<prompt> The nerve root value of knee jerk reflex isT12 L1 L2L2 L3 L4L5S1S2 S3  <response>: B L2 L3 L4 Knee Jerk reflex: Striking the patellar tendon with a tendon hammer just below the patella stretches the sensory nerve fiber of the femoral nerve which synapses without interneurons at the level of L4 in the spinal cord, completely independent of higher centres. From there, an alpha-motor neuron conducts an efferent impulse back to the quadriceps femoris muscle, triggering contraction. This contraction, coordinated with the relaxation of the antagonistic flexor hamstring muscle causes the leg to kick This reflex helps maintain posture and balance, allowing one to walk without consciously thinking about each step. The patellar reflex is a clinical and classic example of the monosynaptic reflex arc. There is no interneuron in the pathway leading to contraction of the quadriceps muscle. Instead the bipolar sensory neuron synapses directly on a motor neuron in the spinal cord. However, there is an inhibitory interneuron used to relax the antagonistic hamstring muscle. It tests L2, L3, and L4. 