Upper and lower motor neuron
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Upper motor neuron (UMN) vs lower motor neuron (LMN)
| Feature | Upper motor neuron lesion | Lower motor neuron lesion |
|---|---|---|
| Site of lesion | Motor cortex, internal capsule, brainstem, or corticospinal tract above anterior horn cell or cranial nerve motor nucleus | Anterior horn cell, motor root, peripheral nerve, neuromuscular motor axon, or cranial nerve after its nucleus |
| Weakness | Spastic weakness, usually affects groups of muscles | Flaccid weakness, affects individual muscles or nerve distribution |
| Tone | Increased tone: spasticity | Decreased tone: flaccidity |
| Deep tendon reflexes | Increased: hyperreflexia, clonus may occur | Decreased or absent: hyporeflexia/areflexia |
| Plantar response | Extensor plantar response: Babinski positive | Flexor plantar response, or absent if weakness is severe |
| Muscle bulk | Mild disuse wasting later | Marked, early neurogenic muscle wasting |
| Fasciculations | Absent | May be present |
| Distribution | Pyramidal pattern, for example arm extensors and leg flexors are often weaker | Segmental, root, plexus, peripheral nerve, or individual-muscle pattern |
| Examples | Stroke, multiple sclerosis, spinal cord compression/myelopathy | Poliomyelitis, peripheral neuropathy, radiculopathy, motor neuron disease |
Why the signs differ
- UMNs normally provide descending control that modulates spinal reflexes. Damage removes inhibition, causing exaggerated reflexes and increased tone.
- LMNs directly connect the spinal cord or cranial nerve nucleus to muscle. Damage disconnects muscle from its nerve supply, causing flaccidity, atrophy, fasciculations, and loss of reflexes.
Typical clinical examples
- Right internal-capsule stroke: left-sided UMN facial weakness and hemiparesis, brisk reflexes, extensor left plantar response.
- L5 radiculopathy: LMN-pattern weakness of ankle/toe dorsiflexion, reduced reflex in the relevant root distribution, possible muscle wasting if chronic.
- Motor neuron disease (ALS): can show both, such as brisk reflexes and extensor plantars with fasciculations and muscle wasting.
Extensor and flxor plantar response
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"plantar reflex" AND Babinski
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plantar reflex Babinski sign extensor flexor examination clinical guidance
Plantar response
It is tested by stroking the lateral border of the sole, from the heel forward toward the ball of the foot, then curving medially across the forefoot.
| Response | What is seen | Meaning in an adult |
|---|---|---|
| Flexor plantar response | Toes flex or curl downwards (plantar-flex), sometimes with no obvious movement | Normal response |
| Extensor plantar response | Great toe extends upwards (dorsiflexes) and the other toes may fan out | Babinski sign: suggests corticospinal tract or UMN dysfunction |
Flexor response: normal in healthy adults.
Extensor response (Babinski): can occur with stroke, spinal cord disease/injury, multiple sclerosis, or other lesions affecting the corticospinal pathway.
Extensor response (Babinski): can occur with stroke, spinal cord disease/injury, multiple sclerosis, or other lesions affecting the corticospinal pathway.
An extensor response is normal in infants and young children, usually up to about 1-2 years, because corticospinal tract myelination is incomplete. NCBI clinical reference
Important: withdrawal of the entire foot because stimulation is painful or ticklish is not a true Babinski response. Look specifically for the slow upward movement of the great toe. The normal plantar response is toe flexion; the extensor response is extension of the great toe. Adams and Victor's Principles of Neurology, 12th Edition, lines 1615-1622.
Components of plantar
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components of plantar response clinical examination great toe other toes ankle knee hip
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"plantar reflex" AND clinical examination
Components of the plantar response
After stroking the lateral sole of the foot, observe:
-
Great toe (hallux)
- Flexor plantar response: great toe moves downward, plantar flexion.
- Extensor plantar response: great toe moves upward, dorsiflexion/extension.
