May 27, 2024
Anatomy Physiology

Lesions of Upper Motor Neurons and Lower Motor Neurons

  • August 27, 2013
  • 3 min read
Lesions of Upper Motor Neurons and Lower Motor Neurons

Upper Motor Neurones (UMN), Lower Motor Neurone (LMN) and their Lesions

Upper and Lower Motor Neurons

All the neurons contributing to the pyramidal and extrapyramidal systems should be called upper motor neurons (UMN).
The anterior horn cells and the related neurons in the motor nuclei of some cranial nerves are called lower motor neurons (LMN). Axons of these cells give rise to the peripheral motor nerves. These are lowest in position in the motor system and recieve all the inputs from higher centers like medulla, pons, mid-brain and cerebral cortex and transmit the same to the target organs. All impulses for motor activity are to be funelled into them and these are also called final common pathway.

Signs of Upper Motor Neuron Lesions (UMNL)

1. Paralysis or weakness of movements of the affected side but gross movements may be produced. No muscle atrophy is seen initially but later on some disuse atrophy may occur.

2. Babinski sign is present: The great toe becomes dorsiflexed and the other toes fan outward in response to sensory stimulation along the lateral aspect of the sole of the foot. The normal response is plantar flexion of all the toes.

Positive Babinski sign
Babinski Reflex

3. Loss of performance of fine-skilled voluntary movements especially at the distal end of the limbs.

4. Superficial abdominal reflexes and cremasteric reflex are absent.

5. Spasticity or hypertonicity of the muscles.

6. Clasp-knife reaction: initial higher resistance to movement is followed by a lesser resistance

7. Exaggerated deep tendon reflexes and clonus may be present.

Signs of Lower Motor Neuron Lesions (LMNL)

1. Flaccid paralysis of muscles supplied.

2. Atrophy of muscles supplied.

3. Loss of reflexes of muscles supplied.

4. Muscles fasciculation (contraction of a group of fibers) due to irritation of the motor neurons – seen with naked eye.

5. Muscle fibrillation (contraction of individual fibers) – detected only by EMG

6. Muscle contracture (shortening of paralyzed muscles)

7. Presence of muscle wasting

8. Reaction of degeneration: When the LMN is cut, a muscle will no longer respond to interrupted electrical stimulation 7 days after nerve section, although it will still respond to direct current. After 10 days, response to direct current also ceases.

Mnemonic for Medical Students

Upper Motor Neuron Lesion vs Lower Motor Neuron Lesion : Difference or comparison between upper motor neuron lesion (UMNL) and lower motor neuron lesion (LMNL)

Mnemonic for basis of difference: STORM Baby
Also remember: In a Lower motor neuron lesion everything lowers

Basis of Difference (STORM Baby) UMNL LMNL
S = Strength Lowers Lowers
T = Tone Increases (spastic) Decreases (flaccid)
O = Others Superficial reflexes absentClonus FasciculationsFibrillationsReaction of degeneration
R = Reflexes = DTR or Deep tendon reflexes Increased Decreased
M = Muscle Mass Slight loss only Decreases / Atrophy
Baby = Babinski Sign Positive (toe up) Negative (toe down)
About Author

Sulabh Shrestha

Intern doctor and Medical Blogger Sulabh Shrestha


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  • cog wheel type of rigidity occurs in parkinsonism due to tremor is super imposed on rigidity.

  • Iliked that pneumonics storm baby and about spasticity:due to lesion of pyramidal tract(corticospinal tract) ,commonest site-internal capsule,only one group of muscles either agonist or antagonist are affected,clasp knife type.. ABOUT RIGIDITY:DUE TO LESION OF THE BASAL GANGLIA,EXTRA PYRAMIDAL RIGIDITY,BOTH AGONIST AND ANTAGONIST AFFECTED,HYPERTONIA(LEAD PIPE RIGIDITY AND COG WHEEL RIGIDITY)

  • thx so much able to understand neurology somehow need more information though if u can help


  • Tanz alot,its quite useful

  • hi thanx a lot that was cul for making it easier for students nice work

  • thanx for this description…. Very useful for comparing the effect

  • thanks it really helped alot in understanding umnl n lmnl……

  • it is very useful to us….
    thanks for publishers…

  • Bahut acche Sulav—very useful description.

  • Thank you for this makes understanding neuro a little easier

    • This is good!
      U hv helped ‘crammers‘!

  • Such a well written post.. Thnkx for sharing this post!

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