What are upper motor neurons (UMNs) and lower motor neurons (LMNs)?

Upper and Lower Motor Neurons: Anatomical and Clinical Distinctions

Anatomical Definitions

Upper motor neurons (UMNs) are neurons located in the brain and spinal cord that control lower motor neurons, while lower motor neurons (LMNs) are neurons whose cell bodies reside in the anterior horn of the spinal cord or brainstem that directly innervate muscles. 1, 2

Upper Motor Neurons (UMNs)

• Location: Cell bodies reside in the motor cortex of the brain and descend through the corticospinal (pyramidal) tract to synapse with lower motor neurons 2
• Function: Control resting muscle tone, help initiate voluntary movement, and modulate the activity of lower motor neurons 2
• Pathway: Extend from the cerebral cortex through the internal capsule, cerebral peduncles, and down the spinal cord along the corticospinal tracts 1

Lower Motor Neurons (LMNs)

• Location: Cell bodies are located in the anterior horn of the spinal cord or in cranial nerve nuclei in the brainstem 1, 3
• Function: Directly innervate skeletal muscle fibers to produce muscle contraction 1
• Classification: Considered peripheral efferent neurons despite their cell bodies being in the central nervous system 1, 3

Clinical Signs of Dysfunction

Upper Motor Neuron Signs

When UMNs are damaged, you will observe:

• Hyperreflexia: Brisk or exaggerated deep tendon reflexes 1
• Spasticity: Increased muscle tone with velocity-dependent resistance to passive movement 4
• Hypertonicity: Increased resting muscle tone 1
• Abnormal plantar reflex: Extensor (Babinski) response 1, 2
• Clonus: Rhythmic muscle contractions in response to sudden, maintained stretch 4
• Slowed movement: Decreased speed of rapid movements and contraction 5
• Weakness without atrophy: Muscle bulk is preserved initially 2

Lower Motor Neuron Signs

When LMNs are damaged, you will observe:

• Fasciculations: Spontaneous, irregular discharges of entire motor units that appear like "raindrops on a tin roof" on examination 1, 4
• Muscle atrophy: Progressive loss of muscle bulk due to denervation 1, 4
• Flaccid paralysis: Weakness with decreased muscle tone 4
• Hypotonia: Decreased resting muscle tone 1, 4
• Hyporeflexia or areflexia: Diminished or absent deep tendon reflexes 1, 4
• Fibrillation potentials: Spontaneous muscle fiber discharges seen on EMG 1

Clinical Significance

Disease Examples

• Both UMN and LMN involvement: Amyotrophic lateral sclerosis (ALS) affects both systems simultaneously, presenting with hyperreflexia and spasticity (UMN) alongside fasciculations, weakness, and atrophy (LMN) 1, 6
• Primarily UMN: Primary lateral sclerosis, hereditary spastic paraplegia, and some cases of multiple sclerosis 2
• Primarily LMN: Spinal muscular atrophy, poliomyelitis, and progressive muscular atrophy 6, 3

Key Diagnostic Pitfall

The distinction between UMN and LMN pathology is critical because they require different diagnostic workups and have different prognoses. 3 For example, fasciculations are a red flag for lower motor neuron disease and warrant EMG evaluation and measurement of creatine phosphokinase levels 1, 4, while isolated hyperreflexia and spasticity suggest upper motor neuron pathology requiring brain and spinal cord imaging 1.

Independent Degeneration

Motor neuron degeneration can occur independently at UMN and LMN levels, even when both are affected in the same disease process 7, 8. In ALS, UMN and LMN signs are maximal in the same body region but are independent of each other in severity and distribution 7.