The patellar reflex tests which nerve and root levels?

The patellar reflex tests which nerve and root levels?

• A. Femoral nerve; L2–L4
• B. Sciatic nerve; L4–S1
• C. Obturator nerve; L2–L3
• D. Lumbosacral trunk; L4–L5

Answer: (A)

The knee-jerk reflex primarily tests the function of the femoral nerve and its L2–L4 root contributions because the quadriceps muscle, which extends the knee, is innervated by the femoral nerve. When the patellar tendon is tapped, muscle spindles in the quadriceps detect the stretch and send an afferent signal via the femoral nerve to the spinal cord at L2–L4. This synapses directly with alpha motor neurons, whose axons travel back through the femoral nerve to the quadriceps, causing a quick contraction and knee extension. If there’s a lesion affecting the femoral nerve or L2–L4 roots, the patellar reflex is diminished or absent. The other nerves listed don’t control the quadriceps: the sciatic nerve handles posterior thigh and leg muscles and would influence knee flexion more than the reflex; the obturator nerve supplies medial thigh adductors; the lumbosacral trunk contributes to plexus formation but isn’t the direct reflex arc for the knee-jerk.

The knee-jerk reflex primarily tests the function of the femoral nerve and its L2–L4 root contributions because the quadriceps muscle, which extends the knee, is innervated by the femoral nerve. When the patellar tendon is tapped, muscle spindles in the quadriceps detect the stretch and send an afferent signal via the femoral nerve to the spinal cord at L2–L4. This synapses directly with alpha motor neurons, whose axons travel back through the femoral nerve to the quadriceps, causing a quick contraction and knee extension. If there’s a lesion affecting the femoral nerve or L2–L4 roots, the patellar reflex is diminished or absent. The other nerves listed don’t control the quadriceps: the sciatic nerve handles posterior thigh and leg muscles and would influence knee flexion more than the reflex; the obturator nerve supplies medial thigh adductors; the lumbosacral trunk contributes to plexus formation but isn’t the direct reflex arc for the knee-jerk.