Upper motor neuron (UMN) injury vs. lower motor neuron (LMN) injury

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Fasciculations are caused by the spontaneous discharges produced by the damaged lower motor neurons.

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In UMN lesions

  • Lower extremity: muscle spasticity is more increased in extensor muscles.
    • When this inhibitory control is lost or reduced due to an UMN lesion, the result is an increase in muscle tone and spasticity, more so in muscles with anti-gravity functions such as the extensors in the legs.
  • Upper Extremities (e.g., Arms): flexor muscles tend to exhibit increased tone and spasticity in UMN lesions
    • This is related to the natural posture and use of these limbs, where flexion is a more dominant and protective posture.
  • Also see Extrapyramidal tracts