Knee-Jerk Reflex: Neural Pathway & Clinical Significance

The patellar reflex (knee-jerk) is a monosynaptic stretch reflex—the simplest spinal reflex arc.

Neural Pathway

1. Stimulus: Tap patellar tendon → stretches quadriceps muscle
2. Sensory neuron: Muscle spindle Ia afferent → dorsal root ganglion → spinal cord (L2–L4)
3. Interneuron: None (monosynaptic) — direct synapse on α-motor neuron
4. Motor neuron: α-motor neuron → ventral root → femoral nerve → quadriceps
5. Response: Quadriceps contraction → knee extension
6. Simultaneous inhibition: Ia inhibitory interneuron → antagonist (hamstrings) relaxation

Clinical Significance

• Normal: Brisk, symmetric contraction
• Absent/hypoactive: Lower motor neuron lesion (peripheral nerve, root, anterior horn)
• Hyperactive + clonus: Upper motor neuron lesion (corticospinal tract)
• Asymmetry: Focal pathology

Testing Tips

• Patient relaxed, leg dangling
• Compare sides
• Reinforcement (Jendrassik maneuver): Clasp hands, pull apart

This reflex tests L2–L4 spinal segments and femoral nerve integrity.