[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Neurophysiology](https://durgical.com/en/physiology/#s-neuro) › Upper versus lower motor neuron lesions [Physiology](https://durgical.com/en/physiology/) · [Neurophysiology](https://durgical.com/en/physiology/#s-neuro) # 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](https://durgical.com/en/anatomy/precentral-gyrus/) · [White matter](https://durgical.com/en/anatomy/white-matter-of-telencephalon/) · [Spinal cord](https://durgical.com/en/anatomy/spinal-cord/) · [Grey matter of the cord](https://durgical.com/en/anatomy/grey-matter-of-spinal-cord/) ## Key points - Upper motor neuron: spasticity (velocity-dependent), hyperreflexia, clonus, Babinski sign, little wasting (disuse only), no fasciculations. - Lower motor neuron: flaccidity, reduced or absent reflexes, marked early wasting, fasciculations, flexor plantar response. - An acute upper lesion (stroke, spinal shock) can look like a lower one at first: flaccid and areflexic. - Upper motor neuron weakness affects arm extensors and leg flexors most (pyramidal pattern). - Mixed upper and lower signs: amyotrophic lateral sclerosis, cervical myelopathy with root compression, B12 deficiency with neuropathy. - Guillain–Barré syndrome: ascending, symmetric lower motor neuron weakness with areflexia; Bell palsy: lower motor neuron lesion of the facial nerve. ## 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. ## Memory aid Upper goes up: tone up, reflexes up, big toe up. Lower goes low: tone low, reflexes low, muscle bulk low. ## 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. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/upper-versus-lower-motor-neuron-lesions/) ## See it in 3D The structures of Durgical's 3D anatomy where it happens. - [Motor cortex](https://durgical.com/en/anatomy/precentral-gyrus/) - [White matter](https://durgical.com/en/anatomy/white-matter-of-telencephalon/) - [Spinal cord](https://durgical.com/en/anatomy/spinal-cord/) - [Grey matter of the cord](https://durgical.com/en/anatomy/grey-matter-of-spinal-cord/) ## Related topics - [Motor cortex and the corticospinal tract](https://durgical.com/en/physiology/motor-cortex-and-the-corticospinal-tract/) - [Spinal cord reflexes](https://durgical.com/en/physiology/spinal-cord-reflexes/) - [Muscle spindle and Golgi tendon organ](https://durgical.com/en/physiology/muscle-spindle-and-golgi-tendon-organ/) - [Motor units, recruitment and summation](https://durgical.com/en/physiology/motor-units-recruitment-and-summation/) - [The neuromuscular junction](https://durgical.com/en/physiology/the-neuromuscular-junction/) ## Related conditions - [Ischaemic stroke](https://durgical.com/en/conditions/ischaemic-stroke/) - [Spinal cord injury](https://durgical.com/en/conditions/spinal-cord-injury/) - [Guillain-Barré syndrome](https://durgical.com/en/conditions/guillain-barre-syndrome/) - [Bell palsy](https://durgical.com/en/conditions/bell-palsy/) - [Degenerative cervical myelopathy](https://durgical.com/en/conditions/degenerative-cervical-myelopathy/) - [Multiple sclerosis](https://durgical.com/en/conditions/multiple-sclerosis/) [← All of physiology](https://durgical.com/en/physiology/) 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](https://openstax.org/books/anatomy-and-physiology-2e/pages/16-4-the-sensory-and-motor-exams) (OpenStax (Rice University), 2022); [14.3 Motor Responses](https://openstax.org/books/anatomy-and-physiology-2e/pages/14-3-motor-responses) (OpenStax (Rice University), 2022); [Spinal Cord Injuries](https://www.ncbi.nlm.nih.gov/books/NBK560721/) (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). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/physiology/upper-versus-lower-motor-neuron-lesions/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/upper-versus-lower-motor-neuron-lesions/