-
Other four toes
- Normal: flexion, often with adduction.
- Extensor response: extension and abduction, called fanning of toes or the fan sign.
-
Withdrawal movements
A true protective flexor withdrawal may include flexion at the:- toes
- ankle
- knee
- hip
In clinical interpretation, the most important component is the movement of the great toe. Upgoing great toe, with or without fanning of the other toes, is an extensor plantar response (Babinski sign). Adams and Victor's Principles of Neurology, 12th Edition, lines 1615-1622.
Descending pathways
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Descending motor pathways
Descending pathways carry signals from the brain to spinal cord motor neurons to control movement, posture, muscle tone, and reflexes.
1. Pyramidal pathways
These arise mainly from the motor cortex and are responsible for voluntary skilled movement.
| Tract | Course and crossing | Main function |
|---|---|---|
| Lateral corticospinal tract | About 85-90% of fibers cross at the pyramidal decussation in the lower medulla, then descend in the lateral spinal cord | Fine, skilled voluntary movement of distal limbs, especially hands and fingers |
| Anterior corticospinal tract | Uncrossed in the medulla; most fibers cross near their spinal level of termination | Voluntary control of axial and proximal trunk muscles |
| Corticobulbar tract | Cortex to cranial-nerve motor nuclei in the brainstem | Voluntary movement of face, jaw, tongue, pharynx, and larynx |
Clinical relevance: A lesion of the corticospinal tract produces upper motor neuron signs: weakness, spasticity, hyperreflexia, clonus, and an extensor plantar response.
2. Extrapyramidal pathways
These arise mainly in the brainstem and regulate posture, balance, muscle tone, and automatic movements.
| Tract | Origin | Main function |
|---|---|---|
| Rubrospinal | Red nucleus, midbrain | Facilitates flexor activity, mainly in upper limb; relatively minor role in humans |
| Vestibulospinal | Vestibular nuclei | Balance, upright posture, and activation of extensor (antigravity) muscles |
| Reticulospinal | Reticular formation of pons and medulla | Posture, gait, muscle tone, automatic movements, and modulation of spinal reflexes |
| Tectospinal | Superior colliculus, midbrain | Reflex turning of head and neck toward visual or auditory stimuli |
Easy classification
- Lateral system: lateral corticospinal and rubrospinal tracts
- Controls distal limb muscles and skilled movement.
- Medial system: anterior corticospinal, vestibulospinal, reticulospinal, and tectospinal tracts
- Controls axial/proximal muscles, posture, equilibrium, and head-neck movements.
Key point
The corticospinal tract is the main voluntary motor pathway. The brainstem extrapyramidal tracts provide the background control needed for that movement: posture, balance, tone, and automatic adjustment.
Root values of all dtr
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deep tendon reflexes root values biceps brachioradialis triceps patellar ankle jaw jerk clinical examination
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"deep tendon reflexes" AND neurologic examination
Root values of deep tendon reflexes (DTRs)
| Reflex | Main root value(s) | Peripheral nerve | Response |
|---|---|---|---|
| Jaw jerk | CN V | Trigeminal nerve | Jaw closes |
| Biceps | C5-C6 mainly C5 | Musculocutaneous | Elbow flexion |
| Brachioradialis (supinator) | C5-C6 mainly C6 | Radial | Elbow flexion with slight supination |
| Triceps | C6-C7 mainly C7 | Radial | Elbow extension |
| Finger flexor | C8-T1 mainly C8 | Median and ulnar | Flexion of fingers/thumb |
| Pectoral | C5-T1 | Medial and lateral pectoral nerves | Adduction/internal rotation of arm |
| Patellar (knee jerk) | L3-L4 mainly L4 | Femoral | Knee extension |
| Adductor reflex | L2-L4 mainly L3 | Obturator | Hip adduction |
| Hamstring reflex | L5-S1 | Sciatic | Knee flexion |
| Achilles (ankle jerk) | S1-S2 mainly S1 | Tibial | Plantar flexion of foot |
High-yield exam values
- Biceps: C5, C6
- Brachioradialis: C5, C6
- Triceps: C6, C7
- Knee jerk: L3, L4
- Ankle jerk: S1, S2
Easy mnemonic
C5-6: biceps, brachioradialis
C7: triceps
L3-4: knee jerk
S1-2: ankle jerk
C7: triceps
L3-4: knee jerk
S1-2: ankle jerk
Some sources name a single predominant root for bedside localization: C5 biceps, C6 brachioradialis, C7 triceps, L4 patellar, and S1 Achilles. Firestein & Kelley's Textbook of Rheumatology, lines 6162-6178. The standard commonly tested ranges are also listed in Harrison's reflex examination.
