Motor Neurons: Upper vs. Lower, Function, and Clinical Significance

Definition

A motor neuron (efferent neuron) transmits signals from the CNS to effector organs (skeletal muscle, smooth muscle, glands).

Two-Tier Organization

Feature	Upper Motor Neurons (UMN)	Lower Motor Neurons (LMN)
Location	Motor cortex (precentral gyrus), brainstem	Spinal cord (anterior horn), brainstem nuclei
Axon	Corticospinal/corticobulbar tracts	Peripheral nerves → muscle
Synapse	On LMN/interneurons	Neuromuscular junction (ACh)
Lesion Signs	Spasticity, hyperreflexia, Babinski, weakness (no atrophy initially)	Flaccidity, hyporeflexia, fasciculations, atrophy, fibrillation

Motor Unit

One LMN + all muscle fibers it innervates. Size principle: small units (slow, fatigue-resistant) recruited first; large (fast, fatigable) for high force.

Clinical Relevance

• UMN lesions: Stroke, SCI, ALS (mixed), MS, cerebral palsy.
• LMN lesions: Peripheral neuropathy, radiculopathy, polio, ALS, SMA.
• EMG/NCS distinguishes.