Monosynaptic Reflex: Understanding the Knee-Jerk Response

The patellar (knee-jerk) reflex is the classic monosynaptic stretch reflex – the simplest neural circuit.

Arc Components

1. Receptor: Muscle spindle (intrafusal fibers) in quadriceps tendon – detects stretch.
2. Sensory neuron: Ia afferent (large, myelinated) → dorsal root ganglion → spinal cord (L2–L4).
3. Single synapse: Directly onto α-motor neuron in ventral horn (no interneuron).
4. Motor neuron: α-efferent → quadriceps (agonist).
5. Effector: Quadriceps contraction → knee extension.
6. Reciprocal inhibition (polysynaptic): Ia inhibitory interneuron → inhibits antagonist (hamstrings).

Clinical Utility

• Tests L2–L4 nerve roots, femoral nerve, neuromuscular junction.
• Hyperreflexia (brisk): Upper motor neuron lesion (corticospinal tract).
• Hyporeflexia/absent: Lower motor neuron lesion, peripheral neuropathy, nerve root compression.
• Asymmetry: Key localizing sign.

Other Monosynaptic Reflexes

• Ankle jerk (S1–S2, gastrocnemius/soleus).
• Biceps jerk (C5–C6).
• Triceps jerk (C7–C8).
• Brachioradialis (C5–C6).

Monosynaptic = fastest reflex (30–50 ms); basis for clinical reflex testing.