

• Ipsilateral loss of: Fine touch, vibration, proprioception (dorsal column), UMN signs below lesion (corticospinal)
• Contralateral loss of: Pain and temperature (spinothalamic) starting 1-2 levels below lesion
• Causes: Trauma, tumor, multiple sclerosis

Anterior Cord Syndrome

• Damage to anterior 2/3 of spinal cord (anterior spinal artery occlusion)
• Bilateral loss of motor + pain/temperature below lesion
• Dorsal columns spared → preserved vibration and proprioception

Posterior Cord Syndrome

• Rare; loss of dorsal columns only
• Loss of vibration and proprioception

Central Cord Syndrome

• Common in elderly with hyperextension injuries
• Greater weakness in upper than lower extremities
• Due to somatotopic organization of corticospinal tract (arms medial, legs lateral)

Key Takeaway

The spinal cord is a precision structure with somatotopic organization. Know the 31 segments, key reflexes (C5-C6 biceps, L2-L4 patellar, S1-S2 Achilles), the difference between vertebral and cord levels (L1-L2 cord end), and the classic syndromes (Brown-Séquard, anterior cord, cauda equina).