Descending pathways
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descending motor pathways corticospinal rubrospinal vestibulospinal reticulospinal tectospinal anatomy function decussation
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"descending motor pathways" AND corticospinal
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Descending motor pathways
They carry motor commands from the cerebral cortex and brainstem to spinal anterior horn cells.
A. Lateral descending motor system
Controls distal limb muscles, especially fine and skilled movement.
| Tract | Origin | Crossing | Main termination | Function |
|---|---|---|---|---|
| Lateral corticospinal tract | Primary motor cortex and other frontal/parietal motor areas | Pyramidal decussation at cervicomedullary junction | Entire spinal cord, mainly cervical and lumbosacral enlargements | Voluntary movement of the contralateral limb, especially fine finger movements |
| Rubrospinal tract | Red nucleus of midbrain | Ventral tegmental decussation in midbrain | Mainly cervical spinal cord | Influences contralateral upper-limb movement, especially flexor activity. Its role is limited/uncertain in humans. |
B. Medial descending motor system
Controls axial and proximal muscles, posture, balance, gait, and head-neck movements.
| Tract | Origin | Crossing | Main termination | Function |
|---|---|---|---|---|
| Anterior corticospinal tract | Motor cortex and supplementary motor area | Mostly uncrossed initially; acts bilaterally near termination | Cervical and upper thoracic cord | Bilateral control of trunk and shoulder/hip girdle muscles |
| Medial vestibulospinal tract | Medial and inferior vestibular nuclei | Mainly bilateral | Cervical and upper thoracic cord | Positions and stabilizes the head and neck |
| Lateral vestibulospinal tract | Lateral vestibular nucleus | Uncrossed | Entire spinal cord | Balance and facilitation of antigravity extensor muscles |
| Reticulospinal tracts | Pontine and medullary reticular formation | Mainly bilateral or uncrossed | Entire spinal cord | Automatic posture, gait-related movements, muscle tone, and reflex modulation |
| Tectospinal tract | Superior colliculus | Dorsal tegmental decussation in midbrain | Cervical spinal cord | Reflex orientation of head and neck toward visual or auditory stimuli |
Corticobulbar tract
- Origin: motor cortex
- Termination: motor nuclei of cranial nerves in the brainstem
- Function: voluntary control of facial, jaw, tongue, pharyngeal, and laryngeal muscles.
- Most cranial motor nuclei receive bilateral cortical input. Important exceptions with predominantly contralateral input include the lower facial nucleus and much of the hypoglossal nucleus.
Quick classification
Lateral system
- Lateral corticospinal
- Rubrospinal
Function: distal limb control and skilled voluntary movement.
Medial system
- Anterior corticospinal
- Vestibulospinal
- Reticulospinal
- Tectospinal
Function: posture, balance, gait, axial/proximal muscle control, and head-neck orientation.
Clinical point
Damage to the corticospinal tract causes upper motor neuron signs below the lesion:
- weakness
- spasticity
- hyperreflexia
- clonus
- extensor plantar response (Babinski sign)
A corticospinal lesion above the pyramidal decussation causes contralateral weakness; a lesion below it, in the spinal cord, causes ipsilateral weakness.
Neuroanatomy through Clinical Cases, 3rd Edition, p. 255.
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