Upper versus lower motor neuron lesions !
- Concept
- Exam priority: high
Weakness from the brain or cord is stiff with brisk reflexes; weakness from the nerve or anterior horn is floppy, wasted and areflexic.
Updated 2026-11-01
Also called: UMN and LMN signs, Pyramidal syndrome
See it in 3D: Motor cortex · White matter · Spinal cord · Grey matter of the cord
What it is
The upper motor neuron runs from the cortex to the brainstem or cord. The lower motor neuron runs from the anterior horn or a cranial nerve nucleus to the muscle and is the final common pathway. Damage to each gives a distinct pattern that lets the examiner localize the lesion: upper lesions release spinal reflexes from descending control, lower lesions disconnect the muscle from all input.
Bedside signs
| Sign | Upper motor neuron | Lower motor neuron |
|---|---|---|
| Tone | Increased (spastic) | Reduced (flaccid) |
| Reflexes | Brisk, clonus | Reduced or absent |
| Plantar response | Extensor (Babinski) | Flexor |
| Wasting | Mild and late (disuse) | Marked and early |
| Fasciculations | Absent | Present |
| Distribution | Whole limb or one side of the body | Segment, root or nerve territory |
Clinical notes
- An internal capsule infarct gives an upper motor neuron hemiparesis on the opposite side.
- Guillain–Barré syndrome often follows a gut or respiratory infection: rapidly ascending weakness and areflexia; breathing must be watched.
- Cervical cord compression at C5–C6: lower motor neuron signs at the level (lost biceps reflex, deltoid wasting) and upper motor neuron signs below (brisk leg reflexes, Babinski).
- Poliomyelitis destroys anterior horn cells and is a pure lower motor neuron disease; the Americas were certified free of wild poliovirus in 1994.
Test yourself
Think of the answer, then tap the question to see it.
Why does an upper motor neuron lesion cause brisk reflexes rather than weak ones?
The reflex arc (spindle, Ia afferent, α motor neuron) is intact. The lesion removes descending inhibition, so gamma drive and interneuron excitability rise and the arc becomes overactive.
Why are fasciculations a lower motor neuron sign?
They are spontaneous discharges of a whole motor unit from a sick, irritable motor neuron or axon. Upper motor neurons do not form motor units, so their loss does not cause them.
Cervical compression at C5–C6: the biceps reflex is lost but the knee and ankle jerks are brisk. Why?
At the level the anterior horn and root are damaged (lower motor neuron: lost biceps reflex), while corticospinal fibres to lower segments are cut (upper motor neuron: brisk leg reflexes).
Why can a patient be flaccid and areflexic in the first hours after a large stroke?
Sudden loss of descending input depresses spinal motor neurons (a shock phase). Spasticity and hyperreflexia appear over days to weeks as the cord circuits adapt.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- Motor cortex and the corticospinal tract
- Spinal cord reflexes
- Muscle spindle and Golgi tendon organ
- Motor units, recruitment and summation
- The neuromuscular junction
Related conditions
- Ischaemic stroke
- Spinal cord injury
- Guillain-Barré syndrome
- Bell palsy
- Degenerative cervical myelopathy
- Multiple sclerosis
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 16.4 The Sensory and Motor Exams (OpenStax (Rice University), 2022); 14.3 Motor Responses (OpenStax (Rice University), 2022); Spinal Cord Injuries (StatPearls (NCBI Bookshelf), 2023); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 55). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.