Lesions of Upper Motor Neurons and Lower Motor Neurons

UMN LMNUpper and Lower Motor Neurons

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

UMN LMN

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)UMNLLMNL
S = StrengthLowersLowers
T = ToneIncreases (spastic)Decreases (flaccid)
O = OthersSuperficial reflexes absentClonusFasciculationsFibrillationsReaction of degeneration
R = Reflexes = DTR or Deep tendon reflexesIncreasedDecreased
M = Muscle MassSlight loss onlyDecreases / Atrophy
Baby = Babinski SignPositive (toe up)Negative (toe down)
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About the Author

Sulabh Shrestha
Intern doctor and Medical Blogger Sulabh Shrestha

15 Comments on "Lesions of Upper Motor Neurons and Lower Motor Neurons"

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

  2. Thank you for this makes understanding neuro a little easier

  3. Bahut acche Sulav—very useful description.

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

  5. thanks it really helped alot in understanding umnl n lmnl……

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

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

  8. Tanz alot,its quite useful

  9. DR. ABDUR RAHIM AATHIR WAZIR | March 28, 2012 at 2:10 pm | Reply

    THANKS A LOT
    ITS HELPFUL FOR MEDICAL STUDENTS
    & USEFUL 4 DOCTORS

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

  11. shabnam salam | June 23, 2012 at 12:34 am | Reply

    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)

  12. dr.palaniswamy | June 23, 2012 at 12:36 pm | Reply

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

  13. thank you sir,,,,,,,,,,,,,very usefull 4 us,,,,,,,,

  14. very very useful

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