Upper vs lower motor neuron syndrome !

Syndrome

Two patterns of weakness: one with stiffness and brisk reflexes (brain or cord) and one with floppiness and wasting (nerves).

Updated 2026-11-01

See it in 3D: Motor cortex · Spinal cord · Grey matter of the cord · Neck nerve roots · Medulla oblongata

What it is

The motor pathway has two neurons: the upper one, from the cortex to the cord or brainstem, and the lower one, from the anterior horn to the muscle. Damage to the upper neuron releases the reflexes and causes spasticity, hyperreflexia and a Babinski sign; damage to the lower neuron cuts the reflex arc and causes flaccidity, areflexia, wasting and fasciculations. This distinction is the first step in localising any weakness.

Key findings

Paresis: partial weakness; plegia: complete paralysis.
Hemiparesis: one side of the body; paraparesis: both legs; quadriparesis: all four limbs.
Upper motor neuron weakness affects muscle groups; lower motor neuron weakness, individual muscles.
Pronator drift is a sensitive upper motor neuron sign.

Pathophysiology

The upper motor neuron starts in the precentral gyrus, descends through the internal capsule and brainstem, crosses at the medullary pyramids and continues in the lateral column of the cord.
This pathway normally damps the spinal reflex arc; when it is damaged, reflexes and tone increase.
The lower motor neuron lies in the anterior horn or brainstem motor nuclei; its axon forms the root and the nerve.
Without its lower motor neuron, the muscle loses its nerve supply: it wastes and its motor units fire on their own (fasciculations).

Upper versus lower motor neuron

Feature	Upper motor neuron	Lower motor neuron
Distribution of weakness	One side of the body or below a level; mostly arm extensors and leg flexors	Territory of a nerve, a root or a segment
Tone	Increased: clasp-knife spasticity	Reduced: flaccidity
Reflexes	Brisk, with clonus	Reduced or absent
Plantar response	Extensor (Babinski)	Flexor or absent
Wasting	Mild, from disuse, and late	Marked and early
Fasciculations	Absent	Present
Examples	Stroke, multiple sclerosis, myelopathy	Guillain–Barré, poliomyelitis, radiculopathy, neuropathy

Common causes

Upper motor neuron: stroke, brain tumour, multiple sclerosis, cervical myelopathy, spinal cord trauma.
Lower motor neuron: Guillain–Barré syndrome, poliomyelitis, radiculopathy from disc herniation, peripheral neuropathies.
Mixed: amyotrophic lateral sclerosis.
Weakness without changes in reflexes or tone points to the neuromuscular junction (myasthenia) or muscle (myopathy).

Tests that confirm it

MRI localises brain and spinal cord lesions.
Electromyography and nerve conduction studies confirm a lower motor neuron lesion and separate it from myopathy.

Test yourself

Think of the answer, then tap the question to see it.

Which findings indicate a lower motor neuron lesion?

Weakness with flaccidity, reduced or absent reflexes, marked wasting and fasciculations.

Why can a very recent stroke produce a floppy limb without reflexes?

In the acute phase spinal activity is transiently depressed; spasticity and hyperreflexia appear after days or weeks.

Which disease combines upper and lower motor neuron signs without sensory loss?

Amyotrophic lateral sclerosis.

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See it in 3D

The structures of Durgical's 3D anatomy where it is found.

← All of the clinical exam

Educational content on the clinical exam; it is learned and practised under supervision. It does not replace assessment by a health professional and is not for self-diagnosis.

Sources: 16.4 The Sensory and Motor Exams (OpenStax, Anatomy and Physiology 2e, 2022); How To Assess the Motor System (MSD Manual Professional Edition, 2024); How To Assess Reflexes (MSD Manual Professional Edition, 2024); Overview of Spinal Cord Disorders (MSD Manual Professional Edition, 2024).

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Representative model, for educational purposes only.