What is knee jerk?
- Definition: A monosynaptic reflex that tests the integrity of the L2-L4 spinal segments and the femoral nerve.
- Stimulus: Tapping the patellar tendon with a reflex hammer
- Response: Contraction of the quadriceps muscle, causing leg extension at the knee.
How does knee jerk work?
Pathway:
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Sensory input via muscle spindle (intrafusal fibers) of quadriceps.
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Afferent impulse travels via the femoral nerve to the L2-L4 spinal cord.
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Synapses directly with an alpha motor neuron (monosynaptic).
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Efferent impulse sent back through the femoral nerve to the quadriceps.
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| physioogical pathway of knee jerk |
What is Use of Knee Jerk in daily life?
- Function: Maintains posture and balance, prevents knee buckling.
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Clinical Significance:
- Hyperreflexia → Suggests upper motor neuron (UMN) lesion.
- Hyporeflexia/Absent reflex → Suggests lower motor neuron (LMN) lesion or peripheral nerve dysfunction.
- Westphal’s Sign → Absence of knee jerk, seen in LMN lesions, neuropathies, or spinal cord damage.
What are Associated Conditions with abnormal Knee Jerk reflex:
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Upper Motor Neuron (UMN) Lesions (Hyperreflexia)
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Stroke (CVA)
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Spinal cord injury (above L2-L4 level)
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Multiple sclerosis (MS)
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Amyotrophic lateral sclerosis (ALS) (UMN phase)
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Traumatic brain injury (TBI)
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Cerebral palsy
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Brain tumors affecting corticospinal tract
Lower Motor Neuron (LMN) Lesions (Hyporeflexia or Absent Reflex)
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Peripheral neuropathy (e.g., diabetic neuropathy)
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Guillain-Barré syndrome
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Poliomyelitis
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Radiculopathy (L2-L4 nerve root compression, herniated disc)
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Amyotrophic lateral sclerosis (ALS) (LMN phase)
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Cauda equina syndrome
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Spinal muscular atrophy (SMA)
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Lumbar plexopathy (e.g., trauma, diabetes, neoplastic infiltration)
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Question for practice
Delayed relaxation of ankle jerk is seen in:
Explanation
MCQ 2:
A 60-year-old male presents with weakness in both lower limbs and increased knee jerk reflexes. Which of the following is the most likely cause?
A) Guillain-Barré syndrome
B) Diabetic neuropathy
C) Spinal cord injury (above L2)
D) Cauda equina syndrome
Answer: ✅ C) Spinal cord injury (above L2)
Explanation:
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The knee jerk reflex (L2-L4) is exaggerated (hyperreflexia) in UMN lesions.
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Spinal cord injury above L2 disrupts the descending inhibitory control from the brain, leading to increased reflexes (hyperreflexia).
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Guillain-Barré syndrome (A) and diabetic neuropathy (B) cause LMN lesions, leading to absent or reduced reflexes (hyporeflexia).
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Cauda equina syndrome (D) affects the lumbar and sacral nerve roots, causing flaccid paralysis and absent knee jerk reflex.
Thus, the most likely diagnosis is spinal cord injury above L2, causing bilateral weakness with hyperreflexia.
