Spinal Cord

Overview

Snapshot

• A 26-year-old male presents with lower back pain that radiates down the side of his right leg and into his right foot. The pain started after he was recently lifting furniture into his new home. On physical exam, there is notable weakness on dorsiflexion of the foot and extension of the toe. Sensory loss is noted between the first and second digit of the foot. Knee and ankle reflexes are normal. There is no saddle anesthesia, and anal wink is appreciated. When raising the extended leg while in the supine position, pain radiates from the lower back, down the leg, and into the foot. (L5 radiculopathy due to disc herniation)

Introduction

• Spinal nerves  
  • 31 pairs
    • 8 cervical
    • 12 thoracic
    • 5 lumbar
    • 5 sacral
    • 1 coccygeal
  • vertebral exit
    • C1-C7 exit via intervertebral foramina above corresponding vertebra
    • all other nerves exit via intervertebral foramina below corresponding vertebra
• Spinal cord termination
  • adults
    • spinal cord ends at L1-L2
      • becomes cauda equina
    • subarachnoid space extends to lower border of S2
    • see Lumbar puncture topic 
• Pathology
  • vertebral disk herniation
    • nucleus pulposus herniates posteriolaterally through annulus fibrosis
    • most commonly occurs between L5 and S1 
    • cauda equina affects S2-S5 nerve roots  
  • spinal cord syndromes
    • anterior cord syndrome
• ischemia affecting region supplied by anterior spinal artery (anterior 2/3 of spinal cord) 

Spinal Cord Pathways

• Sensory pathways
  • dorsal columns 
    • function
      • ascending pressure, vibration, discriminative touch, and proprioceptive sensation
    • subdivisions
      • fasciculus cuneatus
        • upper body and extremities
      • fasciculus gracilis 
        • lower body and extremities
    • pathway
      • 1st order neuron
        • sensory nerve ending → dorsal root ganglion (location of cell body) → spinal cord → ipsilateral ascent in dorsal column → synapse in ipsilateral nucleus in medulla (cuneatus or gracilis)
      • 2nd order neuron
        • ipsilateral nucleus in medulla → decussates in medulla → contralateral ascent in medial lemniscus → synapse in VPL of thalamus
      • 3rd order neuron
        • VPL of thalamus → sensory cortex
  • Spinothalamic tract
    • function
      • ascending pain and temperature sensation 
    • pathway
      • 1st order neuron
        • sensory nerve ending → dorsal root ganglion (location of cell body) → spinal cord → synapse in ipsilateral gray matter of spinal cord
      • 2nd order neuron
        • ipsilateral gray matter of spinal cord → decussates at anterior white commissure → contralateral ascent in spinothalamic tract → synapse in VPL of thalamus
      • 3rd order neuron
        • VPL of thalamus → sensory cortex
• Motor pathways
  • lateral corticospinal tract   
    • function
      • descending voluntary motor control of contralateral limbs
    • pathway
      • 1st order neuron (upper motor neuron)
        • cell body in primary motor cortex → ipsilateral descent through internal capsule → decussation at caudal medulla (pyramidal decussation) → contralateral descent in lateral corticospinal tract → synapse at cell body of anterior horn in spinal cord
      • 2nd order neuron (lower motor neuron)
        • leaves anterior horn of spinal cord → synapses at neuromuscular junction
    • anterior spinal artery strokes leads to
      • loss of deep tendon reflexes and flaccid weakness acutely after the stroke 
      • hyperreflexia and spascity days to weeks following a stroke