Upper Motor Neuron vs Lower Motor Neuron Lesion
Upper Motor Neuron (UMN) Lesion vs Lower Motor Neuron (LMN) Lesion
Distinguishing between upper motor neuron (UMN) and lower motor neuron (LMN) lesions is fundamental to neurological localization. UMN lesions affect the corticospinal tract from cortex to spinal cord, causing spastic weakness. LMN lesions affect the anterior horn cell or peripheral nerve, causing flaccid weakness with atrophy.
Comparison Table
Key Differences
- →Tone: Spastic (UMN) vs Flaccid (LMN)
- →Reflexes: Hyperreflexia (UMN) vs Hyporeflexia (LMN)
- →Babinski: Present (UMN) vs Absent (LMN)
- →Atrophy: Minimal (UMN) vs Significant with fasciculations (LMN)
Clinical Relevance
- •Bell's palsy (LMN): Entire face affected including forehead; Stroke (UMN): Forehead spared
- •Spinal cord lesions above the lesion level may have LMN signs at the level, UMN signs below
- •ALS has mixed UMN and LMN signs (both anterior horn cells and corticospinal tracts affected)
- •Acute UMN lesions may initially present with hypotonia/hyporeflexia (spinal shock)
Study Tips
- ✓Mnemonic: UMN = "U" for Up (upgoing toe, up reflexes, up tone)
- ✓LMN = "L" for Low (low reflexes, low tone) and "L" for Little (atrophy)
- ✓Fasciculations are a hallmark of LMN disease - visible muscle twitching from denervation
- ✓Think of the reflex arc: UMN lesion removes inhibition (hyperreflexia); LMN lesion breaks the arc (hyporeflexia)
FAQs
Common questions about this comparison
The corticospinal tract normally has inhibitory influence on spinal reflexes. UMN lesion removes this inhibition, resulting in exaggerated reflexes. LMN lesion disrupts the reflex arc itself (either the sensory or motor limb), so the reflex cannot be completed.
The forehead muscles receive bilateral upper motor neuron input from both cerebral hemispheres. A unilateral stroke leaves the intact hemisphere's input to the forehead. In LMN lesion (like Bell's palsy), the facial nerve itself is damaged, paralyzing all ipsilateral facial muscles including forehead.