Upper Motor Neuron Lesion

Aka: Upper Motor Neuron Lesion, Upper Motor Neuron Deficit, Upper Motor Neuron

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II. Physiology

  1. Corticospinal tract (motor, lateral cord) transmits motor signals from the cerebral cortex
    1. Fibers cross the midline at the Brain Stem and course to the contralateral Muscles
  2. Motor fibers Synapse in the anterior horn (grey matter of the spinal cord)
    1. Upper Motor Neurons (UMN) are from the cerebral cortex to the Synapse
      1. Affect the cerebral cortex, Brainstem and Corticospinal tracts
    2. Lower Motor Neurons (LMN) are from the Synapse to the Muscle
      1. Affect the anterior horn (spinal cord), nerve roots, and Peripheral Nerves

III. Causes: Examples

  1. See Muscle Weakness
  2. CNS Lesion
  3. Spinal Cord Syndrome
  4. Multiple Sclerosis

IV. Signs: Upper Motor Neuron Lesion findings

  1. Muscle group weakness (not complete paralysis)
  2. Minimal Muscle atrophy
  3. "Clasp-knife" spasticity
    1. Initial resistance to motion is strongest
    2. Once initial resistance gives-way, resistance to motion is less for remaining range of motion
  4. Hyperreflexia with or without Clonus
  5. Babinski Reflex positive response
  6. Pathologic spread of reflexes
    1. Muscle Contractions produce motions in Muscles other than the one tested
  7. Precaution
    1. Cauda equina are all Peripheral Nerves
    2. Upper Motor Neuron findings for levels below T12 or L1 must be central lesions in the higher cord
      1. Strongly consider severe Cervical Spinal Stenosis
  8. Reference
    1. Robbins (2012) Board Review Express, San Jose

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