Upper and lower motor neurons control skeletal muscle contraction through a two-neuron pathway. Upper motor neurons originate in the brain and direct the activity of lower motor neurons in the spinal cord and brainstem. Lesions of upper motor neurons cause spastic weakness, hyperreflexia, and loss of fine motor skills. Lesions of lower motor neurons result in flaccid paralysis, loss of reflexes, and muscular atrophy. Differentiating between upper and lower motor neuron lesions is important for determining the neurological level and prognosis.

1. UPPER & LOWER
MOTOR NEURON
LESIONS
Prepared by: Anish Dhakal
MBBS 3rd Year
anishdhakal718@gmail.com

2. OBJECTIVES
• To define Upper motor neurons and Lower motor neurons
• To describe and differentiate between Upper and Lower
motor neuron lesions

3. • Lower Motor Neurons:
Ventral horn of spinal cord and in cranial nerve nuclei in
brain stem
Integral motor component of the spinal reflexes
• Upper Motor Neurons:
Cerebral cortex and brain stem
Direct the activity of lower motor neurons
A least two neuron pathway needed for skeletal muscle
contraction

5. LESIONS OF THE CORTICOSPINAL
(PYRAMIDAL) TRACTS
• Babinski sign: Present in Newborns: (Incomplete
myelination)
• Inability to suppress usual withdrawal reflex

6. UMN LESIONS(PYRAMIDAL)
• Muscle weakness
• Loss of superficial
abdominal reflex
• Loss of cremasteric reflex
• Absence of voluntary fine
skilled movements

7. CLINICAL SIGNIFICANCE OF
LEVEL OF DECUSSATION
• Variable ipsilateral
or contralateral
motor weakness
based on the level
of corticospinal
tract lesion

8. LESIONS OF THE
EXTRAPYRAMIDAL TRACTS
• Severe paralysis
• Spasticity
• Exaggerated deep muscle reflex

9. • Upper motoneurons provide descending control over the
reflexes
• Results in hyperactive muscle stretch reflexes (tendon jerks &
Clonus)
(Upper motor neurons have Net inhibitory effect on
muscle stretch reflexes)

10. CLASP KNIFE REFLEX
• Oversensitive Golgi tendon
organs (loss of inhibitory
commands from UMN)

11. LOWER MOTOR NEURON
LESIONS
• Flaccid paralysis
• Loss of reflexes
• Muscular contracture
• Reaction of degeneration

12. LOWER MOTOR NEURON
LESIONS
• Muscular fasciculation & fibrillations (denervation super
sensitivity)
• Atrophy secondary to denervation of LMN
(Neither stretch reflex nor voluntary contraction)
In UMN lesions, can still be contracted by stretch reflexes
Disuse atrophy may be evident

13. POSSIBLE MOTOR LESION SITES
(CLINICAL SIGNIFICANCE)

14. UMN & LMN LESIONS
DIFFERENCES