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)
  • : Motor (corticospinal, vestibulospinal, reticulospinal)