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

IV. Signs: Lower Motor Neuron Lesion findings

  1. Flaccid Paralysis
  2. Muscular atrophy
  3. Muscle Fasciculation (Muscle Twitch)
    1. Involuntary fine Muscle Contractions and relaxations of an entire motor unit synchronously
    2. May be a benign finding, in the absence of a lower motor lesion
  4. Muscle fibrillations
    1. Small, local Muscle Contractions of single Muscle fibers that contract independently of surrounding fibers
    2. Contractions are typically rapid, irregular and unsynchronized
  5. Hyporeflexia
  6. Absent Babinski Reflex

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