Medicine & USMLE

UMN vs. LMN Lesions

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Spinal Lesions
  1. UMN vs. LMN Lesions
  2. Werdnig-Hoffman Disease (SMA Type 1)
  3. Amyotrophic Lateral Sclerosis (ALS)
  4. Tabes Dorsalis
  5. Cauda Equina Syndrome
  6. Syringomyelia
  7. Brown-Sequard Syndrome

Summary

Upper motor neurons have cell bodies originating in the cerebral cortex or brainstem that terminate in the spinal cord. They synapse with lower motor neurons, which receives this information and innervates the muscle. UMN lesions can cause increased muscle tone, spasticity and rigid paralysis, hyperreflexia, and a positive Babinski sign. LMN lesions result in decreased muscle tone, flaccid paralysis, hyporeflexia, muscle atrophy, and fasciculations. Both types of lesions cause muscle weakness.

Key Points

  • UMN vs. LMN Lesions
    • UMN Lesions
      • Increased muscle tone, likely due to loss of descending inhibition
      • Muscle weakness (non-specific)
      • Spastic (rigid) paralysis
        • clasp-knife rigidity/spasticity
        • Increased muscle tone
      • Hyperreflexia (including clonus)
        • positive Babinski sign (upwards plantar reflex)
    • LMN Lesion
      • Decreased muscle tone, due to direct loss of innervation
      • Muscle weakness (non-specific)
      • Flaccid paralysis
        • Hypotonia; decreased muscle tone
      • Hyporeflexia
      • Muscle atrophy
      • Fasciculations