Lower motor neuron lesion

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Lower motor neuron lesion
Spinal nerve.svg
Lower motor neuron in red
Classification and external resources
DiseasesDB 22143

A lower motor neuron lesion is a lesion which affects nerve fibers traveling from the ventral horn or anterior grey column of the spinal cord to the relevant muscle(s) – the lower motor neuron.[1]

One major characteristic used to identify a lower motor neuron lesion is flaccid paralysis – paralysis accompanied by loss of muscle tone. This is in contrast to an upper motor neuron lesion, which often presents with spastic paralysis – paralysis accompanied by severe hypertonia.


The extensor Babinski reflex is usually absent. Muscle paresis/paralysis, hypotonia/atonia, and hyporeflexia/areflexia are usually seen immediately following an insult. Muscle wasting, fasciculations and fibrillations are typically signs of end-stage muscle denervation and are seen over a longer time period. Another feature is the segmentation of symptoms - only muscles innervated by the damaged nerves will be symptomatic.


Most common causes of lower motor neuron injuries are trauma to peripheral nerves that sever the axons - a virus that selectively attacks ventral horn cells. Disuse atrophy of the muscle occurs i.e., shrinkage of muscle fibre finally replaced by fibrous tissue (fibrous muscle) Other causes include Guillain-Barré syndrome, C. botulism, polio, and cauda equina syndrome; another common cause of Lower Motor neuron degeneration is Amyotrophic lateral sclerosis.

Differential Diagnosis[edit]

  • Myasthenia gravis - synaptic transmission at motor end-plate is impaired
  • Amyotrophic lateral sclerosis - causes death of motor neurons, although exact cause is unknown it has been suggested that abnormal built-up of proteins proves toxic for the neurons.

See also[edit]


  1. ^ James D. Fix (1 October 2007). Neuroanatomy. Lippincott Williams & Wilkins. pp. 120–. ISBN 978-0-7817-7245-7. Retrieved 17 November 2010. 

External links[edit]