The nerve root value of knee jerk reflex is
(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.
T12 L1 L2
L2 L3 L4
L5S1
S2 S3
Nervous System