Bit · Neuro
UMN vs LMN signs
Two patterns of motor neuron damage with opposite physical findings. The pivot is tone and reflexes.
Mechanism
The motor pathway is two neurons in series:
- Upper motor neuron (UMN) — originates in the motor cortex, descends through the corticospinal tract, synapses in the spinal cord (or brainstem) on the LMN. UMNs inhibit spinal reflexes. Lose them → reflexes disinhibited: hyperreflexia, spasticity, Babinski sign.
- Lower motor neuron (LMN) — anterior horn cell + its peripheral axon to muscle. The 'final common pathway.' Damage cuts the muscle off from all input → flaccid weakness, atrophy, fasciculations, hyporeflexia.
Some diseases give both — ALS is the classic mixed UMN+LMN disease.
Differentiator Table
| Sign | UMN lesion | LMN lesion |
|---|---|---|
| Tone | ↑ Spasticity (clasp-knife) | ↓ Flaccidity |
| Reflexes (DTRs) | ↑ Hyperreflexia, clonus | ↓ Hyporeflexia or absent |
| Babinski sign | PRESENT (upgoing toes) | Absent (downgoing toes) |
| Muscle bulk | Mild disuse atrophy late | Significant atrophy early |
| Fasciculations | Absent | PRESENT |
| Weakness pattern | Pyramidal (extensors weaker in arms; flexors weaker in legs) | Segmental — follows nerve / nerve root |
| Classic example | Stroke (corticospinal tract), spinal cord injury above conus, MS | Polio, Guillain-Barré, peripheral nerve injury, cauda equina, spinal muscular atrophy |
| Mixed (BOTH UMN and LMN) | Amyotrophic lateral sclerosis (ALS) — pathognomonic combination |
The Pivot
Four findings tell you which one:
- Tone — spastic → UMN. Flaccid → LMN.
- Reflexes — brisk/clonus → UMN. Absent → LMN.
- Babinski sign present → UMN (in an adult).
- Fasciculations and atrophy → LMN.
If you see both patterns in different muscle groups in the same patient — think ALS. Hyperreflexic biceps in an arm that also has atrophy and fasciculations is the classic finding.
NBME-Style Stem
A 58-year-old man presents with 8 months of progressive weakness. Examination shows atrophy and fasciculations of the tongue and intrinsic hand muscles, brisk reflexes throughout, bilateral Babinski signs, and a spastic gait. Sensation is intact. Which of the following is the most likely diagnosis?
Concept Anchor
UMN damage disinhibits the spinal reflex (everything goes up — tone, reflexes, plantar response); LMN damage cuts the muscle off from all input (everything goes down — tone, bulk, reflexes — plus fasciculations as denervated fibres twitch). ALS is the disease that does both at once.