The patellar reflex primarily tests which spinal nerve root?

The patellar reflex primarily tests which spinal nerve root?

• A. L2
• B. L4
• C. L3
• D. L5

Answer: (B)

The knee-jerk reflex mainly assesses the L4 nerve root. When the patellar tendon is tapped, the stretch of the quadriceps is detected by muscle spindles, sending an afferent signal through the femoral nerve into the spinal cord at L4. There, it directly synapses on the alpha motor neurons that innervate the quadriceps, causing the muscle to contract and the knee to extend. The quadriceps muscle group is primarily supplied by nerves from the L2–L4 levels, with L4 being the most closely tied to this reflex. So this reflex is the strongest bedside indicator of L4 integrity. If this reflex is diminished or absent, L4 involvement is a primary consideration. The other roots are related to different functions (for example, L5 with dorsiflexion and toe movements, etc.), so they do not primarily govern this reflex.

The knee-jerk reflex mainly assesses the L4 nerve root. When the patellar tendon is tapped, the stretch of the quadriceps is detected by muscle spindles, sending an afferent signal through the femoral nerve into the spinal cord at L4. There, it directly synapses on the alpha motor neurons that innervate the quadriceps, causing the muscle to contract and the knee to extend. The quadriceps muscle group is primarily supplied by nerves from the L2–L4 levels, with L4 being the most closely tied to this reflex. So this reflex is the strongest bedside indicator of L4 integrity. If this reflex is diminished or absent, L4 involvement is a primary consideration. The other roots are related to different functions (for example, L5 with dorsiflexion and toe movements, etc.), so they do not primarily govern this reflex.