Upper Motor Neurones (UMN) vs Lower Motor Neurone (LMN) Lesions

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.

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.

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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

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 absentClonusFasciculationsFibrillations

Reaction 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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