Detailed Anatomy of the Human Spinal Cord

1. Introduction to Spinal Cord Anatomy

The spinal cord is a cylindrical bundle of nerve fibers and associated tissue that extends from the medulla oblongata down through the vertebral column to the lumbar region. It serves as the primary communication pathway between the brain and the peripheral nervous system, facilitating both sensory input and motor output. The spinal cord is approximately 40-50 cm long in adults and occupies the upper two-thirds of the vertebral canal.

2. External Anatomy and Protection

• Meninges: The spinal cord is surrounded by three protective layers:
  • Dura mater: Tough, outermost fibrous membrane
  • Arachnoid mater: Middle web-like layer containing cerebrospinal fluid
  • Pia mater: Innermost delicate layer that closely follows spinal cord contours
• Spaces:
  • Epidural space: Between dura mater and vertebral canal (contains fat and veins)
  • Subdural space: Between dura and arachnoid mater
  • Subarachnoid space: Between arachnoid and pia mater (contains CSF)
• Vertebral Protection: Encased within the vertebral foramen formed by stacked vertebrae
• Denticulate Ligaments: Lateral extensions of pia mater that anchor spinal cord to dura mater

3. Internal Gray Matter Organization

The spinal cord's gray matter forms an H-shaped or butterfly-shaped region in cross-section:

• Anterior (Ventral) Horn: Contains motor neuron cell bodies that innervate skeletal muscles
  • Medial motor neuron pools: Axial muscles
  • Lateral motor neuron pools: Limb muscles
• Posterior (Dorsal) Horn: Receives sensory input from dorsal root ganglia
  • Marginal zone (Lamina I): Pain and temperature
  • Substantia gelatinosa (Lamina II): Modulation of pain and temperature
  • Nucleus proprius (Lamina III-IV): Touch and pressure
• Intermediate Zone (Lateral Horn): Contains preganglionic sympathetic neurons (T1-L2) and parasynaptic neurons (S2-S4)
• Central Canal: CSF-filled remnant of neural tube, surrounded by ependymal cells

4. White Matter Tracts and Funiculi

White matter surrounds the gray matter and is divided into three funiculi:

• Anterior (Ventral) Funiculus:
  • Anterior corticospinal tract: Bilateral voluntary motor control
  • Anterior spinothalamic tract: Crude touch and pressure
• Lateral Funiculus:
  • Lateral corticospinal tract: ~90% of voluntary motor fibers (contralateral)
  • Corticobulbar tracts: Cortical input to cranial nerve nuclei
  • Spinothalamic tract: Pain and temperature (contralateral)
  • Spinocerebellar tracts: Proprioception to cerebellum (ipsilateral)
• Posterior (Dorsal) Funiculus:
  • Fasciculus gracilis: Fine touch, vibration, proprioception from lower body
  • Fasciculus cuneatus: Fine touch, vibration, proprioception from upper body
  • Both ascend to nucleus gracilis and cuneatus in medulla

5. Spinal Cord Segments and Enlargements

The spinal cord is divided into segments corresponding to vertebral levels:

• Cervical Enlargement (C4-T1): Increased gray matter for upper limb innervation
• Lumbar Enlargement (L1-S2): Increased gray matter for lower limb innervation
• Conus Medullaris: Terminal tapered end around L1-L2 vertebral level
• Cauda Equina: Collection of spinal nerve roots (L2-Co) resembling horse's tail
• Filum Terminale: Pia mater extension anchoring cord to coccyx

6. Blood Supply

• Anterior Spinal Artery: Single midline artery running in anterior median fissure (from vertebral arteries)
• Posterior Spinal Arteries: Paired arteries running in posterolateral sulci (from vertebral arteries)
• Radicular Arteries: Segmental feeders from aorta, intercostal, lumbar arteries
• Artery of Adamkiewicz: Largest radicular artery (usually T9-L2 on left), crucial for lumbar cord

7. Clinical Correlations

• Complete Transection: Loss of all motor, sensory, and autonomic function below lesion
• Brown-Séquard Syndrome: Hemisection causing ipsilateral motor/proprioception loss and contralateral pain/temperature loss
• Anterior Cord Syndrome: Damage to anterior cord causing motor loss and pain/temperature loss below lesion
• Central Cord Syndrome: Greater upper extremity weakness than lower (often hyperextension injury)
• Tabes Dorsalis: Degeneration of posterior columns (sensory ataxia, lightning pains)
• Amyotrophic Lateral Sclerosis: Degeneration of corticospinal tracts and anterior horn cells
• Multiple Sclerosis: Demyelination plaques in white matter, often periventricular
• Syringomyelia: CSF-filled cavity within spinal cord, often expanding syrinx
• Spinal Cord Infarction: Often due to aortic surgery disrupting artery of Adamkiewicz

8. Review Questions

1. What are the three meninges surrounding the spinal cord, from superficial to deep?
2. Which spinal cord tracts decussate (cross midline) and where does this occur?
3. What is the functional difference between the fasciculus gracilis and fasciculus cuneatus?
4. Where is the lateral horn located and what type of neurons does it contain?
5. What vertebral level typically corresponds to the conus medullaris?
6. Which artery is most critical for the blood supply of the lumbar spinal cord?
7. What syndrome is characterized by ipsilateral motor loss and contralateral pain/temperature loss?
8. Which tracts are primarily affected in central cord syndrome?
9. What condition results from degeneration of the posterior columns of the spinal cord?
10. What is the cauda equina and at what vertebral level does it begin?